Table of Contents

This is the specification for the D Programming Language. For more information see dlang.org.

- Introduction

- Lexical

- Grammar

- Modules

- Declarations

- Types

- Properties

- Attributes

- Pragmas

- Expressions

- Statements

- Arrays

- Associative Arrays

- Structs and Unions

- Classes

- Interfaces

- Enums

- Type Qualifiers

- Functions

- Operator Overloading

- Templates

- Template Mixins

- Contract Programming

- Conditional Compilation

- Traits

- Error Handling

- Unit Tests

- Garbage Collection

- Floating Point

- D x86 Inline Assembler

- Embedded Documentation

- Interfacing to C

- Interfacing to C++

- Interfacing to Objective-C

- Portability Guide

- Named Character Entities

- Memory Safety

- Application Binary Interface

- Vector Extensions

- Better C

- Import C

- Live Functions

Introduction

D is a general-purpose systems programming language with a C-like syntax that compiles to native code. It is statically typed and supports both automatic (garbage collected) and manual memory management. D programs are structured as modules that can be compiled separately and linked with external libraries to create native libraries or executables.

This document is the reference manual for the D Programming Language. For more information and other documents, see The D Language Website.

Phases of Compilation

The process of compiling is divided into multiple phases. Each phase is independent of subsequent phases. For example, the scanner is not affected by the semantic analyzer. This separation of passes makes language tools like syntax-directed editors relatively easy to create.

1. source character set
The source file is checked to determine its encoding and the appropriate scanner is loaded. 7-bit ASCII and UTF encodings are accepted.

2. script line
If the first line starts with "#!", then that line is ignored.

3. lexical analysis
The source file is divided into a sequence of tokens. Special tokens are replaced with other tokens. SpecialTokenSequences are processed and removed.

4. syntax analysis
The sequence of tokens is parsed to form syntax trees.

5. semantic analysis
The syntax trees are traversed to declare variables, load symbol tables, assign types, and determine the meaning of the program.

6. optimization
Optimization is an optional pass that attempts to rewrite the program in a semantically equivalent, more performant, version.

7. code generation
Instructions are selected from the target architecture to implement the semantics of the program. The typical result will be an object file suitable for input to a linker.

Memory Model

The byte is the fundamental unit of storage. Each byte has 8 bits and is stored at a unique address. A memory location is a sequence of one or more bytes of the exact size required to hold a scalar type. Multiple threads can access separate memory locations without interference.

Memory locations come in three groups:

1. Thread-local memory locations are accessible from only one thread at a time.

2. Immutable memory locations cannot be written to during their lifetime. Immutable memory locations can be read from by multiple threads without synchronization.

3. Shared memory locations are accessible from multiple threads.

Undefined Behavior: Allowing multiple threads to access a thread-local memory location results in undefined behavior.

Undefined Behavior: Writing to an immutable memory location during its lifetime results in undefined behavior.

Undefined Behavior: Writing to a shared memory location in one thread while one or more additional threads read from or write to the same location is undefined behavior unless all of the reads and writes are synchronized.

Execution of a single thread on thread-local and immutable memory locations is sequentially consistent. This means the collective result of the operations is the same as if they were executed in the same order that the operations appear in the program.

A memory location can be transferred from thread-local to immutable or shared if there is only one reference to the location.

A memory location can be transferred from shared to immutable or thread-local if there is only one reference to the location.

A memory location can be temporarily transferred from shared to local if synchronization is used to prevent any other threads from accessing the memory location during the operation.

Object Model

An object is created in the following circumstances:

- a definition

- a NewExpression

- a temporary is created

- changing which field of a union is active

An object spans a sequence of memory locations which may or may not be contiguous. Its lifetime encompasses construction, destruction, and the period in between. Each object has a type which is determined either statically or by runtime type information. The object's memory locations may include any combination of thread-local, immutable, or shared.

Objects can be composed into a composed object. Objects that make up a composed object are subobjects. An object that is not the subobject of another object is a complete object. The lifetime of a subobject is always within the lifetime of the complete object to which it belongs.

An object's address is the address of the first byte of the first memory location for that object. Object addresses are distinct unless one object is nested within the other.

Arithmetic

Integer Arithmetic

Integer arithmetic is performed using two's complement math. Integer overflow is not checked for.

Floating Point Arithmetic

Floating point arithmetic is performed using IEEE-754 floating point math.

Lexical

The lexical analysis is independent of the syntax parsing and the semantic analysis. The lexical analyzer splits the source text into tokens. The lexical grammar describes the syntax of these tokens. The grammar is designed to be suitable for high-speed scanning and to facilitate the implementation of a correct scanner. It has a minimum of special case rules and there is only one phase of translation.

Source Text

SourceFile:
ByteOrderMark Moduleopt
Shebang Moduleopt
Moduleopt

ByteOrderMark:
\uFEFF

Shebang:
#! Charactersopt EndOfShebang

EndOfShebang:
\u000A
EndOfFile

Source text can be encoded as any one of the following:

- ASCII (strictly, 7-bit ASCII)

- UTF-8

- UTF-16BE

- UTF-16LE

- UTF-32BE

- UTF-32LE

One of the following UTF BOMs (Byte Order Marks) can be present at the beginning of the source text:

UTF Byte Order Marks

Format

BOM

UTF-8

EF BB BF

UTF-16BE

FE FF

UTF-16LE

FF FE

UTF-32BE

00 00 FE FF

UTF-32LE

FF FE 00 00

ASCII

no BOM

If the source file does not begin with a BOM, then the first character must be less than or equal to U+0000007F.

The source text is decoded from its source representation into Unicode Characters. The Characters are further divided into: WhiteSpace, EndOfLine, Comments, SpecialTokenSequences, and Tokens, with the source terminated by an EndOfFile.

The source text is split into tokens using the maximal munch algorithm, i.e., the lexical analyzer assumes the longest possible token. For example, >> is a right-shift token rather than two greater-than tokens. There are two exceptions to this rule:

- A .. embedded between what appear to be two floating point literals, as in 1..2, is interpreted as if the .. were separated by a space from the first integer.

- A 1.a is interpreted as the three tokens 1, ., and a, whereas 1. a is interpreted as the two tokens 1. and a.

Character Set

Character:
any Unicode character

End of File

EndOfFile:
physical end of the file
\u0000
\u001A

The source text is terminated by whichever comes first.

End of Line

EndOfLine:
\u000D
\u000A
\u000D \u000A
\u2028
\u2029
EndOfFile

White Space

WhiteSpace:
Space
Space WhiteSpace

Space:
\u0020
\u0009
\u000B
\u000C

Comments

Comment:
BlockComment
LineComment
NestingBlockComment

BlockComment:
/* Charactersopt */

LineComment:
// Charactersopt EndOfLine

NestingBlockComment:
/+ NestingBlockCommentCharactersopt +/

NestingBlockCommentCharacters:
NestingBlockCommentCharacter
NestingBlockCommentCharacter NestingBlockCommentCharacters

NestingBlockCommentCharacter:
Character
NestingBlockComment

Characters:
Character
Character Characters

There are three kinds of comments:

1. Block comments can span multiple lines, but do not nest.

2. Line comments terminate at the end of the line.

3. Nesting block comments can span multiple lines and can nest.

The contents of strings and comments are not tokenized. Consequently, comment openings occurring within a string do not begin a comment, and string delimiters within a comment do not affect the recognition of comment closings and nested /+ comment openings. With the exception of /+ occurring within a /+ comment, comment openings within a comment are ignored.

a = /+ // +/ 1;    // parses as if 'a = 1;'
a = /+ "+/" +/ 1"; // parses as if 'a = " +/ 1";'
a = /+ /* +/ */ 3; // parses as if 'a = */ 3;'

Comments cannot be used as token concatenators, for example, abc/**/def is two tokens, abc and def, not one abcdef token.

Tokens

Tokens:
Token
Token Tokens

Token:
{
}
TokenNoBraces

TokenNoBraces:
Identifier
StringLiteral
CharacterLiteral
IntegerLiteral
FloatLiteral
Keyword
/
/=
.
..
...
&
&=
&&
|
|=
||
-
-=
--
+
+=
++
<
<=
<<
<<=
>
>=
>>=
>>>=
>>
>>>
!
!=
(
)
[
]
?
,
;
:
$
=
==
*
*=
%
%=
^
^=
^^
^^=
~
~=
@
=>
#

Identifiers

Identifier:
IdentifierStart
IdentifierStart IdentifierChars

IdentifierChars:
IdentifierChar
IdentifierChar IdentifierChars

IdentifierStart:
_
Letter
UniversalAlpha

IdentifierChar:
IdentifierStart
0
NonZeroDigit

Identifiers start with a letter, _, or universal alpha, and are followed by any number of letters, _, digits, or universal alphas. Universal alphas are as defined in ISO/IEC 9899:1999(E) Appendix D of the C99 Standard. Identifiers can be arbitrarily long, and are case sensitive.

Implementation Defined: Identifiers starting with __ (two underscores) are reserved.

String Literals

StringLiteral:
WysiwygString
AlternateWysiwygString
DoubleQuotedString

HexString
DelimitedString
TokenString

WysiwygString:
r" WysiwygCharactersopt " StringPostfixopt

AlternateWysiwygString:
` WysiwygCharactersopt ` StringPostfixopt

WysiwygCharacters:
WysiwygCharacter
WysiwygCharacter WysiwygCharacters

WysiwygCharacter:
Character
EndOfLine

DoubleQuotedString:
" DoubleQuotedCharactersopt " StringPostfixopt

DoubleQuotedCharacters:
DoubleQuotedCharacter
DoubleQuotedCharacter DoubleQuotedCharacters

DoubleQuotedCharacter:
Character
EscapeSequence
EndOfLine

EscapeSequence:
\'
\"
\?
\\
\0
\a
\b
\f
\n
\r
\t
\v
\x HexDigit HexDigit
\ OctalDigit
\ OctalDigit OctalDigit
\ OctalDigit OctalDigit OctalDigit
\u HexDigit HexDigit HexDigit HexDigit
\U HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit HexDigit
\ NamedCharacterEntity

HexString:
x" HexStringCharsopt " StringPostfixopt

HexStringChars:
HexStringChar
HexStringChar HexStringChars

HexStringChar:
HexDigit
WhiteSpace
EndOfLine

StringPostfix:
c
w
d

DelimitedString:
q" Delimiter WysiwygCharactersopt MatchingDelimiter " StringPostfixopt
q"( ParenDelimitedCharactersopt )" StringPostfixopt
q"[ BracketDelimitedCharactersopt ]" StringPostfixopt
q"{ BraceDelimitedCharactersopt }" StringPostfixopt
q"< AngleDelimitedCharactersopt >" StringPostfixopt

Delimiter:
Identifier

MatchingDelimiter:
Identifier

ParenDelimitedCharacters:
WysiwygCharacter
WysiwygCharacter ParenDelimitedCharacters
( ParenDelimitedCharactersopt )

BracketDelimitedCharacters:
WysiwygCharacter
WysiwygCharacter BracketDelimitedCharacters
[ BracketDelimitedCharactersopt ]

BraceDelimitedCharacters:
WysiwygCharacter
WysiwygCharacter BraceDelimitedCharacters
{ BraceDelimitedCharactersopt }

AngleDelimitedCharacters:
WysiwygCharacter
WysiwygCharacter AngleDelimitedCharacters
< AngleDelimitedCharactersopt >

TokenString:
q{ TokenStringTokensopt } StringPostfixopt

TokenStringTokens:
TokenStringToken
TokenStringToken TokenStringTokens

TokenStringToken:
TokenNoBraces
{ TokenStringTokensopt }

A string literal is either a double quoted string, a wysiwyg quoted string, a delimited string, a token string, or a hex string.

In all string literal forms, an EndOfLine is regarded as a single \n character.

String literals are read only.

Undefined Behavior: Writes to string literals cannot always be detected, but cause undefined behavior.

Wysiwyg Strings

Wysiwyg ("what you see is what you get") quoted strings can be defined using either of two syntaxes.

In the first form, they are enclosed between r" and ". All characters between the r" and " are part of the string. There are no escape sequences inside wysiwyg strings.

r"I am Oz"
r"c:\games\Sudoku.exe"
r"ab\n" // string is 4 characters,
// 'a', 'b', '\', 'n'

Alternatively, wysiwyg strings can be enclosed by backquotes, using the ` character.

`the Great and Powerful.`
`c:\games\Empire.exe`
`The "lazy" dog`
`a"b\n`  // string is 5 characters,
// 'a', '"', 'b', '\', 'n'

Double Quoted Strings

Double quoted strings are enclosed by "". EscapeSequences can be embedded in them.

"Who are you?"
"c:\\games\\Doom.exe"
"ab\n"   // string is 3 characters,
// 'a', 'b', and a linefeed
"ab
"        // string is 3 characters,
// 'a', 'b', and a linefeed

Hex Strings

Hex strings allow string literals to be created using hex data. The hex data need not form valid UTF characters.

x"0A"              // same as "\x0A"
x"00 FBCD 32FD 0A" // same as
// "\x00\xFB\xCD\x32\xFD\x0A"

Whitespace and newlines are ignored, so the hex data can be easily formatted. The number of hex characters must be a multiple of 2.

Note: Hex Strings are deprecated. Please use std.conv.hexString instead.

Delimited Strings

Delimited strings use various forms of delimiters. The delimiter, whether a character or identifier, must immediately follow the " without any intervening whitespace. The terminating delimiter must immediately precede the closing " without any intervening whitespace. A nesting delimiter nests, and is one of the following characters:

Nesting Delimiters

Delimiter

Matching Delimiter

[

]

(

)

<

>

{

}

q"(foo(xxx))"   // "foo(xxx)"
q"[foo{]"       // "foo{"

If the delimiter is an identifier, the identifier must be immediately followed by a newline, and the matching delimiter must be the same identifier starting at the beginning of the line:

writeln(q"EOS
This
is a multi-line
heredoc string
EOS"
);

The newline following the opening identifier is not part of the string, but the last newline before the closing identifier is part of the string. The closing identifier must be placed on its own line at the leftmost column.

Otherwise, the matching delimiter is the same as the delimiter character:

q"/foo]/"          // "foo]"
// q"/abc/def/"    // error

Token Strings

Token strings open with the characters q{ and close with the token }. In between must be valid D tokens. The { and } tokens nest. The string is formed of all the characters between the opening and closing of the token string, including comments.

q{this is the voice of} // "this is the voice of"
q{/*}*/ }               // "/*}*/ "
q{ world(q{control}); } // " world(q{control}); "
q{ __TIME__ }           // " __TIME__ "
// i.e. it is not replaced with the time
// q{ __EOF__ }         // error
// __EOF__ is not a token, it's end of file

String Postfix

The optional StringPostfix character gives a specific type to the string, rather than it being inferred from the context. The types corresponding to the postfix characters are:

String Literal Postfix Characters

Postfix

Type

Alias

c

immutable(char)[]

string

w

immutable(wchar)[]

wstring

d

immutable(dchar)[]

dstring

"hello"c  // string
"hello"w  // wstring
"hello"d  // dstring

The string literals are assembled as UTF-8 char arrays, and the postfix is applied to convert to wchar or dchar as necessary as a final step.

Escape Sequences

The escape sequences listed in EscapeSequence are:

Escape Sequences

Sequence

Meaning

'

Literal single-quote: '

"

Literal double-quote: "

?

Literal question mark: ?

\

Literal backslash:

\0

Binary zero (NUL, U+0000).

\a

BEL (alarm) character (U+0007).

\b

Backspace (U+0008).

\f

Form feed (FF) (U+000C).

\n

End-of-line (U+000A).

\r

Carriage return (U+000D).

\t

Horizontal tab (U+0009).

\v

Vertical tab (U+000B).

\xnn

Byte value in hexadecimal, where nn is specified as two hexadecimal digits.
For example: \xFF represents the character with the value 255.

n
nn
nnn

Byte value in octal.
For example: \101 represents the character with the value 65 ('A'). Analogous to hexadecimal characters, the largest byte value is \377 (= \xFF in hexadecimal or 255 in decimal)

\unnnn

Unicode character U+nnnn, where nnnn are four hexadecimal digits.
For example, \u03B3 represents the Unicode character γ (U+03B3 - GREEK SMALL LETTER GAMMA).

\Unnnnnnnn

Unicode character U+nnnnnnnn, where nnnnnnnn are 8 hexadecimal digits.
For example, \U0001F603 represents the Unicode character U+1F603 (SMILING FACE WITH OPEN MOUTH).

name

Named character entity from the HTML5 specification. These names begin with & and end with ;

e.g., &euro;. See NamedCharacterEntity.

Character Literals

CharacterLiteral:
' SingleQuotedCharacter '

SingleQuotedCharacter:
Character
EscapeSequence

Character literals are a single character or escape sequence enclosed by single quotes.

'h'   // the letter h
'\n'  // newline
'\\'  // the backslash character

Integer Literals

IntegerLiteral:
Integer
Integer IntegerSuffix

Integer:
DecimalInteger
BinaryInteger
HexadecimalInteger

IntegerSuffix:
L
u
U
Lu
LU
uL
UL

DecimalInteger:
0
NonZeroDigit
NonZeroDigit DecimalDigitsUS

BinaryInteger:
BinPrefix BinaryDigitsNoSingleUS

BinPrefix:
0b
0B

HexadecimalInteger:
HexPrefix HexDigitsNoSingleUS

NonZeroDigit:
1
2
3
4
5
6
7
8
9

DecimalDigits:
DecimalDigit
DecimalDigit DecimalDigits

DecimalDigitsUS:
DecimalDigitUS
DecimalDigitUS DecimalDigitsUS

DecimalDigitsNoSingleUS:
DecimalDigit
DecimalDigit DecimalDigitsUS
DecimalDigitsUS DecimalDigit

DecimalDigitsNoStartingUS:
DecimalDigit
DecimalDigit DecimalDigitsUS

DecimalDigit:
0
NonZeroDigit

DecimalDigitUS:
DecimalDigit
_

BinaryDigitsNoSingleUS:
BinaryDigit
BinaryDigit BinaryDigitsUS
BinaryDigitsUS BinaryDigit
BinaryDigitsUS BinaryDigit BinaryDigitsUS

BinaryDigitsUS:
BinaryDigitUS
BinaryDigitUS BinaryDigitsUS

BinaryDigit:
0
1

BinaryDigitUS:
BinaryDigit
_

OctalDigit:
0
1
2
3
4
5
6
7

HexDigits:
HexDigit
HexDigit HexDigits

HexDigitsUS:
HexDigitUS
HexDigitUS HexDigitsUS

HexDigitsNoSingleUS:
HexDigit
HexDigit HexDigitsUS
HexDigitsUS HexDigit

HexDigitsNoStartingUS:
HexDigit
HexDigit HexDigitsUS

HexDigit:
DecimalDigit
HexLetter

HexDigitUS:
HexDigit
_

HexLetter:
a
b
c
d
e
f
A
B
C
D
E
F

Integers can be specified in decimal, binary, or hexadecimal.

Decimal integers are a sequence of decimal digits.

Binary integers are a sequence of binary digits preceded by a ‘0b’ or ‘0B’.

C-style octal integer notation was deemed too easy to mix up with decimal notation; it is only fully supported in string literals. D still supports octal integer literals interpreted at compile time through the std.conv.octal template, as in octal!167.

Hexadecimal integers are a sequence of hexadecimal digits preceded by a ‘0x’ or ‘0X’.

Integers can have embedded ‘_’ characters to improve readability, and which are ignored.

20_000        // leagues under the sea
867_5309      // number on the wall
1_522_000     // thrust of F1 engine (lbf sea level)

Integers can be immediately followed by one ‘L’ or one of ‘u’ or ‘U’ or both. Note that there is no ‘l’ suffix.

The type of the integer is resolved as follows:

Decimal Literal Types

Literal

Type

Usual decimal notation

0 .. 2_147_483_647

int

2_147_483_648 .. 9_223_372_036_854_775_807

long

9_223_372_036_854_775_808 .. 18_446_744_073_709_551_615

ulong

Explicit suffixes

0L .. 9_223_372_036_854_775_807L

long

0U .. 4_294_967_295U

uint

4_294_967_296U .. 18_446_744_073_709_551_615U

ulong

0UL .. 18_446_744_073_709_551_615UL

ulong

Hexadecimal notation

0x0 .. 0x7FFF_FFFF

int

0x8000_0000 .. 0xFFFF_FFFF

uint

0x1_0000_0000 .. 0x7FFF_FFFF_FFFF_FFFF

long

0x8000_0000_0000_0000 .. 0xFFFF_FFFF_FFFF_FFFF

ulong

Hexadecimal notation with explicit suffixes

0x0L .. 0x7FFF_FFFF_FFFF_FFFFL

long

0x8000_0000_0000_0000L .. 0xFFFF_FFFF_FFFF_FFFFL

ulong

0x0U .. 0xFFFF_FFFFU

uint

0x1_0000_0000U .. 0xFFFF_FFFF_FFFF_FFFFU

ulong

0x0UL .. 0xFFFF_FFFF_FFFF_FFFFUL

ulong

An integer literal may not exceed these values.

Best Practices: Octal integer notation is not supported for integer literals. However, octal integer literals can be interpreted at compile time through the std.conv.octal template, as in octal!167.

Floating Point Literals

FloatLiteral:
Float
Float Suffix
Integer FloatSuffix
Integer ImaginarySuffix
Integer FloatSuffix ImaginarySuffix
Integer RealSuffix ImaginarySuffix

Float:
DecimalFloat
HexFloat

DecimalFloat:
LeadingDecimal .
LeadingDecimal . DecimalDigits
DecimalDigits . DecimalDigitsNoStartingUS DecimalExponent
. DecimalInteger
. DecimalInteger DecimalExponent
LeadingDecimal DecimalExponent

DecimalExponent:
DecimalExponentStart DecimalDigitsNoSingleUS

DecimalExponentStart:
e
E
e+
E+
e-
E-

HexFloat:
HexPrefix HexDigitsNoSingleUS . HexDigitsNoStartingUS HexExponent
HexPrefix . HexDigitsNoStartingUS HexExponent
HexPrefix HexDigitsNoSingleUS HexExponent

HexPrefix:
0x
0X

HexExponent:
HexExponentStart DecimalDigitsNoSingleUS

HexExponentStart:
p
P
p+
P+
p-
P-

Suffix:
FloatSuffix
RealSuffix
ImaginarySuffix
FloatSuffix ImaginarySuffix
RealSuffix ImaginarySuffix

FloatSuffix:
f
F

RealSuffix:
L

ImaginarySuffix:
i

LeadingDecimal:
DecimalInteger
0 DecimalDigitsNoSingleUS

Floats can be in decimal or hexadecimal format.

Hexadecimal floats are preceded by a 0x or 0X and the exponent is a p or P followed by a decimal number serving as the exponent of 2.

Floating literals can have embedded ‘_’ characters to improve readability, and which are ignored.

2.645_751
6.022140857E+23
6_022.140857E+20
6_022_.140_857E+20_

Floating literals with no suffix are of type double. Floating literals followed by f or F are of type float, and those followed by L are of type real.

Examples:

0x1.FFFFFFFFFFFFFp1023 // double.max
0x1p-52                // double.epsilon
1.175494351e-38F       // float.min

The literal may not exceed the range of the type. The literal is rounded to fit into the significant digits of the type.

If a floating literal has a . and a type suffix, at least one digit must be in-between:

1f;  // OK, float
1.f; // error
1.;  // OK, double

NOTE: Floating literals followed by i to denote imaginary floating point values have been deprecated.

Keywords

Keywords are reserved identifiers.

Keyword:
abstract
alias
align
asm
assert
auto

body
bool
break
byte

case
cast
catch
cdouble
cent
cfloat
char
class
const
continue
creal

dchar
debug
default
delegate
delete
deprecated
do
double

else
enum
export
extern

false
final
finally
float
for
foreach
foreach_reverse
function

goto

idouble
if
ifloat
immutable
import
in
inout
int
interface
invariant
ireal
is

lazy
long

macro
mixin
module

new
nothrow
null

out
override

package
pragma
private
protected
public
pure

real
ref
return

scope
shared
short
static
struct
super
switch
synchronized

template
this
throw
true
try
typeid
typeof

ubyte
ucent
uint
ulong
union
unittest
ushort

version
void

wchar
while
with

__FILE__
__FILE_FULL_PATH__
__MODULE__
__LINE__
__FUNCTION__
__PRETTY_FUNCTION__

__gshared
__traits
__vector
__parameters

Special Tokens

These tokens are replaced with other tokens according to the following table:

Special Tokens

Special Token

Replaced with

__DATE__

string literal of the date of compilation "mmm dd yyyy"

__EOF__

tells the scanner to ignore everything after this token

__TIME__

string literal of the time of compilation "hh:mm:ss"

__TIMESTAMP__

string literal of the date and time of compilation "www mmm dd hh:mm:ss yyyy"

__VENDOR__

Compiler vendor string

__VERSION__

Compiler version as an integer

Implementation Defined: The replacement string literal for __VENDOR__ and the replacement integer value for __VERSION__.

Special Token Sequences

SpecialTokenSequence:
# line IntegerLiteral Filespecopt EndOfLine
# line __LINE__ Filespecopt EndOfLine

Filespec:
" Charactersopt "

Special token sequences are processed by the lexical analyzer, may appear between any other tokens, and do not affect the syntax parsing.

There is currently only one special token sequence, #line.

This sets the current source line number to IntegerLiteral, and optionally the current source file name to Filespec, beginning with the next line of source text.

The backslash character is not treated specially inside Filespec strings.

For example:

int #line 6 "pkg/mod.d"
x;  // this is now line 6 of file pkg/mod.d

Implementation Defined: The source file and line number is typically used for printing error messages and for mapping generated code back to the source for the symbolic debugging output.

Grammar

Lexical Syntax

Refer to the page for lexical syntax.

Modules

Module:
ModuleDeclaration
ModuleDeclaration DeclDefs
DeclDefs

DeclDefs:
DeclDef
DeclDef DeclDefs

DeclDef:
AttributeSpecifier
Declaration
Constructor
Destructor
Postblit
Allocator
Deallocator
Invariant
UnitTest
AliasThis
StaticConstructor
StaticDestructor
SharedStaticConstructor
SharedStaticDestructor
ConditionalDeclaration
DebugSpecification
VersionSpecification
StaticAssert
TemplateDeclaration
TemplateMixinDeclaration
TemplateMixin
MixinDeclaration
EmptyDeclaration

EmptyDeclaration:
;

ModuleDeclaration:
ModuleAttributesopt module ModuleFullyQualifiedName ;

ModuleAttributes:
ModuleAttribute
ModuleAttribute ModuleAttributes

ModuleAttribute:
DeprecatedAttribute
UserDefinedAttribute

ModuleFullyQualifiedName:
ModuleName
Packages . ModuleName

ModuleName:
Identifier

Packages:
PackageName
Packages . PackageName

PackageName:
Identifier

ImportDeclaration:
import ImportList ;
static import ImportList ;

ImportList:
Import
ImportBindings
Import , ImportList

Import:
ModuleFullyQualifiedName
ModuleAliasIdentifier = ModuleFullyQualifiedName

ImportBindings:
Import : ImportBindList

ImportBindList:
ImportBind
ImportBind , ImportBindList

ImportBind:
Identifier
Identifier = Identifier

ModuleAliasIdentifier:
Identifier

MixinDeclaration:
mixin ( ArgumentList ) ;

Declarations

Declaration:
FuncDeclaration
VarDeclarations
AliasDeclaration
AliasAssign
AggregateDeclaration
EnumDeclaration
ImportDeclaration
ConditionalDeclaration
StaticForeachDeclaration
StaticAssert

VarDeclarations:
StorageClassesopt BasicType Declarators ;
AutoDeclaration

Declarators:
DeclaratorInitializer
DeclaratorInitializer , DeclaratorIdentifierList

DeclaratorInitializer:
VarDeclarator
VarDeclarator TemplateParametersopt = Initializer
AltDeclarator
AltDeclarator = Initializer

DeclaratorIdentifierList:
DeclaratorIdentifier
DeclaratorIdentifier , DeclaratorIdentifierList

DeclaratorIdentifier:
VarDeclaratorIdentifier
AltDeclaratorIdentifier

VarDeclaratorIdentifier:
Identifier
Identifier TemplateParametersopt = Initializer

AltDeclaratorIdentifier:
TypeSuffixes Identifier AltDeclaratorSuffixesopt
TypeSuffixes Identifier AltDeclaratorSuffixesopt = Initializer
TypeSuffixesopt Identifier AltDeclaratorSuffixes
TypeSuffixesopt Identifier AltDeclaratorSuffixes = Initializer

Declarator:
VarDeclarator
AltDeclarator

VarDeclarator:
TypeSuffixesopt Identifier

AltDeclarator:
TypeSuffixesopt Identifier AltDeclaratorSuffixes
TypeSuffixesopt ( AltDeclaratorInner )
TypeSuffixesopt ( AltDeclaratorInner ) AltFuncDeclaratorSuffix
TypeSuffixesopt ( AltDeclaratorInner ) AltDeclaratorSuffixes

AltDeclaratorInner:
TypeSuffixesopt Identifier
TypeSuffixesopt Identifier AltFuncDeclaratorSuffix
AltDeclarator

AltDeclaratorSuffixes:
AltDeclaratorSuffix
AltDeclaratorSuffix AltDeclaratorSuffixes

AltDeclaratorSuffix:
[ ]
[ AssignExpression ]
[ Type ]

AltFuncDeclaratorSuffix:
Parameters MemberFunctionAttributesopt

StorageClasses:
StorageClass
StorageClass StorageClasses

StorageClass:
LinkageAttribute
AlignAttribute
AtAttribute
deprecated
enum
static
extern
abstract
final
override
synchronized
auto
scope
const
immutable
inout
shared
__gshared
Property
nothrow
pure
ref

Initializer:
VoidInitializer
NonVoidInitializer

NonVoidInitializer:
ExpInitializer
ArrayInitializer
StructInitializer

ExpInitializer:
AssignExpression

ArrayInitializer:
[ ArrayMemberInitializationsopt ]

ArrayMemberInitializations:
ArrayMemberInitialization
ArrayMemberInitialization ,
ArrayMemberInitialization , ArrayMemberInitializations

ArrayMemberInitialization:
NonVoidInitializer
AssignExpression : NonVoidInitializer

StructInitializer:
{ StructMemberInitializersopt }

StructMemberInitializers:
StructMemberInitializer
StructMemberInitializer ,
StructMemberInitializer , StructMemberInitializers

StructMemberInitializer:
NonVoidInitializer
Identifier : NonVoidInitializer

AutoDeclaration:
StorageClasses AutoAssignments ;

AutoAssignments:
AutoAssignment
AutoAssignments , AutoAssignment

AutoAssignment:
Identifier TemplateParametersopt = Initializer

AliasDeclaration:
alias StorageClassesopt BasicType Declarators ;
alias StorageClassesopt BasicType FuncDeclarator ;
alias AliasAssignments ;

AliasAssignments:
AliasAssignment
AliasAssignments , AliasAssignment

AliasAssignment:
Identifier TemplateParametersopt = StorageClassesopt Type
Identifier TemplateParametersopt = FunctionLiteral
Identifier TemplateParametersopt = StorageClassesopt BasicType Parameters MemberFunctionAttributesopt

AliasAssign:
Identifier = Type

AliasReassignment:
Identifier = StorageClassesopt Type
Identifier = FunctionLiteral
Identifier = StorageClassesopt BasicType Parameters MemberFunctionAttributesopt

VoidInitializer:
void

Types

Type:
TypeCtorsopt BasicType TypeSuffixesopt

TypeCtors:
TypeCtor
TypeCtor TypeCtors

TypeCtor:
const
immutable
inout
shared

BasicType:
FundamentalType
. QualifiedIdentifier
QualifiedIdentifier
Typeof
Typeof . QualifiedIdentifier
TypeCtor ( Type )
Vector
TraitsExpression
MixinType

Vector:
__vector ( VectorBaseType )

VectorBaseType:
Type

FundamentalType:
bool
byte
ubyte
short
ushort
int
uint
long
ulong
cent
ucent
char
wchar
dchar
float
double
real
ifloat
idouble
ireal
cfloat
cdouble
creal
void

TypeSuffixes:
TypeSuffix TypeSuffixesopt

TypeSuffix:
*
[ ]
[ AssignExpression ]
[ AssignExpression .. AssignExpression ]
[ Type ]
delegate Parameters MemberFunctionAttributesopt
function Parameters FunctionAttributesopt

QualifiedIdentifier:
Identifier
Identifier . QualifiedIdentifier
TemplateInstance
TemplateInstance . QualifiedIdentifier
Identifier [ AssignExpression ]
Identifier [ AssignExpression ] . QualifiedIdentifier

Typeof:
typeof ( Expression )
typeof ( return )

MixinType:
mixin ( ArgumentList )

Attributes

AttributeSpecifier:
Attribute :
Attribute DeclarationBlock

Attribute:
LinkageAttribute
AlignAttribute
DeprecatedAttribute
VisibilityAttribute
Pragma
static
extern
abstract
final
override
synchronized
auto
scope
const
immutable
inout
shared
__gshared
AtAttribute
FunctionAttributeKwd
ref
return

FunctionAttributeKwd:
nothrow
pure

AtAttribute:
@ disable
@ nogc
@ live
Property
@ safe
@ system
@ trusted
UserDefinedAttribute

Property:
@ property

DeclarationBlock:
DeclDef
{ DeclDefsopt }

LinkageAttribute:
extern ( LinkageType )
extern ( C++ , )
extern ( C++ , QualifiedIdentifier )
extern ( C++ , NamespaceList )

LinkageType:
C
C++
D
Windows
System
Objective-C

NamespaceList:
ConditionalExpression
ConditionalExpression,
ConditionalExpression, NamespaceList

AlignAttribute:
align
align ( AssignExpression )

DeprecatedAttribute:
deprecated
deprecated ( AssignExpression )

VisibilityAttribute:
private
package
package ( QualifiedIdentifier )
protected
public
export

UserDefinedAttribute:
@ ( ArgumentList )
@ Identifier
@ Identifier ( ArgumentListopt )
@ TemplateInstance
@ TemplateInstance ( ArgumentListopt )

Pragmas

PragmaDeclaration:
Pragma ;
Pragma DeclarationBlock

PragmaStatement:
Pragma ;
Pragma NoScopeStatement

Pragma:
pragma ( Identifier )
pragma ( Identifier , ArgumentList )

Expressions

Expression:
CommaExpression

CommaExpression:
AssignExpression
CommaExpression , AssignExpression

AssignExpression:
ConditionalExpression
ConditionalExpression = AssignExpression
ConditionalExpression += AssignExpression
ConditionalExpression -= AssignExpression
ConditionalExpression *= AssignExpression
ConditionalExpression /= AssignExpression
ConditionalExpression %= AssignExpression
ConditionalExpression &= AssignExpression
ConditionalExpression |= AssignExpression
ConditionalExpression ^= AssignExpression
ConditionalExpression ~= AssignExpression
ConditionalExpression <<= AssignExpression
ConditionalExpression >>= AssignExpression
ConditionalExpression >>>= AssignExpression
ConditionalExpression ^^= AssignExpression

ConditionalExpression:
OrOrExpression
OrOrExpression ? Expression : ConditionalExpression

OrOrExpression:
AndAndExpression
OrOrExpression || AndAndExpression

AndAndExpression:
OrExpression
AndAndExpression && OrExpression

OrExpression:
XorExpression
OrExpression | XorExpression

XorExpression:
AndExpression
XorExpression ^ AndExpression

AndExpression:
CmpExpression
AndExpression & CmpExpression

CmpExpression:
ShiftExpression
EqualExpression
IdentityExpression
RelExpression
InExpression

EqualExpression:
ShiftExpression == ShiftExpression
ShiftExpression != ShiftExpression

IdentityExpression:
ShiftExpression is ShiftExpression
ShiftExpression ! is ShiftExpression

RelExpression:
ShiftExpression < ShiftExpression
ShiftExpression <= ShiftExpression
ShiftExpression > ShiftExpression
ShiftExpression >= ShiftExpression

InExpression:
ShiftExpression in ShiftExpression
ShiftExpression ! in ShiftExpression

ShiftExpression:
AddExpression
ShiftExpression << AddExpression
ShiftExpression >> AddExpression
ShiftExpression >>> AddExpression

AddExpression:
MulExpression
AddExpression + MulExpression
AddExpression - MulExpression
CatExpression

CatExpression:
AddExpression ~ MulExpression

MulExpression:
UnaryExpression
MulExpression * UnaryExpression
MulExpression / UnaryExpression
MulExpression % UnaryExpression

UnaryExpression:
& UnaryExpression
++ UnaryExpression
-- UnaryExpression
* UnaryExpression
- UnaryExpression
+ UnaryExpression
! UnaryExpression
ComplementExpression
DeleteExpression
CastExpression
PowExpression

ComplementExpression:
~ UnaryExpression

NewExpression:
new AllocatorArgumentsopt Type
new AllocatorArgumentsopt Type [ AssignExpression ]
new AllocatorArgumentsopt Type ( ArgumentListopt )
NewAnonClassExpression

AllocatorArguments:
( ArgumentListopt )

ArgumentList:
AssignExpression
AssignExpression ,
AssignExpression , ArgumentList

DeleteExpression:
delete UnaryExpression

CastExpression:
cast ( Type ) UnaryExpression
cast ( TypeCtorsopt ) UnaryExpression

PowExpression:
PostfixExpression
PostfixExpression ^^ UnaryExpression

PostfixExpression:
PrimaryExpression
PostfixExpression . Identifier
PostfixExpression . TemplateInstance
PostfixExpression . NewExpression
PostfixExpression ++
PostfixExpression --
PostfixExpression ( ArgumentListopt )
TypeCtorsopt BasicType ( ArgumentListopt )
IndexExpression
SliceExpression

IndexExpression:
PostfixExpression [ ArgumentList ]

SliceExpression:
PostfixExpression [ ]
PostfixExpression [ Slice ,opt ]

Slice:
AssignExpression
AssignExpression , Slice
AssignExpression .. AssignExpression
AssignExpression .. AssignExpression , Slice

PrimaryExpression:
Identifier
. Identifier
TemplateInstance
. TemplateInstance
this
super
null
true
false
$
IntegerLiteral
FloatLiteral
CharacterLiteral
StringLiterals
ArrayLiteral
AssocArrayLiteral
FunctionLiteral
AssertExpression
MixinExpression
ImportExpression
NewExpression
FundamentalType . Identifier
( Type ) . Identifier
( Type ) . TemplateInstance
FundamentalType ( ArgumentListopt )
TypeCtor ( Type ) . Identifier
TypeCtor ( Type ) ( ArgumentListopt )
Typeof
TypeidExpression
IsExpression
( Expression )
SpecialKeyword
TraitsExpression

StringLiterals:
StringLiteral
StringLiterals StringLiteral

ArrayLiteral:
[ ArgumentListopt ]

AssocArrayLiteral:
[ KeyValuePairs ]

KeyValuePairs:
KeyValuePair
KeyValuePair , KeyValuePairs

KeyValuePair:
KeyExpression : ValueExpression

KeyExpression:
AssignExpression

ValueExpression:
AssignExpression

FunctionLiteral:
function refopt Typeopt ParameterWithAttributes opt FunctionLiteralBody2
delegate refopt Typeopt ParameterWithMemberAttributes opt FunctionLiteralBody2
refopt ParameterWithMemberAttributes FunctionLiteralBody2
FunctionLiteralBody
Identifier => AssignExpression

ParameterWithAttributes:
Parameters FunctionAttributesopt

ParameterWithMemberAttributes:
Parameters MemberFunctionAttributesopt

FunctionLiteralBody2:
=> AssignExpression
FunctionLiteralBody

AssertExpression:
assert ( AssertArguments )

AssertArguments:
AssignExpression ,opt
AssignExpression , AssignExpression ,opt

MixinExpression:
mixin ( ArgumentList )

ImportExpression:
import ( AssignExpression )

TypeidExpression:
typeid ( Type )
typeid ( Expression )

IsExpression:
is ( Type )
is ( Type : TypeSpecialization )
is ( Type == TypeSpecialization )
is ( Type : TypeSpecialization , TemplateParameterList )
is ( Type == TypeSpecialization , TemplateParameterList )
is ( Type Identifier )
is ( Type Identifier : TypeSpecialization )
is ( Type Identifier == TypeSpecialization )
is ( Type Identifier : TypeSpecialization , TemplateParameterList )
is ( Type Identifier == TypeSpecialization , TemplateParameterList )

TypeSpecialization:
Type
struct
union
class
interface
enum
__vector
function
delegate
super
const
immutable
inout
shared
return
__parameters
module
package

SpecialKeyword:
__FILE__
__FILE_FULL_PATH__
__MODULE__
__LINE__
__FUNCTION__
__PRETTY_FUNCTION__

Statements

Statement:
EmptyStatement
NonEmptyStatement
ScopeBlockStatement

EmptyStatement:
;

NoScopeNonEmptyStatement:
NonEmptyStatement
BlockStatement

NoScopeStatement:
EmptyStatement
NonEmptyStatement
BlockStatement

NonEmptyOrScopeBlockStatement:
NonEmptyStatement
ScopeBlockStatement

NonEmptyStatement:
NonEmptyStatementNoCaseNoDefault
CaseStatement
CaseRangeStatement
DefaultStatement

NonEmptyStatementNoCaseNoDefault:
LabeledStatement
ExpressionStatement
DeclarationStatement
IfStatement
WhileStatement
DoStatement
ForStatement
ForeachStatement
SwitchStatement
FinalSwitchStatement
ContinueStatement
BreakStatement
ReturnStatement
GotoStatement
WithStatement
SynchronizedStatement
TryStatement
ScopeGuardStatement
ThrowStatement
AsmStatement
MixinStatement
ForeachRangeStatement
PragmaStatement
ConditionalStatement
StaticForeachStatement
StaticAssert
TemplateMixin
ImportDeclaration

ScopeStatement:
NonEmptyStatement
BlockStatement

ScopeBlockStatement:
BlockStatement

LabeledStatement:
Identifier :
Identifier : Statement

BlockStatement:
{ }
{ StatementList }

StatementList:
Statement
Statement StatementList

ExpressionStatement:
Expression ;

DeclarationStatement:
StorageClassesopt Declaration

IfStatement:
if ( IfCondition ) ThenStatement
if ( IfCondition ) ThenStatement else ElseStatement

IfCondition:
Expression
auto Identifier = Expression
scope Identifier = Expression
TypeCtors Identifier = Expression
TypeCtorsopt BasicType Declarator = Expression

ThenStatement:
ScopeStatement

ElseStatement:
ScopeStatement

WhileStatement:
while ( IfCondition ) ScopeStatement

DoStatement:
do ScopeStatement  while ( Expression ) ;

ForStatement:
for ( Initialize Testopt ; Incrementopt ) ScopeStatement

Initialize:
;
NoScopeNonEmptyStatement

Test:
Expression

Increment:
Expression

AggregateForeach:
Foreach ( ForeachTypeList ; ForeachAggregate )

ForeachStatement:
AggregateForeach NoScopeNonEmptyStatement

Foreach:
foreach
foreach_reverse

ForeachTypeList:
ForeachType
ForeachType , ForeachTypeList

ForeachType:
ForeachTypeAttributesopt BasicType Declarator
ForeachTypeAttributesopt Identifier
ForeachTypeAttributesopt alias Identifier

ForeachTypeAttributes:
ForeachTypeAttribute
ForeachTypeAttribute ForeachTypeAttributes

ForeachTypeAttribute:
enum
ref
scope
TypeCtor

ForeachAggregate:
Expression

RangeForeach:
Foreach ( ForeachType ; LwrExpression .. UprExpression )

LwrExpression:
Expression

UprExpression:
Expression

ForeachRangeStatement:
RangeForeach ScopeStatement

SwitchStatement:
switch ( Expression ) ScopeStatement

CaseStatement:
case ArgumentList : ScopeStatementListopt

CaseRangeStatement:
case FirstExp : .. case LastExp : ScopeStatementListopt

FirstExp:
AssignExpression

LastExp:
AssignExpression

DefaultStatement:
default : ScopeStatementListopt

ScopeStatementList:
StatementListNoCaseNoDefault

StatementListNoCaseNoDefault:
StatementNoCaseNoDefault
StatementNoCaseNoDefault StatementListNoCaseNoDefault

StatementNoCaseNoDefault:
EmptyStatement
NonEmptyStatementNoCaseNoDefault
ScopeBlockStatement

FinalSwitchStatement:
final switch ( Expression ) ScopeStatement

ContinueStatement:
continue Identifieropt ;

BreakStatement:
break Identifieropt ;

ReturnStatement:
return Expressionopt ;

GotoStatement:
goto Identifier ;
goto default ;
goto case ;
goto case Expression ;

WithStatement:
with ( Expression ) ScopeStatement
with ( Symbol ) ScopeStatement
with ( TemplateInstance ) ScopeStatement

SynchronizedStatement:
synchronized ScopeStatement
synchronized ( Expression ) ScopeStatement

TryStatement:
try ScopeStatement Catches
try ScopeStatement Catches FinallyStatement
try ScopeStatement FinallyStatement

Catches:
Catch
Catch Catches

Catch:
catch ( CatchParameter ) NoScopeNonEmptyStatement

CatchParameter:
BasicType Identifieropt

FinallyStatement:
finally NoScopeNonEmptyStatement

ThrowStatement:
throw Expression ;

ScopeGuardStatement:
scope ( exit ) NonEmptyOrScopeBlockStatement
scope ( success ) NonEmptyOrScopeBlockStatement
scope ( failure ) NonEmptyOrScopeBlockStatement

AsmStatement:
asm FunctionAttributesopt { AsmInstructionListopt }

AsmInstructionList:
AsmInstruction ;
AsmInstruction ; AsmInstructionList

MixinStatement:
mixin ( ArgumentList ) ;

Structs and Unions

AggregateDeclaration:
ClassDeclaration
InterfaceDeclaration
StructDeclaration
UnionDeclaration

StructDeclaration:
struct Identifier ;
struct Identifier AggregateBody
StructTemplateDeclaration
AnonStructDeclaration

AnonStructDeclaration:
struct AggregateBody

UnionDeclaration:
union Identifier ;
union Identifier AggregateBody
UnionTemplateDeclaration
AnonUnionDeclaration

AnonUnionDeclaration:
union AggregateBody

AggregateBody:
{ DeclDefsopt }

Postblit:
this ( this ) MemberFunctionAttributesopt ;
this ( this ) MemberFunctionAttributesopt FunctionBody

Invariant:
invariant ( ) BlockStatement
invariant BlockStatement
invariant ( AssertArguments ) ;

Classes

ClassDeclaration:
class Identifier ;
class Identifier BaseClassListopt AggregateBody
ClassTemplateDeclaration

BaseClassList:
: SuperClassOrInterface
: SuperClassOrInterface , Interfaces

SuperClassOrInterface:
BasicType

Interfaces:
Interface
Interface , Interfaces

Interface:
BasicType

Constructor:
this Parameters MemberFunctionAttributesopt FunctionBody
ConstructorTemplate

Destructor:
~ this ( ) MemberFunctionAttributesopt FunctionBody

StaticConstructor:
static this ( ) MemberFunctionAttributesopt ;
static this ( ) MemberFunctionAttributesopt FunctionBody

StaticDestructor:
static ~ this ( ) MemberFunctionAttributesopt ;
static ~ this ( ) MemberFunctionAttributesopt FunctionBody

SharedStaticConstructor:
shared static this ( ) MemberFunctionAttributesopt ;
shared static this ( ) MemberFunctionAttributesopt FunctionBody

SharedStaticDestructor:
shared static ~ this ( ) MemberFunctionAttributesopt ;
shared static ~ this ( ) MemberFunctionAttributesopt FunctionBody

Invariant:
invariant ( ) BlockStatement
invariant BlockStatement
invariant ( AssertArguments ) ;

Allocator:
new Parameters FunctionBody

AliasThis:
alias Identifier this ;

NewAnonClassExpression:
new AllocatorArgumentsopt class ConstructorArgsopt SuperClassOrInterfaceopt Interfacesopt AggregateBody

ConstructorArgs:
( ArgumentListopt )

class Identifier : SuperClassOrInterface Interfaces AggregateBody
// ...
new AllocatorArguments Identifier ConstructorArgs

Interfaces

InterfaceDeclaration:
interface Identifier ;
interface Identifier BaseInterfaceListopt AggregateBody
InterfaceTemplateDeclaration

BaseInterfaceList:
: Interfaces

Enums

EnumDeclaration:
enum Identifier EnumBody
enum Identifier : EnumBaseType EnumBody
AnonymousEnumDeclaration

EnumBaseType:
Type

EnumBody:
{ EnumMembers }
;

EnumMembers:
EnumMember
EnumMember ,
EnumMember , EnumMembers

EnumMemberAttributes:
EnumMemberAttribute
EnumMemberAttribute EnumMemberAttributes

EnumMemberAttribute:
DeprecatedAttribute
UserDefinedAttribute
@disable

EnumMember:
EnumMemberAttributesopt Identifier
EnumMemberAttributesopt Identifier = AssignExpression

AnonymousEnumDeclaration:
enum : EnumBaseType { EnumMembers }
enum { EnumMembers }
enum { AnonymousEnumMembers }

AnonymousEnumMembers:
AnonymousEnumMember
AnonymousEnumMember ,
AnonymousEnumMember , AnonymousEnumMembers

AnonymousEnumMember:
EnumMember
Type Identifier = AssignExpression

Functions

FuncDeclaration:
StorageClassesopt BasicType FuncDeclarator FunctionBody
AutoFuncDeclaration

AutoFuncDeclaration:
StorageClasses Identifier FuncDeclaratorSuffix FunctionBody

FuncDeclarator:
TypeSuffixesopt Identifier FuncDeclaratorSuffix

FuncDeclaratorSuffix:
Parameters MemberFunctionAttributesopt
TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt

Parameters:
( ParameterListopt )

ParameterList:
Parameter
Parameter ,
Parameter , ParameterList
VariadicArgumentsAttributesopt ...

Parameter:
ParameterAttributesopt BasicType Declarator
ParameterAttributesopt BasicType Declarator ...
ParameterAttributesopt BasicType Declarator = AssignExpression
ParameterAttributesopt Type
ParameterAttributesopt Type ...
ParameterAttributesopt Type = AssignExpression

ParameterAttributes:
InOut
UserDefinedAttribute
ParameterAttributes InOut
ParameterAttributes UserDefinedAttribute

InOut:
auto
TypeCtor
final
in
lazy
out
ref
return
scope

VariadicArgumentsAttributes:
VariadicArgumentsAttribute
VariadicArgumentsAttribute VariadicArgumentsAttributes

VariadicArgumentsAttribute:
const
immutable
return
scope
shared

FunctionAttributes:
FunctionAttribute
FunctionAttribute FunctionAttributes

FunctionAttribute:
FunctionAttributeKwd
Property
AtAttribute

MemberFunctionAttributes:
MemberFunctionAttribute
MemberFunctionAttribute MemberFunctionAttributes

MemberFunctionAttribute:
const
immutable
inout
return
scope
shared
FunctionAttribute

FunctionBody:
SpecifiedFunctionBody
MissingFunctionBody
ShortenedFunctionBody

FunctionLiteralBody:
SpecifiedFunctionBody

SpecifiedFunctionBody:
doopt BlockStatement
FunctionContractsopt InOutContractExpression doopt BlockStatement
FunctionContractsopt InOutStatement do BlockStatement

MissingFunctionBody:
;
FunctionContractsopt InOutContractExpression ;
FunctionContractsopt InOutStatement

ShortenedFunctionBody:
=> AssignExpression ;

FunctionContracts:
FunctionContract
FunctionContract FunctionContracts

FunctionContract:
InOutContractExpression
InOutStatement

InOutContractExpression:
InContractExpression
OutContractExpression

InOutStatement:
InStatement
OutStatement

InContractExpression:
in ( AssertArguments )

OutContractExpression:
out ( ; AssertArguments )
out ( Identifier ; AssertArguments )

InStatement:
in BlockStatement

OutStatement:
out BlockStatement
out ( Identifier ) BlockStatement

Templates

TemplateDeclaration:
template Identifier TemplateParameters Constraintopt { DeclDefsopt }

TemplateParameters:
( TemplateParameterListopt )

TemplateParameterList:
TemplateParameter
TemplateParameter ,
TemplateParameter , TemplateParameterList

TemplateParameter:
TemplateTypeParameter
TemplateValueParameter
TemplateAliasParameter
TemplateSequenceParameter
TemplateThisParameter

TemplateInstance:
Identifier TemplateArguments

TemplateArguments:
! ( TemplateArgumentListopt )
! TemplateSingleArgument

TemplateArgumentList:
TemplateArgument
TemplateArgument ,
TemplateArgument , TemplateArgumentList

TemplateArgument:
Type
AssignExpression
Symbol

Symbol:
SymbolTail
. SymbolTail

SymbolTail:
Identifier
Identifier . SymbolTail
TemplateInstance
TemplateInstance . SymbolTail

TemplateSingleArgument:
Identifier
FundamentalType
CharacterLiteral
StringLiteral
IntegerLiteral
FloatLiteral
true
false
null
this
SpecialKeyword

TemplateTypeParameter:
Identifier
Identifier TemplateTypeParameterSpecialization
Identifier TemplateTypeParameterDefault
Identifier TemplateTypeParameterSpecialization TemplateTypeParameterDefault

TemplateTypeParameterSpecialization:
: Type

TemplateTypeParameterDefault:
= Type

TemplateThisParameter:
this TemplateTypeParameter

TemplateValueParameter:
BasicType Declarator
BasicType Declarator TemplateValueParameterSpecialization
BasicType Declarator TemplateValueParameterDefault
BasicType Declarator TemplateValueParameterSpecialization TemplateValueParameterDefault

TemplateValueParameterSpecialization:
: ConditionalExpression

TemplateValueParameterDefault:
= AssignExpression
= SpecialKeyword

TemplateAliasParameter:
alias Identifier TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt
alias BasicType Declarator TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt

TemplateAliasParameterSpecialization:
: Type
: ConditionalExpression

TemplateAliasParameterDefault:
= Type
= ConditionalExpression

TemplateSequenceParameter:
Identifier ...

ConstructorTemplate:
this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt :
this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt FunctionBody

ClassTemplateDeclaration:
class Identifier TemplateParameters ;
class Identifier TemplateParameters Constraintopt BaseClassListopt AggregateBody
class Identifier TemplateParameters BaseClassListopt Constraintopt AggregateBody

InterfaceTemplateDeclaration:
interface Identifier TemplateParameters ;
interface Identifier TemplateParameters Constraintopt BaseInterfaceListopt AggregateBody
interface Identifier TemplateParameters BaseInterfaceList Constraint AggregateBody

StructTemplateDeclaration:
struct Identifier TemplateParameters ;
struct Identifier TemplateParameters Constraintopt AggregateBody

UnionTemplateDeclaration:
union Identifier TemplateParameters ;
union Identifier TemplateParameters Constraintopt AggregateBody

Constraint:
if ( Expression )

Template Mixins

TemplateMixinDeclaration:
mixin template Identifier TemplateParameters Constraintopt { DeclDefsopt }

TemplateMixin:
mixin MixinTemplateName TemplateArgumentsopt Identifieropt ;

MixinTemplateName:
. MixinQualifiedIdentifier
MixinQualifiedIdentifier
Typeof . MixinQualifiedIdentifier

MixinQualifiedIdentifier:
Identifier
Identifier . MixinQualifiedIdentifier
TemplateInstance . MixinQualifiedIdentifier

Conditional Compilation

ConditionalDeclaration:
Condition DeclarationBlock
Condition DeclarationBlock else DeclarationBlock
Condition : DeclDefsopt
Condition DeclarationBlock else : DeclDefsopt

ConditionalStatement:
Condition NoScopeNonEmptyStatement
Condition NoScopeNonEmptyStatement else NoScopeNonEmptyStatement

Condition:
VersionCondition
DebugCondition
StaticIfCondition

VersionCondition:
version ( IntegerLiteral )
version ( Identifier )
version ( unittest )
version ( assert )

VersionSpecification:
version = Identifier ;
version = IntegerLiteral ;

DebugCondition:
debug
debug ( IntegerLiteral )
debug ( Identifier )

DebugSpecification:
debug = Identifier ;
debug = IntegerLiteral ;

StaticIfCondition:
static if ( AssignExpression )

StaticForeach:
static AggregateForeach
static RangeForeach

StaticForeachDeclaration:
StaticForeach DeclarationBlock
StaticForeach : DeclDefsopt

StaticForeachStatement:
StaticForeach NoScopeNonEmptyStatement

StaticAssert:
static assert ( AssertArguments ) ;

Traits

TraitsExpression:
__traits ( TraitsKeyword , TraitsArguments )

TraitsKeyword:
isAbstractClass
isArithmetic
isAssociativeArray
isFinalClass
isPOD
isNested
isFuture
isDeprecated
isFloating
isIntegral
isScalar
isStaticArray
isUnsigned
isDisabled
isVirtualFunction
isVirtualMethod
isAbstractFunction
isFinalFunction
isStaticFunction
isOverrideFunction
isTemplate
isRef
isOut
isLazy
isReturnOnStack
isCopyable
isZeroInit
isModule
isPackage
hasMember
hasCopyConstructor
hasPostblit
identifier
getAliasThis
getAttributes
getFunctionAttributes
getFunctionVariadicStyle
getLinkage
getLocation
getMember
getOverloads
getParameterStorageClasses
getPointerBitmap
getCppNamespaces
getVisibility
getProtection
getTargetInfo
getVirtualFunctions
getVirtualMethods
getUnitTests
parent
child
classInstanceSize
getVirtualIndex
allMembers
derivedMembers
isSame
compiles
toType

TraitsArguments:
TraitsArgument
TraitsArgument , TraitsArguments

TraitsArgument:
AssignExpression
Type

Unit Tests

UnitTest:
unittest BlockStatement

D x86 Inline Assembler

AsmStatement:
asm FunctionAttributesopt { AsmInstructionListopt }

AsmInstructionList:
AsmInstruction ;
AsmInstruction ; AsmInstructionList

AsmInstruction:
Identifier : AsmInstruction
align IntegerExpression
even
naked
db Operands
ds Operands
di Operands
dl Operands
df Operands
dd Operands
de Operands
db StringLiteral
ds StringLiteral
di StringLiteral
dl StringLiteral
dw StringLiteral
dq StringLiteral
Opcode
Opcode Operands

Opcode:
Identifier

Operands:
Operand
Operand , Operands

IntegerExpression:
IntegerLiteral
Identifier

Register:
AL
AH
AX
EAX

BL
BH
BX
EBX

CL
CH
CX
ECX

DL
DH
DX
EDX

BP
EBP

SP
ESP

DI
EDI

SI
ESI

ES
CS
SS
DS
GS
FS

CR0
CR2
CR3
CR4

DR0
DR1
DR2
DR3
DR6
DR7

TR3
TR4
TR5
TR6
TR7

ST

ST(0)
ST(1)
ST(2)
ST(3)
ST(4)
ST(5)
ST(6)
ST(7)

MM0
MM1
MM2
MM3
MM4
MM5
MM6
MM7

XMM0
XMM1
XMM2
XMM3
XMM4
XMM5
XMM6
XMM7

Register64:
RAX
RBX
RCX
RDX

BPL
RBP

SPL
RSP

DIL
RDI

SIL
RSI

R8B
R8W
R8D
R8

R9B
R9W
R9D
R9

R10B
R10W
R10D
R10

R11B
R11W
R11D
R11

R12B
R12W
R12D
R12

R13B
R13W
R13D
R13

R14B
R14W
R14D
R14

R15B
R15W
R15D
R15

XMM8
XMM9
XMM10
XMM11
XMM12
XMM13
XMM14
XMM15

YMM0
YMM1
YMM2
YMM3
YMM4
YMM5
YMM6
YMM7

YMM8
YMM9
YMM10
YMM11
YMM12
YMM13
YMM14
YMM15

Operand:
AsmExp

AsmExp:
AsmLogOrExp
AsmLogOrExp ? AsmExp : AsmExp

AsmLogOrExp:
AsmLogAndExp
AsmLogOrExp || AsmLogAndExp

AsmLogAndExp:
AsmOrExp
AsmLogAndExp && AsmOrExp

AsmOrExp:
AsmXorExp
AsmOrExp | AsmXorExp

AsmXorExp:
AsmAndExp
AsmXorExp ^ AsmAndExp

AsmAndExp:
AsmEqualExp
AsmAndExp & AsmEqualExp

AsmEqualExp:
AsmRelExp
AsmEqualExp == AsmRelExp
AsmEqualExp != AsmRelExp

AsmRelExp:
AsmShiftExp
AsmRelExp < AsmShiftExp
AsmRelExp <= AsmShiftExp
AsmRelExp > AsmShiftExp
AsmRelExp >= AsmShiftExp

AsmShiftExp:
AsmAddExp
AsmShiftExp << AsmAddExp
AsmShiftExp >> AsmAddExp
AsmShiftExp >>> AsmAddExp

AsmAddExp:
AsmMulExp
AsmAddExp + AsmMulExp
AsmAddExp - AsmMulExp

AsmMulExp:
AsmBrExp
AsmMulExp * AsmBrExp
AsmMulExp / AsmBrExp
AsmMulExp % AsmBrExp

AsmBrExp:
AsmUnaExp
AsmBrExp [ AsmExp ]

AsmUnaExp:
AsmTypePrefix AsmExp
offsetof AsmExp
seg AsmExp
+ AsmUnaExp
- AsmUnaExp
! AsmUnaExp
~ AsmUnaExp
AsmPrimaryExp

AsmPrimaryExp:
IntegerLiteral
FloatLiteral
__LOCAL_SIZE
$
Register
Register : AsmExp
Register64
Register64 : AsmExp
DotIdentifier
this

DotIdentifier:
Identifier
Identifier . DotIdentifier
FundamentalType . Identifier

AsmTypePrefix:
near ptr
far ptr
word ptr
dword ptr
qword ptr
FundamentalType ptr

GccAsmStatement:
asm FunctionAttributesopt { GccAsmInstructionList }

GccAsmInstructionList:
GccAsmInstruction ;
GccAsmInstruction ; GccAsmInstructionList

GccAsmInstruction:
GccBasicAsmInstruction
GccExtAsmInstruction
GccGotoAsmInstruction

GccBasicAsmInstruction:
AssignExpression

GccExtAsmInstruction:
AssignExpression : GccAsmOperandsopt
AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt
AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt : GccAsmClobbersopt

GccGotoAsmInstruction:
AssignExpression : : GccAsmOperandsopt : GccAsmClobbersopt : GccAsmGotoLabelsopt

GccAsmOperands:
GccSymbolicNameopt StringLiteral ( AssignExpression )
GccSymbolicNameopt StringLiteral ( AssignExpression ) , GccAsmOperands

GccSymbolicName:
[ Identifier ]

GccAsmClobbers:
StringLiteral
StringLiteral , GccAsmClobbers

GccAsmGotoLabels:
Identifier
Identifier , GccAsmGotoLabels

Named Character Entities

NamedCharacterEntity:
& Identifier ;

Application Binary Interface

MangledName:
_D QualifiedName Type
_D QualifiedName Z        // Internal

QualifiedName:
SymbolFunctionName
SymbolFunctionName QualifiedName

SymbolFunctionName:
SymbolName
SymbolName TypeFunctionNoReturn
SymbolName M TypeModifiersopt TypeFunctionNoReturn

SymbolName:
LName
TemplateInstanceName
IdentifierBackRef
0                         // anonymous symbols

TemplateInstanceName:
TemplateID LName TemplateArgs Z

TemplateID:
__T
__U        // for symbols declared inside template constraint

TemplateArgs:
TemplateArg
TemplateArg TemplateArgs

TemplateArg:
TemplateArgX
H TemplateArgX

TemplateArgX:
T Type
V Type Value
S QualifiedName
X Number ExternallyMangledName

Values:
Value
Value Values

Value:
n
i Number
N Number
e HexFloat
c HexFloat c HexFloat
CharWidth Number _ HexDigits
A Number Values
S Number Values
f MangledName

HexFloat:
NAN
INF
NINF
N HexDigits P Exponent
HexDigits P Exponent

Exponent:
N Number
Number

HexDigits:
HexDigit
HexDigit HexDigits

HexDigit:
Digit
A
B
C
D
E
F

CharWidth:
a
w
d

Name:
Namestart
Namestart Namechars

Namestart:
_
Alpha

Namechar:
Namestart
Digit

Namechars:
Namechar
Namechar Namechars

LName:
Number Name

Number:
Digit
Digit Number

Digit:
0
1
2
3
4
5
6
7
8
9

TypeBackRef:
Q NumberBackRef

IdentifierBackRef:
Q NumberBackRef

NumberBackRef:
lower-case-letter
upper-case-letter NumberBackRef

Type:
TypeModifiersopt TypeX
TypeBackRef

TypeX:
TypeArray
TypeStaticArray
TypeAssocArray
TypePointer
TypeFunction
TypeIdent
TypeClass
TypeStruct
TypeEnum
TypeTypedef
TypeDelegate
TypeVoid
TypeByte
TypeUbyte
TypeShort
TypeUshort
TypeInt
TypeUint
TypeLong
TypeUlong
TypeCent
TypeUcent
TypeFloat
TypeDouble
TypeReal
TypeIfloat
TypeIdouble
TypeIreal
TypeCfloat
TypeCdouble
TypeCreal
TypeBool
TypeChar
TypeWchar
TypeDchar
TypeNull
TypeTuple
TypeVector

TypeModifiers:
Const
Wild
Wild Const
Shared
Shared Const
Shared Wild
Shared Wild Const
Immutable

Shared:
O

Const:
x

Immutable:
y

Wild:
Ng

TypeArray:
A Type

TypeStaticArray:
G Number Type

TypeAssocArray:
H Type Type

TypePointer:
P Type

TypeVector:
Nh Type

TypeFunction:
TypeFunctionNoReturn Type

TypeFunctionNoReturn:
CallConvention FuncAttrsopt Parametersopt ParamClose

CallConvention:
F       // D
U       // C
W       // Windows
R       // C++
Y       // Objective-C

FuncAttrs:
FuncAttr
FuncAttr FuncAttrs

FuncAttr:
FuncAttrPure
FuncAttrNothrow
FuncAttrRef
FuncAttrProperty
FuncAttrNogc
FuncAttrReturn
FuncAttrScope
FuncAttrTrusted
FuncAttrSafe
FuncAttrLive

FuncAttrPure:
Na

FuncAttrNogc:
Ni

FuncAttrNothrow:
Nb

FuncAttrProperty:
Nd

FuncAttrRef:
Nc

FuncAttrReturn:
Nj

FuncAttrScope:
Nl

FuncAttrTrusted:
Ne

FuncAttrSafe:
Nf

FuncAttrLive:
Nm

Parameters:
Parameter
Parameter Parameters

Parameter:
Parameter2
M Parameter2     // scope
Nk Parameter2    // return

Parameter2:
Type
I Type     // in
J Type     // out
K Type     // ref
L Type     // lazy

ParamClose:
X     // variadic T t...) style
Y     // variadic T t,...) style
Z     // not variadic

TypeIdent:
I QualifiedName

TypeClass:
C QualifiedName

TypeStruct:
S QualifiedName

TypeEnum:
E QualifiedName

TypeTypedef:
T QualifiedName

TypeDelegate:
D TypeModifiersopt TypeFunction

TypeVoid:
v

TypeByte:
g

TypeUbyte:
h

TypeShort:
s

TypeUshort:
t

TypeInt:
i

TypeUint:
k

TypeLong:
l

TypeUlong:
m

TypeCent:
zi

TypeUcent:
zk

TypeFloat:
f

TypeDouble:
d

TypeReal:
e

TypeIfloat:
o

TypeIdouble:
p

TypeIreal:
j

TypeCfloat:
q

TypeCdouble:
r

TypeCreal:
c

TypeBool:
b

TypeChar:
a

TypeWchar:
u

TypeDchar:
w

TypeNull:
n

TypeTuple:
B Parameters Z

Modules

Module:
ModuleDeclaration
ModuleDeclaration DeclDefs
DeclDefs

DeclDefs:
DeclDef
DeclDef DeclDefs

DeclDef:
AttributeSpecifier
Declaration
Constructor
Destructor
Postblit
Allocator
Deallocator
Invariant
UnitTest
AliasThis
StaticConstructor
StaticDestructor
SharedStaticConstructor
SharedStaticDestructor
ConditionalDeclaration
DebugSpecification
VersionSpecification
StaticAssert
TemplateDeclaration
TemplateMixinDeclaration
TemplateMixin
MixinDeclaration
EmptyDeclaration

EmptyDeclaration:
;

Modules have a one-to-one correspondence with source files. When not explicitly set via a ModuleDeclaration, a module's name defaults to the name of the file stripped of its path and extension.

A module's name automatically acts as a namespace scope for its contents. Modules superficially resemble classes, but differ in that:

- Only one instance of a module exists, and it is statically allocated.

- Modules do not have virtual tables.

- Modules do not inherit, do not have super modules, etc.

- A source file may contain only one module.

- Symbols in a module can be imported.

- Modules are always compiled at global scope and are unaffected by surrounding attributes or other modifiers.

Modules can be grouped into hierarchies called packages.

Modules offer several guarantees:

- The order in which modules are imported does not affect their semantics.

- The semantics of a module are not affected by the scope in which it is imported.

- If a module C imports modules A and B, any modifications to B will not silently change code in C that is dependent on A.

Module Declaration

The ModuleDeclaration sets the name of the module and what package it belongs to. If absent, the module name is taken to be the same name (stripped of path and extension) of the source file name.

ModuleDeclaration:
ModuleAttributesopt module ModuleFullyQualifiedName ;

ModuleAttributes:
ModuleAttribute
ModuleAttribute ModuleAttributes

ModuleAttribute:
DeprecatedAttribute
UserDefinedAttribute

ModuleFullyQualifiedName:
ModuleName
Packages . ModuleName

ModuleName:
Identifier

Packages:
PackageName
Packages . PackageName

PackageName:
Identifier

The Identifiers preceding the rightmost Identifier are the Packages that the module is in. The packages correspond to directory names in the source file path. Package and module names cannot be Keywords.

If present, the ModuleDeclaration must be the first and only such declaration in the source file, and may be preceded only by comments and #line directives.

Example:

module c.stdio; // module stdio in the c package

By convention, package and module names are all lower case. This is because these names have a one-to-one correspondence with the operating system's directory and file names, and many file systems are not case sensitive. Using all lower case package and module names will avoid or minimize problems when moving projects between dissimilar file systems.

If the file name of a module is an invalid module name (e.g. foo-bar.d), use a module declaration to set a valid module name:

module foo_bar;

A ModuleDeclaration can have an optional DeprecatedAttribute. The compiler will produce a message when the deprecated module is imported.

deprecated module foo;

module bar;
import foo;  // Deprecated: module foo is deprecated

A DeprecatedAttribute can have an optional AssignExpression argument to provide a more informative message. An AssignExpression must evaluate to a string at compile time.

deprecated("Please use foo2 instead.")
module foo;

module bar;
import foo;  // Deprecated: module foo is deprecated - Please use foo2 instead.

Implementation Defined:

1. The mapping of package and module identifiers to directory and file names.

2. How the deprecation messages are presented to the user.

Best Practices:

1. PackageNames and ModuleNames should be composed of the ASCII characters lower case letters, digits or _ to ensure maximum portability and compatibility with various file systems.

2. The file names for packages and modules should be composed only of the ASCII lower case letters, digits, and _s, and should not be a Keyword.

Import Declaration

Symbols from one module are made available in another module by using the ImportDeclaration:

ImportDeclaration:
import ImportList ;
static import ImportList ;

ImportList:
Import
ImportBindings
Import , ImportList

Import:
ModuleFullyQualifiedName
ModuleAliasIdentifier = ModuleFullyQualifiedName

ImportBindings:
Import : ImportBindList

ImportBindList:
ImportBind
ImportBind , ImportBindList

ImportBind:
Identifier
Identifier = Identifier

ModuleAliasIdentifier:
Identifier

There are several forms of the ImportDeclaration, from generalized to fine-grained importing.

The order in which ImportDeclarations occur has no significance.

ModuleFullyQualifiedNames in the ImportDeclaration must be fully qualified with whatever packages they are in. They are not considered to be relative to the module that imports them.

Symbol Name Lookup

The simplest form of importing is to just list the modules being imported:

module myapp.main;

import std.stdio; // import module stdio from package std

class Foo : BaseClass
{
import myapp.foo;  // import module myapp.foo in this class' scope
void bar ()
{
import myapp.bar;  // import module myapp.bar in this function' scope
writeln("hello!");  // calls std.stdio.writeln
}
}

When a symbol name is used unqualified, a two-phase lookup is used. First, the module scope is searched, starting from the innermost scope. For example, in the previous example, while looking for writeln, the order will be:

- Declarations inside bar.

- Declarations inside Foo.

- Declarations inside BaseClass.

- Declarations at module scope.

If the first lookup isn't successful, a second one is performed on imports. In the second lookup phase inherited scopes are ignored. This includes the scope of base classes and interfaces (in this example, BaseClass's imports would be ignored), as well as imports in mixed-in template.

Symbol lookup stops as soon as a matching symbol is found. If two symbols with the same name are found at the same lookup phase, this ambiguity will result in a compilation error.

module A;
void foo();
void bar();

module B;
void foo();
void bar();

module C;
import A;
void foo();
void test()
{
foo(); // C.foo() is called, it is found before imports are searched
bar(); // A.bar() is called, since imports are searched
}

module D;
import A;
import B;
void test()
{
foo();   // error, A.foo() or B.foo() ?
A.foo(); // ok, call A.foo()
B.foo(); // ok, call B.foo()
}

module E;
import A;
import B;
alias foo = B.foo;
void test()
{
foo();   // call B.foo()
A.foo(); // call A.foo()
B.foo(); // call B.foo()
}

Public Imports

By default, imports are private. This means that if module A imports module B, and module B imports module C, then names inside C are visible only inside B and not inside A.

An import can be explicitly declared public, which will cause names from the imported module to be visible to further imports. So in the above example where module A imports module B, if module B publicly imports module C, names from C will be visible in A as well.

All symbols from a publicly imported module are also aliased in the importing module. Thus in the above example if C contains the name foo, it will be accessible in A as foo, B.foo and C.foo.

For another example:

module W;
void foo() { }

module X;
void bar() { }

module Y;
import W;
public import X;
...
foo();  // calls W.foo()
bar();  // calls X.bar()

module Z;
import Y;
...
foo();   // error, foo() is undefined
bar();   // ok, calls X.bar()
X.bar(); // ditto
Y.bar(); // ok, Y.bar() is an alias to X.bar()

Static Imports

A static import requires the use of a fully qualified name to reference the module's names:

static import std.stdio;

void main()
{
writeln("hello!");           // error, writeln is undefined
std.stdio.writeln("hello!"); // ok, writeln is fully qualified
}

Renamed Imports

A local name for an import can be given, through which all references to the module's symbols must be qualified with:

import io = std.stdio;

void main()
{
io.writeln("hello!");        // ok, calls std.stdio.writeln
std.stdio.writeln("hello!"); // error, std is undefined
writeln("hello!");           // error, writeln is undefined
}

Best Practices: Renamed imports are handy when dealing with very long import names.

Selective Imports

Specific symbols can be exclusively imported from a module and bound into the current namespace:

import std.stdio : writeln, foo = write;

void main()
{
std.stdio.writeln("hello!"); // error, std is undefined
writeln("hello!");           // ok, writeln bound into current namespace
write("world");              // error, write is undefined
foo("world");                // ok, calls std.stdio.write()
fwritefln(stdout, "abc");    // error, fwritefln undefined
}

static cannot be used with selective imports.

Renamed and Selective Imports

When renaming and selective importing are combined:

import io = std.stdio : foo = writeln;

void main()
{
writeln("bar");           // error, writeln is undefined
std.stdio.foo("bar");     // error, foo is bound into current namespace
std.stdio.writeln("bar"); // error, std is undefined
foo("bar");               // ok, foo is bound into current namespace,
// FQN not required
io.writeln("bar");        // ok, io=std.stdio bound the name io in
// the current namespace to refer to the entire
//   module
io.foo("bar");            // error, foo is bound into current namespace,
// foo is not a member of io
}

Scoped Imports

Import declarations may be used at any scope. For example:

void main()
{
import std.stdio;
writeln("bar");
}

The imports are looked up to satisfy any unresolved symbols at that scope. Imported symbols may hide symbols from outer scopes.

In function scopes, imported symbols only become visible after the import declaration lexically appears in the function body. In other words, imported symbols at function scope cannot be forward referenced.

void main()
{
void writeln(string) {}
void foo()
{
writeln("bar"); // calls main.writeln
import std.stdio;
writeln("bar"); // calls std.stdio.writeln
void writeln(string) {}
writeln("bar"); // calls main.foo.writeln
}
writeln("bar"); // calls main.writeln
std.stdio.writeln("bar");  // error, std is undefined
}

Module Scope Operator

A leading dot (.) causes the identifier to be looked up in the module scope.

int x;

int foo(int x)
{
if (y)
return x;  // returns foo.x, not global x
else
return .x; // returns global x
}

Static Construction and Destruction

Static constructors are executed to initialize a module's state. Static destructors terminate a module's state.

A module may have multiple static constructors and static destructors. The static constructors are run in lexical order, the static destructors are run in reverse lexical order.

Non-shared static constructors and destructors are run whenever threads are created or destroyed, including the main thread.

Shared static constructors are run once before main() is called. Shared static destructors are run after the main() function returns.

import resource;

Resource x;
shared Resource y;
__gshared Resource z;

static this()  // non-shared static constructor
{
x = acquireResource();
}

shared static this()  // shared static constructor
{
y = acquireSharedResource();
z = acquireSharedResource();
}

static ~this()  // non-shared static destructor
{
releaseResource(x);
}

shared static ~this()   // shared static destructor
{
releaseSharedResource(y);
releaseSharedResource(z);
}

Best Practices:

1. Shared static constructors and destructors are used to initialize and terminate shared global data.

2. Non-shared static constructors and destructors are used to initialize and terminate thread local data.

Order of Static Construction

Shared static constructors on all modules are run before any non-shared static constructors.

The order of static initialization is implicitly determined by the import declarations in each module. Each module is assumed to depend on any imported modules being statically constructed first. There is no other order imposed on the execution of module static constructors.

Cycles (circular dependencies) in the import declarations are allowed so long as neither, or one, but not both, of the modules, contains static constructors or static destructors. Violation of this rule will result in a runtime exception.

Implementation Defined:

1. An implementation may provide a means of overriding the cycle detection abort. A typical method uses the D Runtime switch --DRT-oncycle=... where the following behaviors are supported:

1. abort The default behavior. The normal behavior as described in the previous section.

2. deprecate This works just like abort, but upon cycle detection the runtime will use a flawed pre-2.072 algorithm to determine if the cycle was previously detected. If no cycles are detected in the old algorithm, execution continues, but a deprecation message is printed.

3. print Print all cycles detected, but do not abort execution. When cycles are present, the order of static construction is implementation defined, and not guaranteed to be valid.

4. ignore Do not abort execution or print any cycles. When cycles are present, the order of static construction is implementation defined, and not guaranteed to be valid.

Best Practices:

1. Avoid cyclical imports where practical. They can be an indication of poor decomposition of a program's structure into independent modules. Two modules that import each other can often be reorganized into three modules without cycles, where the third contains the declarations needed by the other two.

Order of Static Construction within a Module

Within a module, static construction occurs in the lexical order in which they appear.

Order of Static Destruction

This is defined to be in exactly the reverse order of static construction. Static destructors for individual modules will only be run if the corresponding static constructor successfully completed.

Shared static destructors are executed after static destructors.

Order of Unit tests

Unit tests are run in the lexical order in which they appear within a module.

Mixin Declaration

MixinDeclaration:
mixin ( ArgumentList ) ;

Each AssignExpression in the ArgumentList is evaluated at compile time, and the result must be representable as a string. The resulting strings are concatenated to form a string. The text contents of the string must be compilable as a valid DeclDefs, and is compiled as such.

Package Module

A package module can be used to publicly import other modules, while providing a simpler import syntax. This enables the conversion of a module into a package of modules, without breaking existing code which uses that module. Example of a set of library modules:
libweb/client.d:

module libweb.client;

void runClient() { }

libweb/server.d:

module libweb.server;

void runServer() { }

libweb/package.d:

module libweb;

public import libweb.client;
public import libweb.server;

The package module's file name must be package.d. The module name is declared to be the fully qualified name of the package. Package modules can be imported just like any other modules:
test.d:

module test;

// import the package module
import libweb;

void main()
{
runClient();
runServer();
}

A package module can be nested inside of a sub-package:
libweb/utils/package.d:

// must be declared as the fully qualified name of the package, not just 'utils'
module libweb.utils;

// publicly import modules from within the 'libweb.utils' package.
public import libweb.utils.conv;
public import libweb.utils.text;

The package module can then be imported with the standard module import declaration:
test.d:

module test;

// import the package module
import libweb.utils;

void main() { }

Declarations

Declaration:
FuncDeclaration
VarDeclarations
AliasDeclaration
AliasAssign
AggregateDeclaration
EnumDeclaration
ImportDeclaration
ConditionalDeclaration
StaticForeachDeclaration
StaticAssert

VarDeclarations:
StorageClassesopt BasicType Declarators ;
AutoDeclaration

Declarators:
DeclaratorInitializer
DeclaratorInitializer , DeclaratorIdentifierList

DeclaratorInitializer:
VarDeclarator
VarDeclarator TemplateParametersopt = Initializer
AltDeclarator
AltDeclarator = Initializer

DeclaratorIdentifierList:
DeclaratorIdentifier
DeclaratorIdentifier , DeclaratorIdentifierList

DeclaratorIdentifier:
VarDeclaratorIdentifier
AltDeclaratorIdentifier

VarDeclaratorIdentifier:
Identifier
Identifier TemplateParametersopt = Initializer

AltDeclaratorIdentifier:
TypeSuffixes Identifier AltDeclaratorSuffixesopt
TypeSuffixes Identifier AltDeclaratorSuffixesopt = Initializer
TypeSuffixesopt Identifier AltDeclaratorSuffixes
TypeSuffixesopt Identifier AltDeclaratorSuffixes = Initializer

Declarator:
VarDeclarator
AltDeclarator

VarDeclarator:
TypeSuffixesopt Identifier

AltDeclarator:
TypeSuffixesopt Identifier AltDeclaratorSuffixes
TypeSuffixesopt ( AltDeclaratorInner )
TypeSuffixesopt ( AltDeclaratorInner ) AltFuncDeclaratorSuffix
TypeSuffixesopt ( AltDeclaratorInner ) AltDeclaratorSuffixes

AltDeclaratorInner:
TypeSuffixesopt Identifier
TypeSuffixesopt Identifier AltFuncDeclaratorSuffix
AltDeclarator

AltDeclaratorSuffixes:
AltDeclaratorSuffix
AltDeclaratorSuffix AltDeclaratorSuffixes

AltDeclaratorSuffix:
[ ]
[ AssignExpression ]
[ Type ]

AltFuncDeclaratorSuffix:
Parameters MemberFunctionAttributesopt

StorageClasses:
StorageClass
StorageClass StorageClasses

StorageClass:
LinkageAttribute
AlignAttribute
AtAttribute
deprecated
enum
static
extern
abstract
final
override
synchronized
auto
scope
const
immutable
inout
shared
__gshared
Property
nothrow
pure
ref

Initializer:
VoidInitializer
NonVoidInitializer

NonVoidInitializer:
ExpInitializer
ArrayInitializer
StructInitializer

ExpInitializer:
AssignExpression

ArrayInitializer:
[ ArrayMemberInitializationsopt ]

ArrayMemberInitializations:
ArrayMemberInitialization
ArrayMemberInitialization ,
ArrayMemberInitialization , ArrayMemberInitializations

ArrayMemberInitialization:
NonVoidInitializer
AssignExpression : NonVoidInitializer

StructInitializer:
{ StructMemberInitializersopt }

StructMemberInitializers:
StructMemberInitializer
StructMemberInitializer ,
StructMemberInitializer , StructMemberInitializers

StructMemberInitializer:
NonVoidInitializer
Identifier : NonVoidInitializer

Declaration Syntax

Declaration syntax generally reads right to left, including arrays:

int x;    // x is an int
int* x;   // x is a pointer to int
int** x;  // x is a pointer to a pointer to int

int[] x;  // x is an array of ints
int*[] x; // x is an array of pointers to ints
int[]* x; // x is a pointer to an array of ints

int[3] x;     // x is a static array of 3 ints
int[3][5] x;  // x is a static array of 5 static arrays of 3 ints
int[3]*[5] x; // x is a static array of 5 pointers to static arrays of 3 ints

Pointers to Functions

Pointers to functions are declared using the function keyword:

int function(char) x; // x is a pointer to
// a function taking a char argument
// and returning an int
int function(char)[] x; // x is an array of
// pointers to functions
// taking a char argument
// and returning an int

C-Style Declarations

C-style array, function pointer and pointer to array declarations are deprecated:

int x[3];          // x is a static array of 3 ints
int x[3][5];       // x is a static array of 3 arrays of 5 ints

int (*x[5])[3];    // x is a static array of 5 pointers to static arrays of 3 ints
int (*x)(char);    // x is a pointer to a function taking a char argument
// and returning an int
int (*[] x)(char); // x is an array of pointers to functions
// taking a char argument and returning an int

Declaring Multiple Symbols

In a declaration declaring multiple symbols, all the declarations must be of the same type:

int x,y;   // x and y are ints
int* x,y;  // x and y are pointers to ints
int[] x,y; // x and y are arrays of ints

// invalid C-style declarations
int x,*y;  // error, multiple types
int x[],y; // error, multiple types

Implicit Type Inference

AutoDeclaration:
StorageClasses AutoAssignments ;

AutoAssignments:
AutoAssignment
AutoAssignments , AutoAssignment

AutoAssignment:
Identifier TemplateParametersopt = Initializer

If a declaration starts with a StorageClass and has a NonVoidInitializer from which the type can be inferred, the type on the declaration can be omitted.

static x = 3;      // x is type int
auto y = 4u;       // y is type uint

auto s = "Apollo"; // s is type immutable(char)[] i.e., string

class C { ... }

auto c = new C();  // c is a handle to an instance of class C

The NonVoidInitializer cannot contain forward references (this restriction may be removed in the future). The implicitly inferred type is statically bound to the declaration at compile time, not run time.

An ArrayLiteral is inferred to be a dynamic array type rather than a static array:

auto v = ["resistance", "is", "useless"]; // type is string[], not string[3]

Alias Declarations

AliasDeclaration:
alias StorageClassesopt BasicType Declarators ;
alias StorageClassesopt BasicType FuncDeclarator ;
alias AliasAssignments ;

AliasAssignments:
AliasAssignment
AliasAssignments , AliasAssignment

AliasAssignment:
Identifier TemplateParametersopt = StorageClassesopt Type
Identifier TemplateParametersopt = FunctionLiteral
Identifier TemplateParametersopt = StorageClassesopt BasicType Parameters MemberFunctionAttributesopt

AliasDeclarations create a symbol that is an alias for another type, and can be used anywhere that other type may appear.

alias myint = abc.Foo.bar;

Aliased types are semantically identical to the types they are aliased to. The debugger cannot distinguish between them, and there is no difference as far as function overloading is concerned. For example:

alias myint = int;

void foo(int x) { ... }
void foo(myint m) { ... } // error, multiply defined function foo

A symbol can be declared as an alias of another symbol. For example:

import planets;

alias myAlbedo = planets.albedo;
...
int len = myAlbedo("Saturn"); // actually calls planets.albedo()

The following alias declarations are valid:

template Foo2(T) { alias t = T; }
alias t1 = Foo2!(int);
alias t2 = Foo2!(int).t;
alias t3 = t1.t;
alias t4 = t2;

t1.t v1;  // v1 is type int
t2 v2;    // v2 is type int
t3 v3;    // v3 is type int
t4 v4;    // v4 is type int

alias Fun = int(string p);
int fun(string){return 0;}
static assert(is(typeof(fun) == Fun));

alias MemberFun1 = int() const;
alias MemberFun2 = const int();
// leading attributes apply to the func, not the return type
static assert(is(MemberFun1 == MemberFun2));

Aliased symbols are useful as a shorthand for a long qualified symbol name, or as a way to redirect references from one symbol to another:

version (Win32)
{
alias myfoo = win32.foo;
}
version (linux)
{
alias myfoo = linux.bar;
}

Aliasing can be used to import a symbol from an import into the current scope:

alias strlen = string.strlen;

Aliases can also import a set of overloaded functions, that can be overloaded with functions in the current scope:

class A
{
int foo(int a) { return 1; }
}

class B : A
{
int foo( int a, uint b ) { return 2; }
}

class C : B
{
int foo( int a ) { return 3; }
alias foo = B.foo;
}

class D : C
{
}

void test()
{
D b = new D();
int i;

i = b.foo(1, 2u);   // calls B.foo
i = b.foo(1);       // calls C.foo
}

Note: Type aliases can sometimes look indistinguishable from alias declarations:

alias abc = foo.bar; // is it a type or a symbol?

The distinction is made in the semantic analysis pass.

Aliases cannot be used for expressions:

struct S
{
static int i;
static int j;
}

alias a = S.i; // OK, `S.i` is a symbol
alias b = S.j; // OK. `S.j` is also a symbol
alias c = a + b; // illegal, `a + b` is an expression
a = 2;         // sets `S.i` to `2`
b = 4;         // sets `S.j` to `4`

Alias Assign

AliasAssign:
Identifier = Type

An AliasDeclaration can have a new value assigned to it with an AliasAssign:

template Gorgon(T)
{
alias A = long;
A = T; // assign new value to A
alias Gorgon = A;
}
pragma(msg, Gorgon!int); // prints int

- The AliasAssign and its corresponding AliasDeclaration must both be declared in the same TemplateDeclaration.

- The corresponding AliasDeclaration must appear lexically before the AliasAssign.

- The corresponding AliasDeclaration may not refer to overloaded symbols.

- The value of an AliasDeclaration or left hand side (lvalue) of an AliasAssign may not be used prior to another AliasAssign to the same lvalue other than in the right hand side of that AliasAssign.

Best Practices: AliasAssign is particularly useful when using an iterative computation rather than a recursive one, as it avoids creating the large number of intermediate templates that the recursive one engenders.

template AliasSeq(T...) { alias AliasSeq = T; }

static if (0) // recursive method for comparison
{
template Reverse(T...)
{
static if (T.length == 0)
alias Reverse = AliasSeq!();
else
alias Reverse = AliasSeq!(Reverse!(T[1 .. T.length]), T[0]);
}
}
else // iterative method minimizes template instantiations
{
template Reverse(T...)
{
alias A = AliasSeq!();
static foreach (t; T)
A = AliasSeq!(t, A); // Alias Assign
alias Reverse = A;
}
}

enum X = 3;
alias TK = Reverse!(int, const uint, X);
pragma(msg, TK); // prints tuple(3, (const(uint)), (int))

Alias Reassignment

AliasReassignment:
Identifier = StorageClassesopt Type
Identifier = FunctionLiteral
Identifier = StorageClassesopt BasicType Parameters MemberFunctionAttributesopt

An alias declaration inside a template can be reassigned a new value.

template staticMap(alias F, T...)
{
alias A = AliasSeq!();
static foreach (t; T)
A = AliasSeq!(A, F!T); // alias reassignment
alias staticMap = A;
}

The Identifier must resolve to a lexically preceding AliasDeclaration. Both must be members of the same TemplateDeclaration.

The right hand side of the AliasReassignment replaces the right hand side of the AliasDeclaration.

Once the AliasDeclaration has been referred to in any context other than the right hand side of an AliasReassignment it can no longer be reassigned.

Rationale: Alias reassignment can result in faster compile times and lowered memory consumption, and requires significantly simpler code than the alternative recursive method.

Extern Declarations

Variable declarations with the storage class extern are not allocated storage within the module. They must be defined in some other object file with a matching name which is then linked in.

An extern declaration can optionally be followed by an extern linkage attribute. If there is no linkage attribute it defaults to extern(D):

// variable allocated and initialized in this module with C linkage
extern(C) int foo;
// variable allocated outside this module with C linkage
// (e.g. in a statically linked C library or another module)
extern extern(C) int bar;

Best Practices:

1. The primary usefulness of Extern Declarations is to connect with global variables declarations and functions in C or C++ files.

Void Initializations

VoidInitializer:
void

Normally, variables are initialized either with an explicit Initializer or are set to the default value for the type of the variable. If the Initializer is void, however, the variable is not initialized. If its value is used before it is set, undefined program behavior will result.

Implementation Defined: If a void initialized variable's value is used before it is set, its value is implementation defined.

void bad()
{
int x = void;
writeln(x);  // print implementation defined value
}

Undefined Behavior: If a void initialized variable's value is used before it is set, and the value is a reference, pointer or an instance of a struct with an invariant, the behavior is undefined.

void muchWorse()
{
char[] p = void;
writeln(p);  // may result in apocalypse
}

Best Practices:

1. Void initializers are useful when a static array is on the stack, but may only be partially used, such as as a temporary buffer. Void initializers will potentially speed up the code, but they introduce risk, since one must ensure that array elements are always set before read.

2. The same is true for structs.

3. Use of void initializers is rarely useful for individual local variables, as a modern optimizer will remove the dead store of its initialization if it is initialized later.

4. For hot code paths, it is worth profiling to see if the void initializer actually improves results.

Global and Static Initializers

The Initializer for a global or static variable must be evaluatable at compile time. Runtime initialization is done with static constructors.

Implementation Defined:

1. Whether some pointers can be initialized with the addresses of other functions or data.

Type Qualifiers vs. Storage Classes

Type qualifer and storage classes are distinct.

A type qualifier creates a derived type from an existing base type, and the resulting type may be used to create multiple instances of that type.

For example, the immutable type qualifier can be used to create variables of immutable type, such as:

immutable(int)   x; // typeof(x) == immutable(int)
immutable(int)[] y; // typeof(y) == immutable(int)[]
// typeof(y[0]) == immutable(int)

// Type constructors create new types that can be aliased:
alias ImmutableInt = immutable(int);
ImmutableInt z;     // typeof(z) == immutable(int)

A storage class, on the other hand, does not create a new type, but describes only the kind of storage used by the variable or function being declared. For example, a member function can be declared with the const storage class to indicate that it does not modify its implicit this argument:

struct S
{
int x;
int method() const
{
//x++;    // Error: this method is const and cannot modify this.x
return x; // OK: we can still read this.x
}
}

Although some keywords can be used both as a type qualifier and a storage class, there are some storage classes that cannot be used to construct new types, such as ref:

// ref declares the parameter x to be passed by reference
void func(ref int x)
{
x++; // so modifications to x will be visible in the caller
}

void main()
{
auto x = 1;
func(x);
assert(x == 2);

// However, ref is not a type qualifier, so the following is illegal:
ref(int) y; // Error: ref is not a type qualifier.
}

Functions can also be declared as ref, meaning their return value is passed by reference:

ref int func2()
{
static int y = 0;
return y;
}

void main()
{
func2() = 2; // The return value of func2() can be modified.
assert(func2() == 2);

// However, the reference returned by func2() does not propagate to
// variables, because the 'ref' only applies to the return value itself,
// not to any subsequent variable created from it:
auto x = func2();
static assert(is(typeof(x) == int)); // N.B.: *not* ref(int);
// there is no such type as ref(int).
x++;
assert(x == 3);
assert(func2() == 2); // x is not a reference to what func2() returned; it
// does not inherit the ref storage class from func2().
}

Some keywords, such as const, can be used both as a type qualifier and a storage class. The distinction is determined by the syntax where it appears.

struct S
{
/* Is const here a type qualifier or a storage class?
* Is the return value const(int), or is this a const function that returns
* (mutable) int?
*/
const int* func() // a const function
{
++p;          // error, this.p is const
return p;     // error, cannot convert const(int)* to int*
}

const(int)* func() // a function returning a pointer to a const int
{
++p;          // ok, this.p is mutable
return p;     // ok, int* can be implicitly converted to const(int)*
}

int* p;
}

Best Practices: To avoid confusion, the type qualifier syntax with parentheses should be used for return types, and function storage classes should be written on the right-hand side of the declaration instead of the left-hand side where it may be visually confused with the return type:

struct S
{
// Now it is clear that the 'const' here applies to the return type:
const(int) func1() { return 1; }

// And it is clear that the 'const' here applies to the function:
int func2() const { return 1; }
}

Types

D is statically typed. Every expression has a type. Types constrain the values an expression can hold, and determine the semantics of operations on those values.

Type:
TypeCtorsopt BasicType TypeSuffixesopt

TypeCtors:
TypeCtor
TypeCtor TypeCtors

TypeCtor:
const
immutable
inout
shared

BasicType:
FundamentalType
. QualifiedIdentifier
QualifiedIdentifier
Typeof
Typeof . QualifiedIdentifier
TypeCtor ( Type )
Vector
TraitsExpression
MixinType

Vector:
__vector ( VectorBaseType )

VectorBaseType:
Type

FundamentalType:
bool
byte
ubyte
short
ushort
int
uint
long
ulong
cent
ucent
char
wchar
dchar
float
double
real
ifloat
idouble
ireal
cfloat
cdouble
creal
void

TypeSuffixes:
TypeSuffix TypeSuffixesopt

TypeSuffix:
*
[ ]
[ AssignExpression ]
[ AssignExpression .. AssignExpression ]
[ Type ]
delegate Parameters MemberFunctionAttributesopt
function Parameters FunctionAttributesopt

QualifiedIdentifier:
Identifier
Identifier . QualifiedIdentifier
TemplateInstance
TemplateInstance . QualifiedIdentifier
Identifier [ AssignExpression ]
Identifier [ AssignExpression ] . QualifiedIdentifier

Basic Data Types are leaf types. Derived Data Types build on leaf types. User-Defined Types are aggregates of basic and derived types.

Basic Data Types

Basic Data Types

Keyword

Default Initializer (.init)

Description

void

no default initializer

void has no value

bool

false

boolean value

byte

0

signed 8 bits

ubyte

0u

unsigned 8 bits

short

0

signed 16 bits

ushort

0u

unsigned 16 bits

int

0

signed 32 bits

uint

0u

unsigned 32 bits

long

0L

signed 64 bits

ulong

0uL

unsigned 64 bits

cent

0

signed 128 bits (reserved for future use)

ucent

0u

unsigned 128 bits (reserved for future use)

float

float.nan

32 bit floating point

double

double.nan

64 bit floating point

real

real.nan

largest floating point size available

ifloat

float.nan*1.0i

imaginary float

idouble

double.nan*1.0i

imaginary double

ireal

real.nan*1.0i

imaginary real

cfloat

float.nan+float.nan*1.0i

a complex number of two float values

cdouble

double.nan+double.nan*1.0i

complex double

creal

real.nan+real.nan*1.0i

complex real

char

'xFF'

unsigned 8 bit (UTF-8 code unit)

wchar

'uFFFF'

unsigned 16 bit (UTF-16 code unit)

dchar

'U0000FFFF'

unsigned 32 bit (UTF-32 code unit)

Implementation Defined: The real floating point type has at least the range and precision of the double type. On x86 CPUs it is often implemented as the 80 bit Extended Real type supported by the x86 FPU.

NOTE: Complex and imaginary types ifloat, idouble, ireal, cfloat, cdouble, and creal have been deprecated in favor of std.complex.Complex.

Derived Data Types

- Pointer

- Static Array

- Dynamic Array

- Associative Array

- Functions

- Delegates

User-Defined Types

- Enums

- Structs and Unions

- Classes

- Interfaces

Type Conversions
See also: CastExpression.
Pointer Conversions

Casting pointers to non-pointers and vice versa is allowed.

Best Practices: do not do this for any pointers that point to data allocated by the garbage collector.

Implicit Conversions

Implicit conversions are used to automatically convert types as required.

An enum can be implicitly converted to its base type, but going the other way requires an explicit conversion. For example:

int i;

enum Foo { E }
Foo f;
i = f;           // OK
f = i;           // error
f = cast(Foo)i;  // OK
f = 0;           // error
f = Foo.E;       // OK

A derived class can be implicitly converted to its base class, but going the other way requires an explicit cast. For example:

class Base {}
class Derived : Base {}
Base bd = new Derived();              // implicit conversion
Derived db = cast(Derived)new Base(); // explicit conversion

A dynamic array, say x, of a derived class can be implicitly converted to a dynamic array, say y, of a base class iff elements of x and y are qualified as being either both const or both immutable.

class Base {}
class Derived : Base {}
const(Base)[] ca = (const(Derived)[]).init; // `const` elements
immutable(Base)[] ia = (immutable(Derived)[]).init; // `immutable` elements

A static array, say x, of a derived class can be implicitly converted to a static array, say y, of a base class iff elements of x and y are qualified as being either both const or both immutable or both mutable (neither const nor immutable).

class Base {}
class Derived : Base {}
Base[3] ma = (Derived[3]).init; // mutable elements
const(Base)[3] ca = (const(Derived)[3]).init; // `const` elements
immutable(Base)[3] ia = (immutable(Derived)[3]).init; // `immutable` elements

Integer Promotions

Integer Promotions are conversions of the following types:

Integer Promotions

from

to

bool

int

byte

int

ubyte

int

short

int

ushort

int

char

int

wchar

int

dchar

uint

If an enum has as a base type one of the types in the left column, it is converted to the type in the right column.

Usual Arithmetic Conversions

The usual arithmetic conversions convert operands of binary operators to a common type. The operands must already be of arithmetic types. The following rules are applied in order, looking at the base type:

1. If either operand is real, the other operand is converted to real.

2. Else if either operand is double, the other operand is converted to double.

3. Else if either operand is float, the other operand is converted to float.

4. Else the integer promotions are done on each operand, followed by:

1. If both are the same type, no more conversions are done.

2. If both are signed or both are unsigned, the smaller type is converted to the larger.

3. If the signed type is larger than the unsigned type, the unsigned type is converted to the signed type.

4. The signed type is converted to the unsigned type.

If one or both of the operand types is an enum after undergoing the above conversions, the result type is:

1. If the operands are the same type, the result will be of that type.

2. If one operand is an enum and the other is the base type of that enum, the result is the base type.

3. If the two operands are different enums, the result is the closest base type common to both. A base type being closer means there is a shorter sequence of conversions to base type to get there from the original type.

Integer values cannot be implicitly converted to another type that cannot represent the integer bit pattern after integral promotion. For example:

ubyte  u1 = -1;       // error, -1 cannot be represented in a ubyte
ushort u2 = -1;       // error, -1 cannot be represented in a ushort
uint   u3 = int(-1);  // ok, -1 can be represented in a uint
ulong  u4 = long(-1); // ok, -1 can be represented in a ulong

Floating point types cannot be implicitly converted to integral types. Complex or imaginary floating point types cannot be implicitly converted to non-complex floating point types. Non-complex floating point types cannot be implicitly converted to imaginary floating point types.

bool

The bool type is a byte-size type that can only hold the value true or false.

The only operators that can accept operands of type bool are: & |, ^, &=, |=, ^=, !, &&, ||, and ?:.

A bool value can be implicitly converted to any integral type, with false becoming 0 and true becoming 1.

The numeric literals 0 and 1 can be implicitly converted to the bool values false and true, respectively. Casting an expression to bool means testing for 0 or !=0 for arithmetic types, and null or !=null for pointers or references.

Delegates

Delegates are an aggregate of two pieces of data: an object reference and a pointer to a non-static member function, or a pointer to a closure and a pointer to a nested function. The object reference forms the this pointer when the function is called.

Delegates are declared similarly to function pointers:

int function(int) fp; // fp is pointer to a function
int delegate(int) dg; // dg is a delegate to a function

A delegate is initialized analogously to function pointers:

int func(int);
fp = &func;   // fp points to func
class OB
{
int member(int);
}
OB o;
dg = &o.member; // dg is a delegate to object o and
// member function member

Delegates cannot be initialized with static member functions or non-member functions.

Delegates are called analogously to function pointers:

fp(3);   // call func(3)
dg(3);   // call o.member(3)

The equivalent of member function pointers can be constructed using anonymous lambda functions:

class C
{
int a;
int foo(int i) { return i + a; }
}

// mfp is the member function pointer
auto mfp = function(C self, int i) { return self.foo(i); };
auto c = new C();  // create an instance of C
mfp(c, 1);  // and call c.foo(1)

The C style syntax for declaring pointers to functions is deprecated:

int (*fp)(int);  // fp is pointer to a function

typeof

Typeof:
typeof ( Expression )
typeof ( return )

typeof is a way to specify a type based on the type of an expression. For example:

void func(int i)
{
typeof(i) j;       // j is of type int
typeof(3 + 6.0) x; // x is of type double
typeof(1)* p;      // p is of type pointer to int
int[typeof(p)] a;  // a is of type int[int*]

writeln(typeof('c').sizeof); // prints 1
double c = cast(typeof(1.0))j; // cast j to double
}

Expression is not evaluated, it is used purely to generate the type:

void func()
{
int i = 1;
typeof(++i) j; // j is declared to be an int, i is not incremented
writeln(i);  // prints 1
}

Special cases:

1. typeof(this) will generate the type of what this would be in a non-static member function, even if not in a member function.

2. Analogously, typeof(super) will generate the type of what super would be in a non-static member function.

3. typeof(return) will, when inside a function scope, give the return type of that function.

class A { }

class B : A
{
typeof(this) x;  // x is declared to be a B
typeof(super) y; // y is declared to be an A
}

struct C
{
static typeof(this) z;  // z is declared to be a C

typeof(super) q; // error, no super struct for C
}

typeof(this) r;   // error, no enclosing struct or class

If the expression is a Property Function, typeof gives its return type.

struct S
{
@property int foo() { return 1; }
}
typeof(S.foo) n;  // n is declared to be an int

Best Practices:

1. Typeof is most useful in writing generic template code.

Mixin Types

MixinType:
mixin ( ArgumentList )

Each AssignExpression in the ArgumentList is evaluated at compile time, and the result must be representable as a string. The resulting strings are concatenated to form a string. The text contents of the string must be compilable as a valid Type, and is compiled as such.

void test(mixin("int")* p) // int* p
{
mixin("int")[] a;      // int[] a;
mixin("int[]") b;      // int[] b;
}

Aliased Types

size_t

size_t is an alias to one of the unsigned integral basic types, and represents a type that is large enough to represent an offset into all addressable memory.

ptrdiff_t

ptrdiff_t is an alias to the signed integral basic type the same size as size_t.

string

A string is a special case of an array.

Properties

Every symbol, type, and expression has properties that can be queried:

Property Examples

Expression

Value

int.sizeof

yields 4

float.nan

yields the floating point nan (Not A Number) value

(float).nan

yields the floating point nan value

(3).sizeof

yields 4 (because 3 is an int)

int.init

default initializer for ints

int.mangleof

yields the string "i"

int.stringof

yields the string "int"

(1+2).stringof

yields the string "1 + 2"

Properties for All Types

Property

Description

.init

initializer

.sizeof

size in bytes

.alignof

alignment size

.mangleof

string representing the ‘mangled’ representation of the type

.stringof

string representing the source representation of the type

Properties for Integral Types

Property

Description

.init

initializer

.max

maximum value

.min

minimum value

Properties for Floating Point Types

Property

Description

.init

initializer (NaN)

.infinity

infinity value

.nan

NaN value

.dig

number of decimal digits of precision

.epsilon

smallest increment to the value 1

.mant_dig

number of bits in mantissa

.max_10_exp

maximum int value such that 10max_10_exp is representable

.max_exp

maximum int value such that 2max_exp-1 is representable

.min_10_exp

minimum int value such that 10min_10_exp is representable as a normalized value

.min_exp

minimum int value such that 2min_exp-1 is representable as a normalized value

.max

largest representable value that's not infinity

.min_normal

smallest representable normalized value that's not 0

.re

real part

.im

imaginary part

Properties for Class Types

Property

Description

.classinfo

Information about the dynamic type of the class

.init Property

.init produces a constant expression that is the default initializer. If applied to a type, it is the default initializer for that type. If applied to a variable or field, it is the default initializer for that variable or field's type.

int a;
int b = 1;

int.init // is 0
a.init   // is 0
b.init   // is 0

struct Foo
{
int a;
int b = 7;
}

Foo.init.a  // is 0
Foo.init.b  // is 7

Note: .init produces a default initialized object, not default constructed. If there is a default constructor for an object, it may produce a different value.

1. If T is a nested struct, the context pointer in T.init is null.

void main()
{
int x;
struct S
{
void foo() { x = 1; }  // access x in enclosing scope via context pointer
}
S s1;           // OK. S() correctly initialize its context pointer.
S s2 = S();     // OK. same as s1
S s3 = S.init;  // Bad. the context pointer in s3 is null
s3.foo();       // Access violation
}

2. If T is a struct which has @disable this();, T.init might return a logically incorrect object.

struct S
{
int x;
@disable this();
this(int n) { x = n; }
invariant { assert(x > 0); }
void check() {}
}

void main()
{
//S s1;           // Error: variable s1 initializer required for type S
//S s2 = S();     // Error: constructor S.this is not callable
// because it is annotated with @disable
S s3 = S.init;  // Bad. s3.x == 0, and it violates the invariant of S
s3.check();     // Assertion failure
}

.stringof Property

.stringof produces a constant string that is the source representation of its prefix. If applied to a type, it is the string for that type. If applied to an expression, it is the source representation of that expression. The expression will not be evaluated.

module test;
import std.stdio;

struct Dog { }

enum Color { Red }

int i = 4;

void main()
{
writeln((1+2).stringof);       // "1 + 2"
writeln(Dog.stringof);         // "Dog"
writeln(test.Dog.stringof);    // "Dog"
writeln(int.stringof);         // "int"
writeln((int*[5][]).stringof); // "int*[5][]"
writeln(Color.Red.stringof);   // "Red"
writeln((5).stringof);         // "5"

writeln((++i).stringof);       // "i += 1"
writeln(i);                    // 4
}

Implementation Defined: The string representation for a type or expression can vary.

Best Practices: Do not use .stringof for code generation. Instead use the identifier trait, or one of the Phobos helper functions such as std.traits.fullyQualifiedName.

.sizeof Property

e.sizeof gives the size in bytes of the expression e.

When getting the size of a member, it is not necessary for there to be a this object:

struct S
{
int a;
static int foo()
{
return a.sizeof; // returns 4
}
}

void test()
{
int x = S.a.sizeof; // sets x to 4
}

.sizeof applied to a class object returns the size of the class reference, not the class instantiation.

.alignof Property

.alignof gives the aligned size of an expression or type. For example, an aligned size of 1 means that it is aligned on a byte boundary, 4 means it is aligned on a 32 bit boundary.

Implementation Defined: the actual aligned size.

Best Practices: Be particularly careful when laying out an object that must line up with an externally imposed layout. Data misalignment can result in particularly pernicious bugs. It's often worth putting in an assert to assure it is correct.

.mangleof Property

Mangling refers to how a symbol is represented in text form in the generated object file. .mangleof returns a string literal of the representation of the type or symbol it is applied to. The mangling of types and symbols with D linkage is defined by Name Mangling.

Implementation Defined:

1. whether a leading underscore is added to a symbol

2. the mangling of types and symbols with non-D linkage. For C and C++ linkage, this will typically match what the associated C or C++ compiler does.

.classinfo Property

.classinfo provides information about the dynamic type of a class object. It returns a reference to type object.TypeInfo_Class.

.classinfo applied to an interface gives the information for the interface, not the class it might be an instance of.

User-Defined Properties

User-defined properties can be created using Property Functions.

Attributes

AttributeSpecifier:
Attribute :
Attribute DeclarationBlock

Attribute:
LinkageAttribute
AlignAttribute
DeprecatedAttribute
VisibilityAttribute
Pragma
static
extern
abstract
final
override
synchronized
auto
scope
const
immutable
inout
shared
__gshared
AtAttribute
FunctionAttributeKwd
ref
return

FunctionAttributeKwd:
nothrow
pure

AtAttribute:
@ disable
@ nogc
@ live
Property
@ safe
@ system
@ trusted
UserDefinedAttribute

Property:
@ property

DeclarationBlock:
DeclDef
{ DeclDefsopt }

Attributes are a way to modify one or more declarations. The general forms are:

attribute declaration; // affects the declaration

attribute:     // affects all declarations until the end of
// the current scope
declaration;
declaration;
...

attribute {    // affects all declarations in the block
declaration;
declaration;
...
}

Linkage Attribute

LinkageAttribute:
extern ( LinkageType )
extern ( C++ , )
extern ( C++ , QualifiedIdentifier )
extern ( C++ , NamespaceList )

LinkageType:
C
C++
D
Windows
System
Objective-C

NamespaceList:
ConditionalExpression
ConditionalExpression,
ConditionalExpression, NamespaceList

D provides an easy way to call C functions and operating system API functions, as compatibility with both is essential. The LinkageType is case sensitive, and is meant to be extensible by the implementation (they are not keywords). C and D must be supplied, the others are what makes sense for the implementation. C++ offers limited compatibility with C++. Objective-C offers compatibility with Objective-C, see the Interfacing to Objective-C documentation for more information. System is the same as Windows on Windows platforms, and C on other platforms. Implementation Note: for Win32 platforms, Windows should exist.

C function calling conventions are specified by:

extern (C):
int foo(); // call foo() with C conventions

Note that extern(C) can be provided for all types of declarations, including struct or class, even though there is no corresponding match on the C side. In that case, the attribute is ignored. This behavior applies for nested functions and nested variables as well. However, for static member methods and static nested functions, adding extern(C) will change the calling convention, but not the mangling.
D conventions are:

extern (D):

Windows API conventions are:

extern (Windows):
void *VirtualAlloc(
void *lpAddress,
uint dwSize,
uint flAllocationType,
uint flProtect
);

The Windows convention is distinct from the C convention only on Win32 platforms, where it is equivalent to the stdcall convention.

Note that a lone extern declaration is used as a storage class.

C++ Namespaces

The linkage form extern (C++, QualifiedIdentifier) creates C++ declarations that reside in C++ namespaces. The QualifiedIdentifier specifies the namespaces.

extern (C++, N) { void foo(); }

refers to the C++ declaration:

namespace N { void foo(); }

and can be referred to with or without qualification:

foo();
N.foo();

Namespaces create a new named scope that is imported into its enclosing scope.

extern (C++, N) { void foo(); void bar(); }
extern (C++, M) { void foo(); }

bar();   // ok
foo();   // error - N.foo() or M.foo() ?
M.foo(); // ok

Multiple identifiers in the QualifiedIdentifier create nested namespaces:

extern (C++, N.M) { extern (C++) { extern (C++, R) { void foo(); } } }
N.M.R.foo();

refers to the C++ declaration:

namespace N { namespace M { namespace R { void foo(); } } }

align Attribute

AlignAttribute:
align
align ( AssignExpression )

Specifies the alignment of:

1. variables

2. struct fields

3. union fields

4. class fields

5. struct, union, and class types

align by itself sets it to the default, which matches the default member alignment of the companion C compiler.

struct S
{
align:
byte a;   // placed at offset 0
int b;    // placed at offset 4
long c;   // placed at offset 8
}
static assert(S.alignof == 8);
static assert(S.sizeof == 16);

AssignExpression specifies the alignment which matches the behavior of the companion C compiler when non-default alignments are used. It must be a positive power of 2.

A value of 1 means that no alignment is done; fields are packed together.

struct S
{
align (1):
byte a;   // placed at offset 0
int b;    // placed at offset 1
long c;   // placed at offset 5
}
static assert(S.alignof == 1);
static assert(S.sizeof == 13);

The natural alignment of an aggregate is the maximum alignment of its fields. It can be overridden by setting the alignment outside of the aggregate.

align (2) struct S
{
align (1):
byte a;   // placed at offset 0
int b;    // placed at offset 1
long c;   // placed at offset 5
}
static assert(S.alignof == 2);
static assert(S.sizeof == 14);

Setting the alignment of a field aligns it to that power of 2, regardless of the size of the field.

struct S
{
byte a;  // placed at offset 0
align (4)  byte b;  // placed at offset 4
align (16) short c; // placed at offset 16
}
static assert(S.alignof == 16);
static assert(S.sizeof == 32);

Do not align references or pointers that were allocated using NewExpression on boundaries that are not a multiple of size_t. The garbage collector assumes that pointers and references to GC allocated objects will be on size_t byte boundaries.

Undefined Behavior: If any pointers and references to GC allocated objects are not aligned on size_t byte boundaries.

The AlignAttribute is reset to the default when entering a function scope or a non-anonymous struct, union, class, and restored when exiting that scope. It is not inherited from a base class.

deprecated Attribute

DeprecatedAttribute:
deprecated
deprecated ( AssignExpression )

It is often necessary to deprecate a feature in a library, yet retain it for backwards compatibility. Such declarations can be marked as deprecated, which means that the compiler can be instructed to produce an error if any code refers to deprecated declarations:

deprecated
{
void oldFoo();
}

oldFoo();   // Deprecated: function test.oldFoo is deprecated

Optionally a string literal or manifest constant can be used to provide additional information in the deprecation message.

deprecated("Don't use bar") void oldBar();
oldBar();   // Deprecated: function test.oldBar is deprecated - Don't use bar

Calling CTFE-able functions or using manifest constants is also possible.

import std.format;
enum Message = format("%s and all its members are obsolete", Foobar.stringof);
deprecated(Message) class Foobar {}
auto f = new Foobar();   // Deprecated: class test.Foobar is deprecated - Foobar
// and all its members are obsolete
deprecated(format("%s is also obsolete", "This class")) class BarFoo {}
auto bf = new BarFoo();  // Deprecated: class test.BarFoo is deprecated - This
// class is also obsolete

Implementation Note: The compiler should have a switch specifying if deprecated should be ignored, cause a warning, or cause an error during compilation.

Visibility Attribute

VisibilityAttribute:
private
package
package ( QualifiedIdentifier )
protected
public
export

Visibility is an attribute that is one of private, package, protected, public, or export. They may be referred to as protection attributes in documents predating DIP22.

Symbols with private visibility can only be accessed from within the same module. Private member functions are implicitly final and cannot be overridden.

package extends private so that package members can be accessed from code in other modules that are in the same package. If no identifier is provided, this applies to the innermost package only, or defaults to private if a module is not nested in a package.

package may have an optional parameter in the form of a dot-separated identifier list which is resolved as the qualified package name. The package must be either the module's parent package or one of its anscestors. If this optional parameter is present, the symbol will be visible in the specified package and all of its descendants.

protected only applies inside classes (and templates as they can be mixed in) and means that a symbol can only be seen by members of the same module, or by a derived class. If accessing a protected instance member through a derived class member function, that member can only be accessed for the object instance which can be implicitly cast to the same type as ‘this’. protected module members are illegal.

public means that any code within the executable can see the member. It is the default visibility attribute.

export means that any code outside the executable can access the member. export is analogous to exporting definitions from a DLL.

Visibility participates in symbol name lookup.

const Attribute

The const attribute changes the type of the declared symbol from T to const(T), where T is the type specified (or inferred) for the introduced symbol in the absence of const.

const int foo = 7;
static assert(is(typeof(foo) == const(int)));

const
{
double bar = foo + 6;
}
static assert(is(typeof(bar) == const(double)));

class C
{
const void foo();
const
{
void bar();
}
void baz() const;
}
pragma(msg, typeof(C.foo)); // const void()
pragma(msg, typeof(C.bar)); // const void()
pragma(msg, typeof(C.baz)); // const void()
static assert(is(typeof(C.foo) == typeof(C.bar)) &&
is(typeof(C.bar) == typeof(C.baz)));

immutable Attribute

The immutable attribute modifies the type from T to immutable(T), the same way as const does.

inout Attribute

The inout attribute modifies the type from T to inout(T), the same way as const does.

shared Attribute

The shared attribute modifies the type from T to shared(T), the same way as const does.

__gshared Attribute

By default, non-immutable global declarations reside in thread local storage. When a global variable is marked with the __gshared attribute, its value is shared across all threads.

int foo;            // Each thread has its own exclusive copy of foo.
__gshared int bar;  // bar is shared by all threads.

__gshared may also be applied to member variables and local variables. In these cases, __gshared is equivalent to static, except that the variable is shared by all threads rather than being thread local.

class Foo
{
__gshared int bar;
}

int foo()
{
__gshared int bar = 0;
return bar++; // Not thread safe.
}

Unlike the shared attribute, __gshared provides no safe-guards against data races or other multi-threaded synchronization issues. It is the responsibility of the programmer to ensure that access to variables marked __gshared is synchronized correctly.

__gshared is disallowed in safe mode.

@disable Attribute

A reference to a declaration marked with the @disable attribute causes a compile time error. This can be used to explicitly disallow certain operations or overloads at compile time rather than relying on generating a runtime error.

@disable void foo() { }

void main() { foo(); /* error, foo is disabled */ }

Disabling struct no-arg constructor disallows default construction of the struct.

Disabling struct postblit makes the struct not copyable.

@safe, @trusted, and @system Attribute

See Function Safety.

@nogc Attribute

See No-GC Functions.

@property Attribute

See Property Functions.

nothrow Attribute

See Nothrow Functions.

pure Attribute

See Pure Functions.

ref Attribute

See Ref Functions.

return Attribute

See Return Ref Parameters.

override Attribute

The override attribute applies to virtual functions. It means that the function must override a function with the same name and parameters in a base class. The override attribute is useful for catching errors when a base class's member function gets its parameters changed, and all derived classes need to have their overriding functions updated.

class Foo
{
int bar();
int abc(int x);
}

class Foo2 : Foo
{
override
{
int bar(char c); // error, no bar(char) in Foo
int abc(int x);  // ok
}
}

static Attribute

The static attribute applies to functions and data. It means that the declaration does not apply to a particular instance of an object, but to the type of the object. In other words, it means there is no this reference. static is ignored when applied to other declarations.

class Foo
{
static int bar() { return 6; }
int foobar() { return 7; }
}

...

Foo f = new Foo;
Foo.bar();      // produces 6
Foo.foobar();   // error, no instance of Foo
f.bar();        // produces 6;
f.foobar();     // produces 7;

Static functions are never virtual.

Static data has one instance per thread, not one per object.

Static does not have the additional C meaning of being local to a file. Use the private attribute in D to achieve that. For example:

module foo;
int x = 3;         // x is global
private int y = 4; // y is local to module foo

auto Attribute

The auto attribute is used when there are no other attributes and type inference is desired.

auto i = 6.8;   // declare i as a double

For functions, the auto attribute means return type inference. See Auto Functions.

scope Attribute

The scope attribute is used for local variables and for class declarations. For class declarations, the scope attribute creates a scope class. For local declarations, scope implements the RAII (Resource Acquisition Is Initialization) protocol. This means that the destructor for an object is automatically called when the reference to it goes out of scope. The destructor is called even if the scope is exited via a thrown exception, thus scope is used to guarantee cleanup.

If there is more than one scope variable going out of scope at the same point, then the destructors are called in the reverse order that the variables were constructed.

scope cannot be applied to globals, statics, data members, ref or out parameters. Arrays of scopes are not allowed, and scope function return values are not allowed. Assignment to a scope, other than initialization, is not allowed. Rationale: These restrictions may get relaxed in the future if a compelling reason to appears.

abstract Attribute

An abstract member function must be overridden by a derived class. Only virtual member functions may be declared abstract; non-virtual member functions and free-standing functions cannot be declared abstract.

Classes become abstract if any of its virtual member functions are declared abstract or if they are defined within an abstract attribute. Note that an abstract class may also contain non-virtual member functions.

Classes defined within an abstract attribute or with abstract member functions cannot be instantiated directly. They can only be instantiated as a base class of another, non-abstract, class.

Member functions declared as abstract can still have function bodies. This is so that even though they must be overridden, they can still provide ‘base class functionality’, e.g. through super.foo() in a derived class. Note that the class is still abstract and cannot be instantiated directly.

User-Defined Attributes

UserDefinedAttribute:
@ ( ArgumentList )
@ Identifier
@ Identifier ( ArgumentListopt )
@ TemplateInstance
@ TemplateInstance ( ArgumentListopt )

User-Defined Attributes (UDA) are compile-time expressions that can be attached to a declaration. These attributes can then be queried, extracted, and manipulated at compile time. There is no runtime component to them.
A user-defined attribute looks like:

@(3) int a;

@("string", 7) int b;

enum Foo;
@Foo int c;

struct Bar
{
int x;
}

@Bar(3) int d;

If there are multiple UDAs in scope for a declaration, they are concatenated:

@(1)
{
@(2) int a;         // has UDAs (1, 2)
@("string") int b;  // has UDAs (1, "string")
}

UDAs can be extracted into an expression tuple using __traits:

@('c') string s;
pragma(msg, __traits(getAttributes, s)); // prints tuple('c')

If there are no user-defined attributes for the symbol, an empty tuple is returned. The expression tuple can be turned into a manipulatable tuple:

enum EEE = 7;
@("hello") struct SSS { }
@(3) { @(4) @EEE @SSS int foo; }

alias TP = __traits(getAttributes, foo);

pragma(msg, TP); // prints tuple(3, 4, 7, (SSS))
pragma(msg, TP[2]); // prints 7

Of course the tuple types can be used to declare things:

TP[3] a; // a is declared as an SSS

The attribute of the type name is not the same as the attribute of the variable:

pragma(msg, __traits(getAttributes, typeof(a))); // prints tuple("hello")

Of course, the real value of UDAs is to be able to create user-defined types with specific values. Having attribute values of basic types does not scale. The attribute tuples can be manipulated like any other tuple, and can be passed as the argument list to a template.

Whether the attributes are values or types is up to the user, and whether later attributes accumulate or override earlier ones is also up to how the user interprets them.

UDAs cannot be attached to template parameters.

Pragmas

PragmaDeclaration:
Pragma ;
Pragma DeclarationBlock

PragmaStatement:
Pragma ;
Pragma NoScopeStatement

Pragma:
pragma ( Identifier )
pragma ( Identifier , ArgumentList )

Pragmas pass special information to the implementation and can add vendor specific extensions. Pragmas can be used by themselves terminated with a ;, and can apply to a statement, a block of statements, a declaration, or a block of declarations.

Pragmas can be either a PragmaDeclaration or a PragmaStatement.

pragma(ident);        // just by itself

pragma(ident) declaration; // influence one declaration

pragma(ident): // influence subsequent declarations
declaration;
declaration;

pragma(ident)   // influence block of declarations
{
declaration;
declaration;
}

pragma(ident) statement; // influence one statement

pragma(ident)   // influence block of statements
{
statement;
statement;
}

The kind of pragma it is determined by the Identifier. ArgumentList is a comma-separated list of AssignExpressions. The AssignExpressions must be parsable as expressions, but their meaning is up to the individual pragma semantics.

Predefined Pragmas

All implementations must support these, even if by just ignoring them:

- pragma crt_constructor

- pragma crt_destructor

- pragma inline

- pragma lib

- pragma linkerDirective

- pragma mangle

- pragma msg

- pragma printf

- pragma scanf

- pragma startaddress

Implementation Defined: An implementation may ignore these pragmas.

pragma crt_constructor

Annotates a function so it is run after the C runtime library is initialized and before the D runtime library is initialized.

The function must:

1. be extern (C)

2. not have any parameters

3. not be a non-static member function

4. be a function definition, not a declaration (i.e. it must have a function body)

5. not return a type that has a destructor

6. not be a nested function

__gshared int initCount;

pragma(crt_constructor)
extern(C) void initializer() { initCount += 1; }

No arguments to the pragma are allowed.

A function may be annotated with both pragma(crt_constructor) and pragma(crt_destructor).

Annotating declarations other than function definitions has no effect.

Annotating a struct or class definition does not affect the members of the aggregate.

Best Practices: Use for system programming and interfacing with C/C++, for example to allow for initialization of the runtime when loading a DSO, or as a simple replacement for shared static this in betterC mode.

Implementation Defined: The order in which functions annotated with pragma(crt_constructor) are run is implementation defined.

Best Practices: to control the order in which the functions are called within one module, write a single function that calls them in the desired order, and only annotate that function.

Implementation Defined: This uses the mechanism C compilers use to run code before main() is called. C++ compilers use it to run static constructors and destructors. For example, GCC's __attribute__((constructor)) is equivalent. Digital Mars C uses _STI and _STD identifier prefixes to mark crt_constructor and crt_destructor functions.

Implementation Defined: A reference to the annotated function will be inserted in the .init_array section for Elf systems, the XI section for Win32 OMF systems, the .CRT$XCU section for Windows MSCOFF systems, and the __mod_init_func section for OSX systems.

Note: crt_constructor and crt_destructor were implemented in v2.078.0 (2018-01-01). Some compilers exposed non-standard, compiler-specific mechanism before.

pragma crt_destructor

pragma(crt_destructor) works the same as pragma(crt_destructor) except:

Annotates a function so it is run after the D runtime library is terminated and before the C runtime library is terminated. Calling C's exit() function also causes the annotated functions to run.

The order in which the annotated functions are run is the reverse of those functions annotated with pragma(crt_constructor).

Implementation Defined: This uses the mechanism C compilers use to run code after main() returns or exit() is called. C++ compilers use it to run static destructors. For example, GCC's __attribute__((destructor)) is equivalent. Digital Mars C uses _STI and _STD identifier prefixes to mark crt_constructor and crt_destructor functions.

Implementation Defined: A reference to the annotated function will be inserted in the .fini_array section for Elf systems, the XC section for Win32 OMF systems, the .CRT$XPU section for Windows MSCOFF systems, and the __mod_term_func section for OSX systems.

__gshared int initCount;

pragma(crt_constructor)
extern(C) void initialize() { initCount += 1; }

pragma(crt_destructor)
extern(C) void deinitialize() { initCount -= 1; }

pragma(crt_constructor)
pragma(crt_destructor)
extern(C) void innuendo() { printf("Inside a constructor... Or destructor?\n"); }

pragma inline

Affects whether functions are inlined or not. If at the declaration level, it affects the functions declared in the block it controls. If inside a function, it affects the function it is enclosed by.

It takes two forms:

1. pragma(inline)

Sets the behavior to match the implementation's default behavior.

2. pragma(inline, AssignExpression)

The AssignExpression is evaluated and must have a type that can be converted to a boolean. If the result is false the functions are never inlined, otherwise they are always inlined.

More than one AssignExpression is not allowed.

If there are multiple pragma inlines in a function, the lexically last one takes effect.

pragma(inline):
int foo(int x) // foo() is never inlined
{
pragma(inline, true);
++x;
pragma(inline, false); // supercedes the others
return x + 3;
}

Implementation Defined:

1. The default inline behavior is typically selectable with a compiler switch such as -inline.

2. Whether a particular function can be inlined or not is implementation defined.

3. What happens for pragma(inline, true) if the function cannot be inlined. An error message is typical.

pragma lib

There must be one AssignExpression and it must evaluate at compile time to a string literal.

pragma(lib, "foo.lib");

Implementation Defined: The string literal specifies the file name of a library file. This name is inserted into the generated object file, or otherwise passed to the linker, so the linker automatically links in that library.

pragma linkerDirective

There must be one AssignExpression and it must evaluate at compile time to a string literal.

pragma(linkerDirective, "/FAILIFMISMATCH:_ITERATOR_DEBUG_LEVEL=2");

Implementation Defined: The string literal specifies a linker directive to be embedded in the generated object file. Linker directives are only supported for MS-COFF output.

pragma mangle

Overrides the default mangling for a symbol.

There must be one AssignExpression and it must evaluate at compile time to a string literal.

It only applies to function and variable symbols. Other symbols are ignored.

Implementation Defined: On macOS and Win32, an extra underscore (_) is prepended to the string since 2.079, as is done by the C/C++ toolchain. This allows using the same pragma(mangle) for all compatible (POSIX in one case, win64 in another) platforms instead of having to special-case.

Rationale:

- Enables linking to symbol names that D cannot represent.

- Enables linking to a symbol which is a D keyword, since an Identifier cannot be a keyword.

pragma(mangle, "body")
extern(C) void body_func();

pragma msg

Each AssignExpression is evaluated at compile time and then all are combined into a message.

pragma(msg, "compiling...", 6, 1.0); // prints "compiling...61.0" at compile time

Implementation Defined: The form the message takes and how it is presented to the user. One way is by printing them to the standard error stream.

Rationale: Analogously to how writeln() performs a role of writing informational messages during runtime, pragma(msg) performs the equivalent role at compile time. For example,

static if (kilroy)
pragma(msg, "Kilroy was here");
else
pragma(msg, "Kilroy got lost");

pragma printf

pragma(printf) specifies that a function declaration is a printf-like function, meaning it is an extern (C) or extern (C++) function with a format parameter accepting a pointer to a 0-terminated char string conforming to the C99 Standard 7.19.6.1, immediately followed by either a ... variadic argument list or a parameter of type va_list as the last parameter.

If the format argument is a string literal, it is verified to be a valid format string per the C99 Standard. If the format parameter is followed by ..., the number and types of the variadic arguments are checked against the format string.

Diagnosed incompatibilities are:

- incompatible sizes which may cause argument misalignment

- deferencing arguments that are not pointers

- insufficient number of arguments

- struct arguments

- array and slice arguments

- non-pointer arguments to s specifier

- non-standard formats

- undefined behavior per C99

Per the C99 Standard, extra arguments are ignored.

Ignored mismatches are:

- sign mismatches, such as printing an int with a %u format

- integral promotion mismatches, where the format specifies a smaller integral type than int or uint, such as printing a short with the %d format rather than %hd

printf("%k\n", value); // error: non-Standard format k
printf("%d\n");        // error: not enough arguments
printf("%d\n", 1, 2);  // ok, extra arguments ignored

Best Practices: In order to use non-Standard printf/scanf formats, an easy workaround is:

const format = "%k\n";
printf(format.ptr, value);  // no error

Best Practices: Most of the errors detected are portability issues. For instance,

string s;
printf("%.*s\n", s.length, s.ptr);
printf("%d\n", s.sizeof);
ulong u;
scanf("%lld%*c\n", &u);

should be replaced with:

string s;
printf("%.*s\n", cast(int) s.length, s.ptr);
printf("%zd\n", s.sizeof);
ulong u;
scanf("%llu%*c\n", &u);

pragma(printf) applied to declarations that are not functions are ignored. In particular, it has no effect on the declaration of a pointer to function type.

pragma scanf

pragma(scanf) specifies that a function declaration is a scanf-like function, meaning it is an extern (C) or extern (C++) function with a format parameter accepting a pointer to a 0-terminated char string conforming to the C99 Standard 7.19.6.2, immediately followed by either a ... variadic argument list or a parameter of type va_list as the last parameter.

If the format argument is a string literal, it is verified to be a valid format string per the C99 Standard. If the format parameter is followed by ..., the number and types of the variadic arguments are checked against the format string.

Diagnosed incompatibilities are:

- argument is not a pointer to the format specified type

- insufficient number of arguments

- non-standard formats

- undefined behavior per C99

Per the C99 Standard, extra arguments are ignored.

pragma(scanf) applied to declarations that are not functions are ignored. In particular, it has no effect on the declaration of a pointer to function type.

pragma startaddress

There must be one AssignExpression and it must evaluate at compile time to a function symbol.

Implementation Defined: The function symbol specifies the start address for the program. The symbol is inserted into the object file or is otherwise presented to the linker to set the start address. This is not normally used for application level programming, but is for specialized systems work. For applications code, the start address is taken care of by the runtime library.

void foo() { ... }
pragma(startaddress, foo);

Vendor Specific Pragmas

Vendor specific pragma Identifiers can be defined if they are prefixed by the vendor's trademarked name, in a similar manner to version identifiers:

pragma(DigitalMars_extension) { ... }

Implementations must diagnose an error for unrecognized Pragmas, even if they are vendor specific ones.

Implementation Defined: Vendor specific pragmas.

Best Practices: vendor specific pragmas should be wrapped in version statements

version (DigitalMars)
{
pragma(DigitalMars_extension)
{   ... }
}

Expressions

An expression is a sequence of operators and operands that specifies an evaluation. The syntax, order of evaluation, and semantics of expressions are as follows.

Expressions are used to compute values with a resulting type. These values can then be assigned, tested, or ignored. Expressions can also have side effects.

Definitions and Terms

Definition (“Full expression”): For any expression expr, the full expression of expr is defined as follows. If expr parses as a subexpression of another expression expr_1, then the full expression of expr is the full expression of expr_1. Otherwise, expr is its own full expression.

Each expression has a unique full expression.

Example: in the statement return f() + g() * 2;, the full expression of g() * 2 is f() + g() * 2, but not the full expression of f() + g() because the latter is not parsed as a subexpression.

Note: Although the definition is straightforward, a few subtleties exist related to function literals. In the statement return (() => x + f())() * g();, the full expression of f() is x + f(), not the expression passed to return. This is because the parent of x + f() has function literal type, not expression type.

Definition (“Lvalue”): The following expressions, and no others, are called lvalue expressions or lvalues:

1. this inside struct and union member functions;

2. a variable or the result of the DotIdentifier grammatical construct . (left side may be missing) when the rightmost side of the dot is a variable, field (direct or static), function name, or invocation of a function that returns by reference;

3. the result of the following expressions:

- built-in unary operators + (when applied to an lvalue), *, ++ (prefix only), -- (prefix only);

- built-in indexing operator [] (but not the slicing operator);

- built-in assignment binary operators, i.e. =, +=, *=, /=, %=, &=, |=, ^=, ~=, <<=, >>=, >>>=, and ^^=;

- the ternary operator e ? e_1 : e_2 under the following circumstances:

1. e_1 and e_2 are lvalues of the same type; OR

2. One of e_1 and e_2 is an lvalue of type T and the other has and alias this converting it to ref T;

- user-defined operators if and only if the function called as a result of lowering returns by reference;

- mixin expressions if and only if the compilation of the expression resulting from compiling the argument(s) to mixin is an lvalue;

- cast(U) expressions applied to lvalues of type T when T* is implicitly convertible to U*;

- cast() and cast(qualifier list) when applied to an lvalue.

Definition (“Rvalue”): Expressions that are not lvalues are rvalues.

Note: Rvalues include all literals, special value keywords such as __FILE__ and __LINE__, enum values, and the result of expressions not defined as lvalues above.

The built-in address-of operator (unary &) may only be applied to lvalues.

Definition (“Smallest short-circuit expression”): Given an expression expr that is a subexpression of a full expression fullexpr, the smallest short-circuit expression, if any, is the shortest subexpression scexpr of fullexpr that is an AndAndExpression (&&) or an OrOrExpression (||), such that expr is a subexpression of scexpr.
Example: in the expression ((f() * 2 && g()) + 1) || h(), the smallest short-circuit expression of the subexpression f() * 2 is f() * 2 && g(). In the expression (f() && g()) + h(), the subexpression h() has no smallest short-circuit expression.
Order Of Evaluation

Built-in prefix unary expressions ++ and -- are evaluated as if lowered (rewritten) to assignments as follows: ++expr becomes ((expr) += 1), and --expr becomes ((expr) -= 1). Therefore, the result of prefix ++ and -- is the lvalue after the side effect has been effected.

Built-in postfix unary expressions ++ and -- are evaluated as if lowered (rewritten) to lambda invocations as follows: expr++ becomes (ref T x){auto t = x; ++x; return t;}(expr), and expr-- becomes (ref T x){auto t = x; --x; return t;}(expr). Therefore, the result of postfix ++ and -- is an rvalue just before the side effect has been effected.

Binary expressions except for AssignExpression, OrOrExpression, and AndAndExpression are evaluated in lexical order (left-to-right). Example:

int i = 2;
i = ++i * i++ + i;
assert(i == 3 * 3 + 4);

OrOrExpression and AndAndExpression evaluate their left-hand side argument first. Then, OrOrExpression evaluates its right-hand side if and only if its left-hand side does not evaluate to nonzero. AndAndExpression evaluates its right-hand side if and only if its left-hand side evaluates to nonzero.

ConditionalExpression evaluates its left-hand side argument first. Then, if the result is nonzero, the second operand is evaluated. Otherwise, the third operand is evaluated.

Calls to functions with extern(D) linkage (which is the default linkage) are evaluated in the following order: first, if necessary, the address of the function to call is evaluated (e.g. in the case of a computed function pointer or delegate). Then, arguments are evaluated left to right. Finally, transfer is passed to the function. Example:

import std.stdio;
void function(int a, int b, int c) fun()
{
writeln("fun() called");
static void r(int a, int b, int c) { writeln("callee called"); }
return &r;
}
int f1() { writeln("f1() called"); return 1; }
int f2() { writeln("f2() called"); return 2; }
int f3(int x) { writeln("f3() called"); return x + 3; }
int f4() { writeln("f4() called"); return 4; }
// evaluates fun() then f1() then f2() then f3() then f4()
// after which control is transferred to the callee
fun()(f1(), f3(f2()), f4());

Implementation Defined:

1. The order of evaluation of the operands of AssignExpression.

2. The order of evaluation of function arguments for functions with linkage other than extern (D).

Best Practices: Even though the order of evaluation is well-defined, writing code that depends on it is rarely recommended.

Lifetime of Temporaries

Expressions and statements may create and/or consume rvalues. Such values are called temporaries and do not have a name or a visible scope. Their lifetime is managed automatically as defined in this section.

For each evaluation that yields a temporary value, the lifetime of that temporary begins at the evaluation point, similarly to creation of a usual named value initialized with an expression.

Termination of lifetime of temporaries does not obey the customary scoping rules and is defined as follows:

- If:

1. the full expression has a smallest short-circuit expression expr; and

2. the temporary is created on the right-hand side of the && or || operator; and

3. the right-hand side is evaluated,

then temporary destructors are evaluated right after the right-hand side expression has been evaluated and converted to bool. Evaluation of destructors proceeds in reverse order of construction.

- For all other cases, the temporaries generated for the purpose of invoking functions are deferred to the end of the full expression. The order of destruction is inverse to the order of construction.

If a subexpression of an expression throws an exception, all temporaries created up to the evaluation of that subexpression will be destroyed per the rules above. No destructor calls will be issued for temporaries not yet constructed.

Note: An intuition behind these rules is that destructors of temporaries are deferred to the end of full expression and in reverse order of construction, with the exception that the right-hand side of && and || are considered their own full expressions even when part of larger expressions.

Note: The ternary expression e_1 ? e_2 : e_3 is not a special case although it evaluates expressions conditionally: e_1 and one of e_2 and e_3 may create temporaries. Their destructors are inserted to the end of the full expression in the reverse order of creation.

Example:

import std.stdio;
struct S
{
int x;
this(int n) { x = n; writefln("S(%s)", x); }
~this() { writefln("~S(%s)", x); }
}
bool b = (S(1) == S(2) || S(3) != S(4)) && S(5) == S(6);

The output of the code above is:

S(1)
S(2)
S(3)
S(4)
~S(4)
~S(3)
S(5)
S(6)
~S(6)
~S(5)
~S(2)
~S(1)

First, S(1) and S(2) are evaluated in lexical order. Per the rules, they will be destroyed at the end of the full expression and in reverse order. The comparison S(1) == S(2) yields false, so the right-hand side of the || is evaluated causing S(3) and S(4) to be evaluated, also in lexical order. However, their destruction is not deferred to the end of the full expression. Instead, S(4) and then S(3) are destroyed at the end of the || expression. Following their destruction, S(5) and S(6) are constructed in lexical order. Again they are not destroyed at the end of the full expression, but right at the end of the && expression. Consequently, the destruction of S(6) and S(5) is carried before that of S(2) and S(1).
Expressions

Expression:
CommaExpression

CommaExpression:
AssignExpression
CommaExpression , AssignExpression

The left operand of the , is evaluated, then the right operand is evaluated. The type of the expression is the type of the right operand, and the result is the result of the right operand. Using the result of comma expressions isn't allowed.

Assign Expressions

AssignExpression:
ConditionalExpression
ConditionalExpression = AssignExpression
ConditionalExpression += AssignExpression
ConditionalExpression -= AssignExpression
ConditionalExpression *= AssignExpression
ConditionalExpression /= AssignExpression
ConditionalExpression %= AssignExpression
ConditionalExpression &= AssignExpression
ConditionalExpression |= AssignExpression
ConditionalExpression ^= AssignExpression
ConditionalExpression ~= AssignExpression
ConditionalExpression <<= AssignExpression
ConditionalExpression >>= AssignExpression
ConditionalExpression >>>= AssignExpression
ConditionalExpression ^^= AssignExpression

For all assign expressions, the left operand must be a modifiable lvalue. The type of the assign expression is the type of the left operand, and the value is the value of the left operand after assignment occurs. The resulting expression is a modifiable lvalue.

Undefined Behavior: If either operand is a reference type and one of the following:

1. the operands have partially overlapping storage

2. the operands' storage overlaps exactly but the types are different

Implementation Defined: If neither operand is a reference type and one of the following:

1. the operands have partially overlapping storage

2. the operands' storage overlaps exactly but the types are different

Simple Assignment Expression

If the operator is = then it is simple assignment. The right operand is implicitly converted to the type of the left operand, and assigned to it.

If the left and right operands are of the same struct type, and the struct type has a Postblit, then the copy operation is as described in Struct Postblit.

If the lvalue is the .length property of a dynamic array, the behavior is as described in Setting Dynamic Array Length.

If the lvalue is a static array or a slice, the behavior is as described in Array Copying and Array Setting.

If the lvalue is a user-defined property, the behavior is as described in Property Functions.

Assignment Operator Expressions

For arguments of built-in types, assignment operator expressions such as

a op= b

are semantically equivalent to:

a = cast(typeof(a))(a op b)

except that

- operand a is only evaluated once,

- overloading op uses a different function than overloading op= does, and

- the left operand of >>>= does not undergo Integer Promotions before shifting.

For user-defined types, assignment operator expressions are overloaded separately from the binary operator. Still the left operand must be an lvalue.

Conditional Expressions

ConditionalExpression:
OrOrExpression
OrOrExpression ? Expression : ConditionalExpression

The first expression is converted to bool, and is evaluated.

If it is true, then the second expression is evaluated, and its result is the result of the conditional expression.

If it is false, then the third expression is evaluated, and its result is the result of the conditional expression.

If either the second or third expressions are of type void, then the resulting type is void. Otherwise, the second and third expressions are implicitly converted to a common type which becomes the result type of the conditional expression.

Note: When a conditional expression is the left operand of an assign expression, parentheses are required for disambiguation:

bool test;
int a, b, c;
...
test ? a = b : c = 2;   // Deprecated
(test ? a = b : c) = 2; // Equivalent

This makes the intent clearer, because the first statement can easily be misread as the following code:

test ? a = b : (c = 2);

OrOr Expressions

OrOrExpression:
AndAndExpression
OrOrExpression || AndAndExpression

The result type of an OrOrExpression is bool, unless the right operand has type void, when the result is type void.

The OrOrExpression evaluates its left operand.

If the left operand, converted to type bool, evaluates to true, then the right operand is not evaluated. If the result type of the OrOrExpression is bool then the result of the expression is true.

If the left operand is false, then the right operand is evaluated. If the result type of the OrOrExpression is bool then the result of the expression is the right operand converted to type bool.

AndAnd Expressions

AndAndExpression:
OrExpression
AndAndExpression && OrExpression

The result type of an AndAndExpression is bool, unless the right operand has type void, when the result is type void.

The AndAndExpression evaluates its left operand.

If the left operand, converted to type bool, evaluates to false, then the right operand is not evaluated. If the result type of the AndAndExpression is bool then the result of the expression is false.

If the left operand is true, then the right operand is evaluated. If the result type of the AndAndExpression is bool then the result of the expression is the right operand converted to type bool.

Bitwise Expressions

Bit wise expressions perform a bitwise operation on their operands. Their operands must be integral types. First, the Usual Arithmetic Conversions are done. Then, the bitwise operation is done.

Or Expressions

OrExpression:
XorExpression
OrExpression | XorExpression

The operands are OR'd together.

Xor Expressions

XorExpression:
AndExpression
XorExpression ^ AndExpression

The operands are XOR'd together.

And Expressions

AndExpression:
CmpExpression
AndExpression & CmpExpression

The operands are AND'd together.

Compare Expressions

CmpExpression:
ShiftExpression
EqualExpression
IdentityExpression
RelExpression
InExpression

Equality Expressions

EqualExpression:
ShiftExpression == ShiftExpression
ShiftExpression != ShiftExpression

Equality expressions compare the two operands for equality (==) or inequality (!=). The type of the result is bool.

Inequality is defined as the logical negation of equality.

If the operands are integral values, the Usual Arithmetic Conversions are applied to bring them to a common type before comparison. Equality is defined as the bit patterns of the common type match exactly.

If the operands are pointers, equality is defined as the bit patterns of the operands match exactly.

For float, double, and real values, the Usual Arithmetic Conversions are applied to bring them to a common type before comparison. The values -0 and +0 are considered equal. If either or both operands are NAN, then == returns false and != returns true. Otherwise, the bit patterns of the common type are compared for equality.

For complex numbers, equality is defined as equivalent to:

x.re == y.re && x.im == y.im

For struct objects, equality means the result of the opEquals() member function. If an opEquals() is not provided, equality is defined as the logical product of all equality results of the corresponding object fields.

Implementation Defined: The contents of any alignment gaps in the struct object.

Best Practices: If there are overlapping fields, which happens with unions, the default equality will compare each of the overlapping fields. An opEquals() can account for which of the overlapping fields contains valid data. An opEquals() can override the default behavior of floating point NaN values always comparing as unequal. Be careful using memcmp() to implement opEquals() if:

- there are any alignment gaps

- if any fields have an opEquals()

- there are any floating point fields that may contain NaN or -0 values

For class and struct objects, the expression (a == b) is rewritten as a.opEquals(b), and (a != b) is rewritten as !a.opEquals(b).

For class objects, the == and != operators are intended to compare the contents of the objects, however an appropriate opEquals override must be defined for this to work. The default opEquals provided by the root Object class is equivalent to the is operator. Comparing against null is invalid, as null has no contents. Use the is and !is operators instead.

class C;
C c;
if (c == null)  // error
...
if (c is null)  // ok
...

For static and dynamic arrays, equality is defined as the lengths of the arrays matching, and all the elements are equal.

Identity Expressions

IdentityExpression:
ShiftExpression is ShiftExpression
ShiftExpression ! is ShiftExpression

The is compares for identity. To compare for nonidentity, use e1 !is e2. The type of the result is bool. The operands undergo the Usual Arithmetic Conversions to bring them to a common type before comparison.

For class objects, identity is defined as the object references are for the same object. Null class objects can be compared with is.

For struct objects and floating point values, identity is defined as the bits in the operands being identical.

For static and dynamic arrays, identity is defined as referring to the same array elements and the same number of elements.

For other operand types, identity is defined as being the same as equality.

The identity operator is cannot be overloaded.

Relational Expressions

RelExpression:
ShiftExpression < ShiftExpression
ShiftExpression <= ShiftExpression
ShiftExpression > ShiftExpression
ShiftExpression >= ShiftExpression

First, the Usual Arithmetic Conversions are done on the operands. The result type of a relational expression is bool.

For class objects, the result of Object.opCmp() forms the left operand, and 0 forms the right operand. The result of the relational expression (o1 op o2) is:

(o1.opCmp(o2) op 0)

It is an error to compare objects if one is null.

For static and dynamic arrays, the result of the relational op is the result of the operator applied to the first non-equal element of the array. If two arrays compare equal, but are of different lengths, the shorter array compares as "less" than the longer array.

Integer comparisons

Integer comparisons happen when both operands are integral types.

Integer comparison operators

Operator

Relation

<

less

>

greater

<=

less or equal

>=

greater or equal

==

equal

!=

not equal

It is an error to have one operand be signed and the other unsigned for a <, <=, > or >= expression. Use casts to make both operands signed or both operands unsigned.

Floating point comparisons

If one or both operands are floating point, then a floating point comparison is performed.

A relational operator can have NaN operands. If either or both operands is NaN, the floating point comparison operation returns as follows:

Floating point comparison operators

Operator

Relation

Returns

<

less

false

>

greater

false

<=

less or equal

false

>=

greater or equal

false

==

equal

false

!=

unordered, less, or greater

true

Class comparisons

For class objects, the relational operators compare the contents of the objects. Therefore, comparing against null is invalid, as null has no contents.

class C;
C c;
if (c < null)  // error
...

In Expressions

InExpression:
ShiftExpression in ShiftExpression
ShiftExpression ! in ShiftExpression

An associative array can be tested to see if an element is in the array:

int foo[string];
...
if ("hello" in foo)
...

The in expression has the same precedence as the relational expressions <, <=, etc. The return value of the InExpression is null if the element is not in the array; if it is in the array it is a pointer to the element.

The !in expression is the logical negation of the in operation.

Shift Expressions

ShiftExpression:
AddExpression
ShiftExpression << AddExpression
ShiftExpression >> AddExpression
ShiftExpression >>> AddExpression

The operands must be integral types, and undergo the Integer Promotions. The result type is the type of the left operand after the promotions. The result value is the result of shifting the bits by the right operand's value.

<< is a left shift. >> is a signed right shift. >>> is an unsigned right shift.

It's illegal to shift by the same or more bits than the size of the quantity being shifted:

int c;
auto x = c << 33;        // error

Add Expressions

AddExpression:
MulExpression
AddExpression + MulExpression
AddExpression - MulExpression
CatExpression

If the operands are of integral types, they undergo the Usual Arithmetic Conversions, and then are brought to a common type using the Usual Arithmetic Conversions.

If either operand is a floating point type, the other is implicitly converted to floating point and they are brought to a common type via the Usual Arithmetic Conversions.

If the operator is + or -, and the first operand is a pointer, and the second is an integral type, the resulting type is the type of the first operand, and the resulting value is the pointer plus (or minus) the second operand multiplied by the size of the type pointed to by the first operand.

If the second operand is a pointer, and the first is an integral type, and the operator is +, the operands are reversed and the pointer arithmetic just described is applied.

If both operands are pointers, and the operator is +, then it is illegal.

If both operands are pointers, and the operator is -, the pointers are subtracted and the result is divided by the size of the type pointed to by the operands. In this calculation the assumed size of void is one byte. It is an error if the pointers point to different types. The type of the result is ptrdiff_t.

If both operands are of integral types and an overflow or underflow occurs in the computation, wrapping will happen. For example, uint.max + 1 == uint.min, uint.min - 1 == uint.max, int.max + 1 == int.min, and int.min - 1 == int.max.

Add expressions for floating point operands are not associative.

Cat Expressions

CatExpression:
AddExpression ~ MulExpression

A CatExpression concatenates arrays, producing a dynamic array with the result. The arrays must be arrays of the same element type. If one operand is an array and the other is of that array's element type, that element is converted to an array of length 1 of that element, and then the concatenation is performed.

Mul Expressions

MulExpression:
UnaryExpression
MulExpression * UnaryExpression
MulExpression / UnaryExpression
MulExpression % UnaryExpression

The operands must be arithmetic types. They undergo the Usual Arithmetic Conversions.

For integral operands, the *, /, and % correspond to multiply, divide, and modulus operations. For multiply, overflows are ignored and simply chopped to fit into the integral type.

For integral operands of the / and % operators, the quotient rounds towards zero and the remainder has the same sign as the dividend.

The following divide or modulus integral operands:

- denominator is 0

- signed int.min is the numerator and -1 is the denominator

- signed long.min is the numerator and -1L is the denominator

are illegal if encountered during Compile Time Execution.

Undefined Behavior: is exhibited if they are encountered during run time. core.checkedint can be used to check for them and select a defined behavior.

For floating point operands, the * and / operations correspond to the IEEE 754 floating point equivalents. % is not the same as the IEEE 754 remainder. For example, 15.0 % 10.0 == 5.0, whereas for IEEE 754, remainder(15.0,10.0) == -5.0.

Mul expressions for floating point operands are not associative.

Unary Expressions

UnaryExpression:
& UnaryExpression
++ UnaryExpression
-- UnaryExpression
* UnaryExpression
- UnaryExpression
+ UnaryExpression
! UnaryExpression
ComplementExpression
DeleteExpression
CastExpression
PowExpression

Complement Expressions

ComplementExpression:
~ UnaryExpression

ComplementExpressions work on integral types (except bool). All the bits in the value are complemented.

Note: the usual Integer Promotions are not performed prior to the complement operation.

New Expressions

NewExpression:
new AllocatorArgumentsopt Type
new AllocatorArgumentsopt Type [ AssignExpression ]
new AllocatorArgumentsopt Type ( ArgumentListopt )
NewAnonClassExpression

AllocatorArguments:
( ArgumentListopt )

ArgumentList:
AssignExpression
AssignExpression ,
AssignExpression , ArgumentList

NewExpressions are used to allocate memory on the garbage collected heap (default) or using a class or struct specific allocator.

To allocate multidimensional arrays, the declaration reads in the same order as the prefix array declaration order.

char[][] foo;   // dynamic array of strings
...
foo = new char[][30]; // allocate array of 30 strings

The above allocation can also be written as:

foo = new char[][](30); // allocate array of 30 strings

To allocate the nested arrays, multiple arguments can be used:

int[][][] bar;
...
bar = new int[][][](5, 20, 30);

The code above is equivalent to:

bar = new int[][][5];
foreach (ref a; bar)
{
a = new int[][20];
foreach (ref b; a)
{
b = new int[30];
}
}

If there is a new ( ArgumentList ), then those arguments are passed to the class or struct specific allocator function after the size argument.

If a NewExpression is used as an initializer for a function local variable with scope storage class, and the ArgumentList to new is empty, then the instance is allocated on the stack rather than the heap or using the class specific allocator.

Delete Expressions

DeleteExpression:
delete UnaryExpression

NOTE: delete has been deprecated. Instead, please use destroy if feasible, or core.memory.__delete as a last resort.

If the UnaryExpression is a class object reference, and there is a destructor for that class, the destructor is called for that object instance.

Next, if the UnaryExpression is a class object reference, or a pointer to a struct instance, and the class or struct has overloaded operator delete, then that operator delete is called for that class object instance or struct instance.

Otherwise, the garbage collector is called to immediately free the memory allocated for the class instance or struct instance.

If the UnaryExpression is a pointer or a dynamic array, the garbage collector is called to immediately release the memory.

The pointer, dynamic array, or reference is set to null after the delete is performed. Any attempt to reference the data after the deletion via another reference to it will result in undefined behavior.

If UnaryExpression is a variable allocated on the stack, the class destructor (if any) is called for that instance. Neither the garbage collector nor any class deallocator is called.

Undefined Behavior:

1. Using delete to free memory not allocated by the garbage collector.

2. Referring to data that has been the operand of delete.

Cast Expressions

CastExpression:
cast ( Type ) UnaryExpression
cast ( TypeCtorsopt ) UnaryExpression

A CastExpression converts the UnaryExpression to Type.

cast(foo) -p; // cast (-p) to type foo
(foo) - p;      // subtract p from foo

Any casting of a class reference to a derived class reference is done with a runtime check to make sure it really is a downcast. null is the result if it isn't.

class A { ... }
class B : A { ... }

void test(A a, B b)
{
B bx = a;         // error, need cast
B bx = cast(B) a; // bx is null if a is not a B
A ax = b;         // no cast needed
A ax = cast(A) b; // no runtime check needed for upcast
}

In order to determine if an object o is an instance of a class B use a cast:

if (cast(B) o)
{
// o is an instance of B
}
else
{
// o is not an instance of B
}

Casting a pointer type to and from a class type is done as a type paint (i.e. a reinterpret cast).

Casting a dynamic array to another dynamic array is done only if the array lengths multiplied by the element sizes match. The cast is done as a type paint, with the array length adjusted to match any change in element size. If there's not a match, a runtime error is generated.

import std.stdio;

int main()
{
byte[] a = [1,2,3];
auto b = cast(int[])a; // runtime array cast misalignment

int[] c = [1, 2, 3];
auto d = cast(byte[])c; // ok
// prints:
// [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0]
writeln(d);
return 0;
}

Casting a static array to another static array is done only if the array lengths multiplied by the element sizes match; a mismatch is illegal. The cast is done as a type paint (aka a reinterpret cast). The contents of the array are not changed.

import core.stdc.stdio;

void main()
{
byte[16] b = 3;
int[4] ia = cast(int[4]) b;
foreach (i; ia)
printf("%x\n", i);
/* prints:
3030303
3030303
3030303
3030303
*/
}

Casting a floating point literal from one type to another changes its type, but internally it is retained at full precision for the purposes of constant folding.

void test()
{
real a = 3.40483L;
real b;
b = 3.40483;     // literal is not truncated to double precision
assert(a == b);
assert(a == 3.40483);
assert(a == 3.40483L);
assert(a == 3.40483F);
double d = 3.40483; // truncate literal when assigned to variable
assert(d != a);     // so it is no longer the same
const double x = 3.40483; // assignment to const is not
assert(x == a);     // truncated if the initializer is visible
}

Casting a floating point value to an integral type is the equivalent of converting to an integer using truncation.

void main()
{
int a = cast(int) 0.8f;
assert(a == 0);
long b = cast(long) 1.5;
assert(b == 1L);
long c = cast(long) -1.5;
assert(c == -1);
}

Casting a value v to a struct S, when value is not a struct of the same type, is equivalent to:

S(v)

Casting to a CastQual replaces the qualifiers to the type of the UnaryExpression.

shared int x;
assert(is(typeof(cast(const)x) == const int));

Casting with no Type or CastQual removes any top level const, immutable, shared or inout type modifiers from the type of the UnaryExpression.

shared int x;
assert(is(typeof(cast()x) == int));

Casting an expression to void type is allowed to mark that the result is unused. On ExpressionStatement, it could be used properly to avoid "has no effect" error.

void foo(lazy void exp) {}
void main()
{
foo(10);            // NG - has no effect in expression '10'
foo(cast(void)10);  // OK
}

Pow Expressions

PowExpression:
PostfixExpression
PostfixExpression ^^ UnaryExpression

PowExpression raises its left operand to the power of its right operand.

Postfix Expressions

PostfixExpression:
PrimaryExpression
PostfixExpression . Identifier
PostfixExpression . TemplateInstance
PostfixExpression . NewExpression
PostfixExpression ++
PostfixExpression --
PostfixExpression ( ArgumentListopt )
TypeCtorsopt BasicType ( ArgumentListopt )
IndexExpression
SliceExpression

Index Expressions

IndexExpression:
PostfixExpression [ ArgumentList ]

PostfixExpression is evaluated. If PostfixExpression is an expression of type static array or dynamic array, the symbol $ is set to be the number of elements in the array. If PostfixExpression is a ValueSeq, the symbol $ is set to be the number of elements in the sequence. A new declaration scope is created for the evaluation of the ArgumentList and $ appears in that scope only.

If PostfixExpression is a ValueSeq, then the ArgumentList must consist of only one argument, and that must be statically evaluatable to an integral constant. That integral constant n then selects the nth expression in the ValueSeq, which is the result of the IndexExpression. It is an error if n is out of bounds of the ValueSeq.

Slice Expressions

SliceExpression:
PostfixExpression [ ]
PostfixExpression [ Slice ,opt ]

Slice:
AssignExpression
AssignExpression , Slice
AssignExpression .. AssignExpression
AssignExpression .. AssignExpression , Slice

PostfixExpression is evaluated. if PostfixExpression is an expression of type static array or dynamic array, the special variable $ is declared and set to be the length of the array. A new declaration scope is created for the evaluation of the AssignExpression..AssignExpression and $ appears in that scope only.

The first AssignExpression is taken to be the inclusive lower bound of the slice, and the second AssignExpression is the exclusive upper bound. The result of the expression is a slice of the PostfixExpression array.

If the [ ] form is used, the slice is of the entire array.

The type of the slice is a dynamic array of the element type of the PostfixExpression.

A SliceExpression is not a modifiable lvalue.

If the slice bounds can be known at compile time, the slice expression is implicitly convertible to an lvalue of static array. For example:

arr[a .. b]     // typed T[]

If both a and b are integers (may be constant-folded), the slice expression can be converted to a static array type T[b - a].

void foo(int[2] a)
{
assert(a == [2, 3]);
}
void bar(ref int[2] a)
{
assert(a == [2, 3]);
a[0] = 4;
a[1] = 5;
assert(a == [4, 5]);
}
void baz(int[3] a) {}

void main()
{
int[] arr = [1, 2, 3];

foo(arr[1 .. 3]);
assert(arr == [1, 2, 3]);

bar(arr[1 .. 3]);
assert(arr == [1, 4, 5]);

//baz(arr[1 .. 3]); // cannot match length
}

The following forms of slice expression can be convertible to a static array type:

e

An expression that contains no side effects.

a, b

Integers (that may be constant-folded).

Computing array lengths during compilation

Form

The length calculated at compile time

arr[]

The compile time length of arr if it's known.

arr[a .. b]

b - a

arr[e-a .. e]

a

arr[e .. e+b]

b

arr[e-a .. e+b]

a + b

arr[e+a .. e+b]

b - a if a <= b

arr[e-a .. e-b]

a - b if a >= b

If PostfixExpression is a ValueSeq, then the result of the slice is a new ValueSeq formed from the upper and lower bounds, which must statically evaluate to integral constants. It is an error if those bounds are out of range.

Primary Expressions

PrimaryExpression:
Identifier
. Identifier
TemplateInstance
. TemplateInstance
this
super
null
true
false
$
IntegerLiteral
FloatLiteral
CharacterLiteral
StringLiterals
ArrayLiteral
AssocArrayLiteral
FunctionLiteral
AssertExpression
MixinExpression
ImportExpression
NewExpression
FundamentalType . Identifier
( Type ) . Identifier
( Type ) . TemplateInstance
FundamentalType ( ArgumentListopt )
TypeCtor ( Type ) . Identifier
TypeCtor ( Type ) ( ArgumentListopt )
Typeof
TypeidExpression
IsExpression
( Expression )
SpecialKeyword
TraitsExpression

.Identifier

See Module Scope Operator.

this

Within a non-static member function, this resolves to a reference to the object for which the function was called. If the object is an instance of a struct, this will be a pointer to that instance. If a member function is called with an explicit reference to typeof(this), a non-virtual call is made:

class A
{
char get() { return 'A'; }

char foo() { return typeof(this).get(); }
char bar() { return this.get(); }
}

class B : A
{
override char get() { return 'B'; }
}

void main()
{
B b = new B();

assert(b.foo() == 'A');
assert(b.bar() == 'B');
}

Assignment to this is not allowed.

super

super is identical to this, except that it is cast to this's base class. It is an error if there is no base class. It is an error to use super within a struct member function. (Only class Object has no base class.) If a member function is called with an explicit reference to super, a non-virtual call is made.

Assignment to super is not allowed.

null

null represents the null value for pointers, pointers to functions, delegates, dynamic arrays, associative arrays, and class objects. If it has not already been cast to a type, it is given the singular type typeof(null) and it is an exact conversion to convert it to the null value for pointers, pointers to functions, delegates, etc. After it is cast to a type, such conversions are implicit, but no longer exact.

true, false

These are of type bool and when cast to another integral type become the values 1 and 0, respectively.

Character Literals

Character literals are single characters and resolve to one of type char, wchar, or dchar. If the literal is a \u escape sequence, it resolves to type wchar. If the literal is a \U escape sequence, it resolves to type dchar. Otherwise, it resolves to the type with the smallest size it will fit into.

String Literals

StringLiterals:
StringLiteral
StringLiterals StringLiteral

String literals can implicitly convert to any of the following types, they have equal weight:

immutable(char)*

immutable(wchar)*

immutable(dchar)*

immutable(char)[]

immutable(wchar)[]

immutable(dchar)[]

By default, a string literal is typed as a dynamic array, but the element count is known at compile time. So all string literals can be implicitly converted to static array types.

void foo(char[2] a)
{
assert(a == "bc");
}
void bar(ref const char[2] a)
{
assert(a == "bc");
}
void baz(const char[3] a) {}

void main()
{
string str = "abc";
foo(str[1 .. 3]);
bar(str[1 .. 3]);
//baz(str[1 .. 3]); // cannot match length
}

String literals have a 0 appended to them, which makes them easy to pass to C or C++ functions expecting a const char* string. The 0 is not included in the .length property of the string literal.

Array Literals

ArrayLiteral:
[ ArgumentListopt ]

Array literals are a comma-separated list of AssignExpressions between square brackets [ and ]. The AssignExpressions form the elements of a dynamic array, the length of the array is the number of elements. The common type of the all elements is taken to be the type of the array element, and all elements are implicitly converted to that type.

auto a1 = [1,2,3];  // type is int[], with elements 1, 2 and 3
auto a2 = [1u,2,3]; // type is uint[], with elements 1u, 2u, and 3u

By default, an array literal is typed as a dynamic array, but the element count is known at compile time. So all array literals can be implicitly converted to static array types.

void foo(long[2] a)
{
assert(a == [2, 3]);
}
void bar(ref long[2] a)
{
assert(a == [2, 3]);
a[0] = 4;
a[1] = 5;
assert(a == [4, 5]);
}
void baz(const char[3] a) {}

void main()
{
long[] arr = [1, 2, 3];

foo(arr[1 .. 3]);
assert(arr == [1, 2, 3]);

bar(arr[1 .. 3]);
assert(arr == [1, 4, 5]);

//baz(arr[1 .. 3]); // cannot match length
}

If any of the arguments in the ArgumentList are a ValueSeq, then the elements of the ValueSeq are inserted as arguments in place of the sequence.

Array literals are allocated on the memory managed heap. Thus, they can be returned safely from functions:

int[] foo()
{
return [1, 2, 3];
}

When array literals are cast to another array type, each element of the array is cast to the new element type. When arrays that are not literals are cast, the array is reinterpreted as the new type, and the length is recomputed:

import std.stdio;

void main()
{
// cast array literal
const short[] ct = cast(short[]) [cast(byte)1, 1];
// this is equivalent with:
// const short[] ct = [cast(short)1, cast(short)1];
writeln(ct);  // writes [1, 1]

// cast other array expression
// --> normal behavior of CastExpression
byte[] arr = [cast(byte)1, cast(byte)1];
short[] rt = cast(short[]) arr;
writeln(rt);  // writes [257]
}

In other words, casting literal expression will change the literal type.
Associative Array Literals

AssocArrayLiteral:
[ KeyValuePairs ]

KeyValuePairs:
KeyValuePair
KeyValuePair , KeyValuePairs

KeyValuePair:
KeyExpression : ValueExpression

KeyExpression:
AssignExpression

ValueExpression:
AssignExpression

Associative array literals are a comma-separated list of key:value pairs between square brackets [ and ]. The list cannot be empty. The common type of the all keys is taken to be the key type of the associative array, and all keys are implicitly converted to that type. The common type of the all values is taken to be the value type of the associative array, and all values are implicitly converted to that type. An AssocArrayLiteral cannot be used to statically initialize anything.

[21u:"he", 38:"ho", 2:"hi"];  // type is string[uint],
// with keys 21u, 38u and 2u
// and values "he", "ho", and "hi"

If any of the keys or values in the KeyValuePairs are a ValueSeq, then the elements of the ValueSeq are inserted as arguments in place of the sequence.

Function Literals

FunctionLiteral:
function refopt Typeopt ParameterWithAttributes opt FunctionLiteralBody2
delegate refopt Typeopt ParameterWithMemberAttributes opt FunctionLiteralBody2
refopt ParameterWithMemberAttributes FunctionLiteralBody2
FunctionLiteralBody
Identifier => AssignExpression

ParameterWithAttributes:
Parameters FunctionAttributesopt

ParameterWithMemberAttributes:
Parameters MemberFunctionAttributesopt

FunctionLiteralBody2:
=> AssignExpression
FunctionLiteralBody

FunctionLiterals (also known as Lambdas) enable embedding anonymous functions and anonymous delegates directly into expressions. Type is the return type of the function or delegate, if omitted it is inferred from any ReturnStatements in the FunctionLiteralBody. ParameterWithAttributes or ParameterWithMemberAttributes can be used to specify the parameters for the function. If these are omitted, the function defaults to the empty parameter list ( ). The type of a function literal is a delegate or a pointer to function.

For example:

int function(char c) fp; // declare pointer to a function

void test()
{
static int foo(char c) { return 6; }

fp = &foo;
}

is exactly equivalent to:

int function(char c) fp;

void test()
{
fp = function int(char c) { return 6;} ;
}

A delegate is necessary if the FunctionLiteralBody2 accesses any non-static local variables in enclosing functions.

int abc(int delegate(int i));

void test()
{
int b = 3;
int foo(int c) { return 6 + b; }

abc(&foo);
}

is exactly equivalent to:

int abc(int delegate(int i));

void test()
{
int b = 3;

abc( delegate int(int c) { return 6 + b; } );
}

The use of ref declares that the return value is returned by reference:

void main()
{
int x;
auto dg = delegate ref int() { return x; }
x = 3;
assert(fp() == 3);
}

If function or delegate is omitted, it is inferred to be delegate if accessing variables in enclosing functions, otherwise function is inferred.

int abc(int delegate(int i));
int def(int function(int s));

void test()
{
int b = 3;

abc( int(int c) { return 6 + b; } );  // inferred to delegate
def( uint(uint c) { return c * 2; } ); // inferred to function
//def( int(int c) { return c * b; } );  // error!
// Because the FunctionLiteralBody accesses b, then the function literal type
// is inferred to delegate. But def cannot accept a delegate argument.
}

If the type of a function literal can be uniquely determined from its context, the parameter type inference is possible.

void foo(int function(int) fp);

void test()
{
int function(int) fp = (n) { return n * 2; };
// The type of parameter n is inferred to int.

foo((n) { return n * 2; });
// The type of parameter n is inferred to int.
}

auto fp1 = function (i) { return 1; } // error, cannot infer type of `i`

If the function literal is assigned to an alias, the inference of the parameter types is done when the types are needed, as the function literal becomes a template.

alias fp3 = function (i) { return 1; };   // ok, infer type of `i` when used
int j = fp3(4);       // `i` is inferred as `int` here
double d = fp3(10.3); // `i` is inferred as `double` here

Anonymous delegates can behave like arbitrary statement literals. For example, here an arbitrary statement is executed by a loop:

double test()
{
double d = 7.6;
float f = 2.3;

void loop(int k, int j, void delegate() statement)
{
foreach (i; k .. j)
{
statement();
}
}

loop(5, 100, { d += 1; });
loop(3, 10,  { f += 3; });

return d + f;
}

The syntax => AssignExpression is equivalent to { return AssignExpression; }.

import std.stdio;

void main()
{
auto i = 3;
auto twice  = function (int x) => x * 2;
auto square = delegate (int x) => x * x;

auto n = 5;
auto mul_n = (int x) => x * n;

writeln(twice(i));   // prints 6
writeln(square(i));  // prints 9
writeln(mul_n(i));   // prints 15
}

The syntax Identifier => AssignExpression is equivalent to (Identifier) { return AssignExpression; }.

// the following two declarations are equivalent
alias fp = i => 1;
alias fp = (i) { return 1; };

Best Practices: The minimal form of the function literal is most useful as an argument to a template alias parameter:

int motor(alias fp)(int i)
{
return fp(i) + 1;
}

int engine()
{
return motor!(i => i * 2)(6); // returns 13
}

Note: The syntax Identifier { statement; } is not supported because because it is easily confused with statements x = Identifier; { statement; }; if the semicolons were accidentally omitted.

Note: When comparing function literals with nested functions, the function form is analogous to static or non-nested functions, and the delegate form is analogous to non-static nested functions. I.e. a delegate literal can access non-static local variables in an enclosing function, a function literal cannot.

Uniform construction syntax for built-in scalar types

The implicit conversions of built-in scalar types can be explicitly represented by using function call syntax. For example:

auto a = short(1);  // implicitly convert an integer literal '1' to short
auto b = double(a); // implicitly convert a short variable 'a' to double
auto c = byte(128); // error, 128 cannot be represented in a byte

If the argument is omitted, it means default construction of the scalar type:

auto a = ushort();  // same as: ushort.init
auto b = wchar();   // same as: wchar.init

Assert Expressions

AssertExpression:
assert ( AssertArguments )

AssertArguments:
AssignExpression ,opt
AssignExpression , AssignExpression ,opt

The first AssignExpression must evaluate to true. If it does not, an Assert Failure has occurred and the program enters an Invalid State.

If the first AssignExpression consists entirely of compile time constants, and evaluates to false, it is a special case; it signifies that it is unreachable code. Compile Time Function Execution (CTFE) is not attempted.

AssertExpression has different semantics if it is in a unittest or in contract.

The second AssignExpression, if present, must be implicitly convertible to type const(char)[].

If the first AssignExpression is a reference to a class instance for which a class Invariant exists, the class Invariant must hold.

If the first AssignExpression is a pointer to a struct instance for which a struct Invariant exists, the struct Invariant must hold.

The type of an AssertExpression is void.

Undefined Behavior: Once in an Invalid State the behavior of the continuing execution of the program is undefined.

Implementation Defined: Whether the first AssertExpression is evaluated or not at runtime is typically set with a compiler switch. If it is not evaluated, any side effects specified by the AssertExpression may not occur. The behavior if the first AssertExpression is evaluated and is false is also typically set with a compiler switch and may include these options:

1. continuing execution

2. immediately halting via execution of a special CPU instruction

3. aborting the program

4. calling the assert failure function in the corresponding C runtime library

5. throwing the AssertError exception in the D runtime library

If the optional second AssignExpression is provided, the implementation may evaluate it and print the resulting message upon assert failure:

void main()
{
assert(0, "an" ~ " error message");
}

When compiled and run, it will produce the message:

core.exception.AssertError@test.d(3) an error message

The implementation may handle the case of the first AssignExpression evaluating at compile time to false differently in that in release mode it may simply generate a HLT instruction or equivalent.

Best Practices:

1. Do not have side effects in either AssignExpression that subsequent code depends on.

2. AssertExpressions are intended to detect bugs in the program, do not use for detecting input or environmental errors.

3. Do not attempt to resume normal execution after an Assert Failure.

Mixin Expressions

MixinExpression:
mixin ( ArgumentList )

Each AssignExpression in the ArgumentList is evaluated at compile time, and the result must be representable as a string. The resulting strings are concatenated to form a string. The text contents of the string must be compilable as a valid Expression, and is compiled as such.

int foo(int x)
{
return mixin("x +", 1) * 7;  // same as ((x + 1) * 7)
}

Import Expressions

ImportExpression:
import ( AssignExpression )

The AssignExpression must evaluate at compile time to a constant string. The text contents of the string are interpreted as a file name. The file is read, and the exact contents of the file become a string literal.

Implementations may restrict the file name in order to avoid directory traversal security vulnerabilities. A possible restriction might be to disallow any path components in the file name.

Note that by default an import expression will not compile unless one or more paths are passed via the -J switch. This tells the compiler where it should look for the files to import. This is a security feature.

void foo()
{
// Prints contents of file foo.txt
writeln(import("foo.txt"));
}

Typeid Expressions

TypeidExpression:
typeid ( Type )
typeid ( Expression )

If Type, returns an instance of class TypeInfo corresponding to Type.

If Expression, returns an instance of class TypeInfo corresponding to the type of the Expression. If the type is a class, it returns the TypeInfo of the dynamic type (i.e. the most derived type). The Expression is always executed.

class A { }
class B : A { }

void main()
{
writeln(typeid(int));        // int
uint i;
writeln(typeid(i++));        // uint
writeln(i);                  // 1
A a = new B();
writeln(typeid(a));          // B
writeln(typeid(typeof(a)));  // A
}

IsExpression

IsExpression:
is ( Type )
is ( Type : TypeSpecialization )
is ( Type == TypeSpecialization )
is ( Type : TypeSpecialization , TemplateParameterList )
is ( Type == TypeSpecialization , TemplateParameterList )
is ( Type Identifier )
is ( Type Identifier : TypeSpecialization )
is ( Type Identifier == TypeSpecialization )
is ( Type Identifier : TypeSpecialization , TemplateParameterList )
is ( Type Identifier == TypeSpecialization , TemplateParameterList )

TypeSpecialization:
Type
struct
union
class
interface
enum
__vector
function
delegate
super
const
immutable
inout
shared
return
__parameters
module
package

IsExpressions are evaluated at compile time and are used for checking for valid types, comparing types for equivalence, determining if one type can be implicitly converted to another, and deducing the subtypes of a type. The result of an IsExpression is a boolean of value true if the condition is satisfied. If the condition is not satisfied, the result is a boolean of value false.

Type is the type being tested. It must be syntactically correct, but it need not be semantically correct. If it is not semantically correct, the condition is not satisfied.

Identifier is declared to be an alias of the resulting type if the condition is satisfied. The Identifier forms can only be used if the IsExpression appears in a StaticIfCondition.

TypeSpecialization is the type that Type is being compared against.

The forms of the IsExpression are:

1. is ( Type )
The condition is satisfied if Type is semantically correct (it must be syntactically correct regardless).

alias int func(int);    // func is a alias to a function type
void foo()
{
if (is(func[]))     // not satisfied because arrays of
// functions are not allowed
writeln("satisfied");
else
writeln("not satisfied");

if (is([][]))       // error, [][] is not a syntactically valid type
...
}

2. is ( Type : TypeSpecialization )
The condition is satisfied if Type is semantically correct and it is the same as or can be implicitly converted to TypeSpecialization. TypeSpecialization is only allowed to be a Type.

alias Bar = short;
void foo()
{
if (is(Bar : int))   // satisfied because short can be
// implicitly converted to int
writeln("satisfied");
else
writeln("not satisfied");
}

3. is ( Type == TypeSpecialization )
The condition is satisfied if Type is semantically correct and is the same type as TypeSpecialization. If TypeSpecialization is one of struct union class interface enum function delegate const immutable shared module package then the condition is satisfied if Type is one of those. Package modules are considered to be both packages and modules.

alias Bar = short;
void foo()
{
if (is(Bar == int))   // not satisfied because short is not
// the same type as int
writeln("satisfied");
else
writeln("not satisfied");
}

4. is ( Type Identifier )
The condition is satisfied if Type is semantically correct. If so, Identifier is declared to be an alias of Type.

alias Bar = short;
void foo()
{
static if (is(Bar T))
alias S = T;
else
alias S = long;

writeln(typeid(S)); // prints "short"
if (is(Bar T))      // error, Identifier T form can
// only be in StaticIfConditions
...
}

5. is ( Type Identifier : TypeSpecialization )
The condition is satisfied if Type is the same as TypeSpecialization, or if Type is a class and TypeSpecialization is a base class or base interface of it. The Identifier is declared to be either an alias of the TypeSpecialization or, if TypeSpecialization is dependent on Identifier, the deduced type.

alias Bar = int;
alias Abc = long*;
void foo()
{
static if (is(Bar T : int))
alias S = T;
else
alias S = long;

writeln(typeid(S));  // prints "int"

static if (is(Abc U : U*))
{
U u;
writeln(typeid(typeof(u)));  // prints "long"
}
}

The way the type of Identifier is determined is analogous to the way template parameter types are determined by TemplateTypeParameterSpecialization.

6. is ( Type Identifier == TypeSpecialization )
The condition is satisfied if Type is semantically correct and is the same as TypeSpecialization. The Identifier is declared to be either an alias of the TypeSpecialization or, if TypeSpecialization is dependent on Identifier, the deduced type. If TypeSpecialization is one of struct union class interface enum function delegate const immutable shared then the condition is satisfied if Type is one of those. Furthermore, Identifier is set to be an alias of the type:

keyword

alias type for Identifier

struct

Type

union

Type

class

Type

interface

Type

super

TypeSeq of base classes and interfaces

enum

the base type of the enum

function

TypeSeq of the function parameter types. For C- and D-style variadic functions, only the non-variadic parameters are included. For typesafe variadic functions, the ... is ignored.

delegate

the function type of the delegate

return

the return type of the function, delegate, or function pointer

__parameters

the parameter sequence of a function, delegate, or function pointer. This includes the parameter types, names, and default values.

const

Type

immutable

Type

shared

Type

alias Bar = short;
enum E : byte { Emember }
void foo()
{
static if (is(Bar T == int))   // not satisfied, short is not int
alias S = T;
alias U = T;                   // error, T is not defined

static if (is(E V == enum))    // satisfied, E is an enum
V v;                       // v is declared to be a byte
}

7. is ( Type : TypeSpecialization , TemplateParameterList )
is ( Type == TypeSpecialization , TemplateParameterList )
is ( Type Identifier : TypeSpecialization , TemplateParameterList )
is ( Type Identifier == TypeSpecialization , TemplateParameterList ) More complex types can be pattern matched; the TemplateParameterList declares symbols based on the parts of the pattern that are matched, analogously to the way implied template parameters are matched.

import std.stdio, std.typecons;

void main()
{
alias Tup = Tuple!(int, string);
alias AA = long[string];

static if (is(Tup : Template!Args, alias Template, Args...))
{
writeln(__traits(isSame, Template, Tuple)); // true
writeln(is(Template!(int, string) == Tup));  // true
writeln(typeid(Args[0]));  // int
writeln(typeid(Args[1]));  // immutable(char)[]
}

static if (is(AA T : T[U], U : string))
{
writeln(typeid(T));  // long
writeln(typeid(U));  // string
}

static if (is(AA A : A[B], B : int))
{
assert(0);  // should not match, as B is not an int
}

static if (is(int[10] W : W[len], int len))
{
writeln(typeid(W));  // int
writeln(len);        // 10
}

static if (is(int[10] X : X[len], int len : 5))
{
assert(0);  // should not match, len should be 10
}
}

Special Keywords

SpecialKeyword:
__FILE__
__FILE_FULL_PATH__
__MODULE__
__LINE__
__FUNCTION__
__PRETTY_FUNCTION__

__FILE__ and __LINE__ expand to the source file name and line number at the point of instantiation. The path of the source file is left up to the compiler.

__FILE_FULL_PATH__ expands to the absolute source file name at the point of instantiation.

__MODULE__ expands to the module name at the point of instantiation.

__FUNCTION__ expands to the fully qualified name of the function at the point of instantiation.

__PRETTY_FUNCTION__ is similar to __FUNCTION__, but also expands the function return type, its parameter types, and its attributes.

Example:

module test;
import std.stdio;

void test(string file = __FILE__, size_t line = __LINE__,
string mod = __MODULE__, string func = __FUNCTION__,
string pretty = __PRETTY_FUNCTION__,
string fileFullPath = __FILE_FULL_PATH__)
{
writefln("file: '%s', line: '%s', module: '%s',\nfunction: '%s', " ~
"pretty function: '%s',\nfile full path: '%s'",
file, line, mod, func, pretty, fileFullPath);
}

int main(string[] args)
{
test();
return 0;
}

Assuming the file was at /example/test.d, this will output:

file: 'test.d', line: '13', module: 'test',
function: 'test.main', pretty function: 'int test.main(string[] args)',
file full path: '/example/test.d'

Associativity and Commutativity

An implementation may rearrange the evaluation of expressions according to arithmetic associativity and commutativity rules as long as, within that thread of execution, no observable difference is possible.

This rule precludes any associative or commutative reordering of floating point expressions.

Statements

The order of execution within a function is controlled by Statements. A function's body consists of a sequence of zero or more Statements. Execution occurs in lexical order, though certain statements may have deferred effects. A Statement has no value; it is executed for its effects.

Statement:
EmptyStatement
NonEmptyStatement
ScopeBlockStatement

EmptyStatement:
;

NoScopeNonEmptyStatement:
NonEmptyStatement
BlockStatement

NoScopeStatement:
EmptyStatement
NonEmptyStatement
BlockStatement

NonEmptyOrScopeBlockStatement:
NonEmptyStatement
ScopeBlockStatement

NonEmptyStatement:
NonEmptyStatementNoCaseNoDefault
CaseStatement
CaseRangeStatement
DefaultStatement

NonEmptyStatementNoCaseNoDefault:
LabeledStatement
ExpressionStatement
DeclarationStatement
IfStatement
WhileStatement
DoStatement
ForStatement
ForeachStatement
SwitchStatement
FinalSwitchStatement
ContinueStatement
BreakStatement
ReturnStatement
GotoStatement
WithStatement
SynchronizedStatement
TryStatement
ScopeGuardStatement
ThrowStatement
AsmStatement
MixinStatement
ForeachRangeStatement
PragmaStatement
ConditionalStatement
StaticForeachStatement
StaticAssert
TemplateMixin
ImportDeclaration

Any ambiguities in the grammar between Statements and Declarations are resolved by the declarations taking precedence. Wrapping such a statement in parentheses will disambiguate it in favor of being a Statement.

Scope Statements

ScopeStatement:
NonEmptyStatement
BlockStatement

A new scope for local symbols is introduced for the NonEmptyStatement or BlockStatement.

Even though a new scope is introduced, local symbol declarations cannot shadow (hide) other local symbol declarations in the same function.

void func1(int x)
{
int x;    // illegal, x shadows parameter x

int y;

{ int y; } // illegal, y shadows enclosing scope's y

void delegate() dg;
dg = { int y; }; // ok, this y is not in the same function

struct S
{
int y;    // ok, this y is a member, not a local
}

{ int z; }
{ int z; }  // ok, this z is not shadowing the other z

{ int t; }
{ t++;   }  // illegal, t is undefined
}

Best Practices: Local declarations within a function should all have unique names, even if they are in non-overlapping scopes.

Scope Block Statements

ScopeBlockStatement:
BlockStatement

A scope block statement introduces a new scope for the BlockStatement.

Labeled Statements

Statements can be labeled. A label is an identifier that precedes a statement.

LabeledStatement:
Identifier :
Identifier : Statement

Any statement can be labeled, including empty statements, and so can serve as the target of a goto statement. Labeled statements can also serve as the target of a break or continue statement.

A label can appear without a following statement at the end of a block.

Labels are in a name space independent of declarations, variables, types, etc. Even so, labels cannot have the same name as local declarations. The label name space is the body of the function they appear in. Label name spaces do not nest, i.e. a label inside a block statement is accessible from outside that block.

Labels in one function cannot be referenced from another function.

Block Statement

BlockStatement:
{ }
{ StatementList }

StatementList:
Statement
Statement StatementList

A block statement is a sequence of statements enclosed by { }. The statements are executed in lexical order, until the end of the block is reached or a statement transfers control elsewhere.

Expression Statement

ExpressionStatement:
Expression ;

The expression is evaluated.

Expressions that have no effect, like (x + x), are illegal as expression statements unless they are cast to void.

int x;
x++;               // ok
x;                 // illegal
1+1;               // illegal
cast(void)(x + x); // ok

Declaration Statement

Declaration statements define variables, and declare types, templates, functions, imports, conditionals, static foreaches, and static asserts.

DeclarationStatement:
StorageClassesopt Declaration

Some declaration statements:

int a;        // declare a as type int and initialize it to 0
struct S { }  // declare struct s
alias myint = int;

If Statement

If statements provide simple conditional execution of statements.

IfStatement:
if ( IfCondition ) ThenStatement
if ( IfCondition ) ThenStatement else ElseStatement

IfCondition:
Expression
auto Identifier = Expression
scope Identifier = Expression
TypeCtors Identifier = Expression
TypeCtorsopt BasicType Declarator = Expression

ThenStatement:
ScopeStatement

ElseStatement:
ScopeStatement

Expression is evaluated and must have a type that can be converted to a boolean. If it's true the ThenStatement is transferred to, else the ElseStatement is transferred to.

The ElseStatementis associated with the innermost if statement which does not already have an associated ElseStatement.

If an auto Identifier is provided, it is declared and initialized to the value and type of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

If a TypeCtors Identifier is provided, it is declared to be of the type specified by TypeCtors and is initialized with the value of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

If a Declarator is provided, it is declared and initialized to the value of the Expression. Its scope extends from when it is initialized to the end of the ThenStatement.

import std.regex;
...
if (auto m = std.regex.matchFirst("abcdef", "b(c)d"))
{
writefln("[%s]", m.pre);    // prints [a]
writefln("[%s]", m.post);   // prints [ef]
writefln("[%s]", m[0]);     // prints [bcd]
writefln("[%s]", m[1]);     // prints [c]
}
else
{
writeln(m.post); // Error: undefined identifier 'm'
}
writeln(m.pre);      // Error: undefined identifier 'm'

While Statement

WhileStatement:
while ( IfCondition ) ScopeStatement

A While Statement implements a simple loop.

If the IfCondition is an Expression, it is evaluated and must have a type that can be converted to a boolean. If it's true the ScopeStatement is executed. After the ScopeStatement is executed, the Expression is evaluated again, and if true the ScopeStatement is executed again. This continues until the Expression evaluates to false.

int i = 0;
while (i < 10)
{
foo(i);
++i;
}

If an auto Identifier is provided, it is declared and initialized to the value and type of the Expression. Its scope extends from when it is initialized to the end of the ScopeStatement.

If a TypeCtors Identifier is provided, it is declared to be of the type specified by TypeCtors and is initialized with the value of the Expression. Its scope extends from when it is initialized to the end of the ScopeStatement.

If a Declarator is provided, it is declared and initialized to the value of the Expression. Its scope extends from when it is initialized to the end of the ScopeStatement.

A BreakStatement will exit the loop.

A ContinueStatement will transfer directly to evaluating IfCondition again.

Do Statement

DoStatement:
do ScopeStatement  while ( Expression ) ;

Do while statements implement simple loops.

ScopeStatement is executed. Then Expression is evaluated and must have a type that can be converted to a boolean. If it's true the loop is iterated again. This continues until the Expression evaluates to false.

int i = 0;
do
{
foo(i);
} while (++i < 10);

A BreakStatement will exit the loop. A ContinueStatement will transfer directly to evaluating Expression again.

For Statement

For statements implement loops with initialization, test, and increment clauses.

ForStatement:
for ( Initialize Testopt ; Incrementopt ) ScopeStatement

Initialize:
;
NoScopeNonEmptyStatement

Test:
Expression

Increment:
Expression

Initialize is executed. Test is evaluated and must have a type that can be converted to a boolean. If it's true the statement is executed. After the statement is executed, the Increment is executed. Then Test is evaluated again, and if true the statement is executed again. This continues until the Test evaluates to false.

A BreakStatement will exit the loop. A ContinueStatement will transfer directly to the Increment.

A ForStatement creates a new scope. If Initialize declares a variable, that variable's scope extends through the end of the for statement. For example:

for (int i = 0; i < 10; i++)
foo(i);

is equivalent to:

{
int i;
for (i = 0; i < 10; i++)
foo(i);
}

Function bodies cannot be empty:

for (int i = 0; i < 10; i++)
;       // illegal

Use instead:

for (int i = 0; i < 10; i++)
{
}

The Initialize may be omitted (although the trailing ; is still required). Test may also be omitted, and if so, it is treated as if it evaluated to true.

Best Practices: Consider replacing ForStatements with Foreach Statements or Foreach Range Statements. Foreach loops are easier to understand, less prone to error, and easier to refactor.

Foreach Statement

A foreach statement loops over the contents of an aggregate.

AggregateForeach:
Foreach ( ForeachTypeList ; ForeachAggregate )

ForeachStatement:
AggregateForeach NoScopeNonEmptyStatement

Foreach:
foreach
foreach_reverse

ForeachTypeList:
ForeachType
ForeachType , ForeachTypeList

ForeachType:
ForeachTypeAttributesopt BasicType Declarator
ForeachTypeAttributesopt Identifier
ForeachTypeAttributesopt alias Identifier

ForeachTypeAttributes:
ForeachTypeAttribute
ForeachTypeAttribute ForeachTypeAttributes

ForeachTypeAttribute:
enum
ref
scope
TypeCtor

ForeachAggregate:
Expression

ForeachAggregate is evaluated. It must evaluate to an expression of type static array, dynamic array, associative array, struct, class, delegate, or sequence. The NoScopeNonEmptyStatement is executed, once for each element of the aggregate. At the start of each iteration, the variables declared by the ForeachTypeList are set to be a copy of the elements of the aggregate. If the ForeachTypeAttribute is ref, it is a reference to the contents of that aggregate. If the ForeachTypeAttribute is scope, the ForeachType declaration will have scope semantics.

The aggregate must be loop invariant, meaning that elements to the aggregate cannot be added or removed from it in the NoScopeNonEmptyStatement.

Foreach over Arrays

If the aggregate is a static or dynamic array, there can be one or two variables declared. If one, then the variable is said to be the value set to the elements of the array, one by one. The type of the variable must match the type of the array contents, except for the special cases outlined below. If there are two variables declared, the first is said to be the index and the second is said to be the value. The index must be of type size_t for dynamic arrays. Static arrays may use any integral type that spans the length of the array. index cannot be ref. It is set to be the index of the array element.

char[] a;
...
foreach (int i, char c; a)
{
writefln("a[%d] = '%c'", i, c);
}

For foreach, the elements for the array are iterated over starting at index 0 and continuing to the maximum of the array. For foreach_reverse, the array elements are visited in the reverse order.

Note: The ForeachTypeAttribute is implicit, and when a type is not specified, it is inferred. In that case, auto is implied, and it is not necessary (and actually forbidden) to use it.

int[] arr;
...
foreach (n; arr) // ok, n is an int
writeln(n);

foreach (auto n; arr) // error, auto is redundant
writeln(n);

Foreach over Arrays of Characters

If the aggregate expression is a static or dynamic array of chars, wchars, or dchars, then the Type of the value can be any of char, wchar, or dchar. In this manner any UTF array can be decoded into any UTF type:

char[] a = "\xE2\x89\xA0".dup;  // \u2260 encoded as 3 UTF-8 bytes

foreach (dchar c; a)
{
writefln("a[] = %x", c); // prints 'a[] = 2260'
}

dchar[] b = "\u2260"d.dup;

foreach (char c; b)
{
writef("%x, ", c);  // prints 'e2, 89, a0, '
}

Aggregates can be string literals, which can be accessed as char, wchar, or dchar arrays:

void test()
{
foreach (char c; "ab")
{
writefln("'%s'", c);
}
foreach (wchar w; "xy")
{
writefln("'%s'", w);
}
}

which would print:

'a'
'b'
'x'
'y'

Foreach over Associative Arrays

If the aggregate expression is an associative array, there can be one or two variables declared. If one, then the variable is said to be the value set to the elements of the array, one by one. The type of the variable must match the type of the array contents. If there are two variables declared, the first is said to be the index and the second is said to be the value. The index must be of the same type as the indexing type of the associative array. It cannot be ref, and it is set to be the index of the array element. The order in which the elements of the array are iterated over is unspecified for foreach. foreach_reverse for associative arrays is illegal.

double[string] a; // index type is string, value type is double
...
foreach (string s, double d; a)
{
writefln("a['%s'] = %g", s, d);
}

Foreach over Structs and Classes with opApply

If the aggregate expression is a struct or class object, the foreach is defined by the special opApply member function, and the foreach_reverse behavior is defined by the special opApplyReverse member function. These functions have the type:

int opApply(scope int delegate(ref Type [, ...]) dg);

int opApplyReverse(scope int delegate(ref Type [, ...]) dg);

where Type matches the Type used in the ForeachType declaration of Identifier. Multiple ForeachTypes correspond with multiple Types in the delegate type passed to opApply or opApplyReverse. There can be multiple opApply and opApplyReverse functions, one is selected by matching the type of dg to the ForeachTypes of the ForeachStatement. The body of the apply function iterates over the elements it aggregates, passing them each to the dg function. If the dg returns 0, then apply goes on to the next element. If the dg returns a nonzero value, apply must cease iterating and return that value. Otherwise, after done iterating across all the elements, apply will return 0.

For example, consider a class that is a container for two elements:

class Foo
{
uint[2] array;

int opApply(scope int delegate(ref uint) dg)
{
int result = 0;

for (int i = 0; i < array.length; i++)
{
result = dg(array[i]);
if (result)
break;
}
return result;
}
}

An example using this might be:

void test()
{
Foo a = new Foo();

a.array[0] = 73;
a.array[1] = 82;

foreach (uint u; a)
{
writefln("%d", u);
}
}

which would print:

73
82

The scope storage class on the dg parameter means that the parameter's value does not escape the scope of the opApply function (an example would be assigning dg to a global). If it cannot be statically guaranteed that dg does not escape, a closure may be allocated for it on the heap instead of the stack. Best practice is to annotate delegate parameters with scope when possible.

opApply can also be a templated function, which will infer the types of parameters based on the ForeachStatement.

For example:

struct S
{
import std.traits : ParameterTypeTuple;  // introspection template

int opApply(Dg)(scope Dg dg)
if (ParameterTypeTuple!Dg.length == 2) // foreach with 2 parameters
{
return 0;
}

int opApply(Dg)(scope Dg dg)
if (ParameterTypeTuple!Dg.length == 3) // foreach with takes 3 parameters
{
return 0;
}
}

void main()
{
foreach (int a, int b; S()) { }  // calls first opApply function
foreach (int a, int b, float c; S()) { }  // calls second opApply function
}

It is important to make sure that, if opApply catches any exceptions, that those exceptions did not originate from the delegate passed to opApply. The user would expect exceptions thrown from a foreach body to both terminate the loop, and propagate outside the foreach body.

Foreach over Structs and Classes with Ranges

If the aggregate expression is a struct or class object, but the opApply for foreach, or opApplyReverse foreach_reverse do not exist, then iteration over struct and class objects can be done with range primitives. For foreach, this means the following properties and methods must be defined:

Foreach Range Properties

Property

Purpose

.empty

returns true if no more elements

.front

return the leftmost element of the range

Foreach Range Methods

Method

Purpose

.popFront()

move the left edge of the range right by one

Meaning:

foreach (e; range) { ... }

translates to:

for (auto __r = range; !__r.empty; __r.popFront())
{
auto e = __r.front;
...
}

Similarly, for foreach_reverse, the following properties and methods must be defined:

Foreach_reverse Range Properties

Property

Purpose

.empty

returns true if no more elements

.back

return the rightmost element of the range

Foreach_reverse Range Methods

Method

Purpose

.popBack()

move the right edge of the range left by one

Meaning:

foreach_reverse (e; range) { ... }

translates to:

for (auto __r = range; !__r.empty; __r.popBack())
{
auto e = __r.back;
...
}

Multiple loop variables are allowed if the front property returns a type that expands to an expression sequence whose size matches the number of variables. Each variable is assigned to the corresponding value in the tuple.

// Common tuple implementation that can decay into its members
import std.typecons : Tuple;

// Range whose elements are tuples
struct TupleRange
{
Tuple!(char, bool, int) front()
{
return typeof(return)('a', true, 2);
}

bool empty() { return false; }

void popFront() {}
}

void main()
{
foreach (a, b, c; TupleRange())
{
assert(a == 'a');
assert(b == true);
assert(c == 2);
}

// Expected 3 arguments, not 1
// foreach (a; TupleRange()) { ... }
}

Foreach over Delegates

If ForeachAggregate is a delegate, the type signature of the delegate is of the same as for opApply. This enables many different named looping strategies to coexist in the same class or struct.

For example:

void main()
{
// Custom loop implementation, that iterates over powers of 2 with
// alternating sign. The loop body is passed in dg.
int myLoop(int delegate(ref int) dg)
{
for (int z = 1; z < 128; z *= -2)
{
auto ret = dg(z);

// If the loop body contains a break, ret will be non-zero.
if (ret != 0)
return ret;
}
return 0;
}

// This example loop simply collects the loop index values into an array.
int[] result;
foreach (ref x; &myLoop)
{
result ~= x;
}
assert(result == [1, -2, 4, -8, 16, -32, 64, -128]);
}

Note: When ForeachAggregate is a delegate, the compiler does not try to implement reverse traversal of the results returned by the delegate when foreach_reverse is used. This may result in code that is confusing to read. Therefore, using foreach_reverse with a delegate is now deprecated, and will be rejected in the future.

Foreach over Sequences

If the aggregate expression is a sequence, there can be one or two iteration symbols declared. If one, then the symbol is an element alias of each element in the sequence in turn.

If the sequence is a TypeSeq, then the foreach statement is executed once for each type, and the element alias is set to each type.

When the sequence is a ValueSeq, the element alias is a variable and is set to each value in the sequence. If the type of the variable is given, it must match the type of the sequence contents. If no type is given, the type of the variable is set to the type of the sequence element, which may change from iteration to iteration.

If there are two symbols declared, the first is the index variable and the second is the element alias. The index must be of int, uint, long or ulong type, it cannot be ref, and it is set to the index of each sequence element.

Example:

import std.meta : AliasSeq;

void main()
{
alias Seq = AliasSeq!(int, "literal", main);

foreach (sym; Seq)
{
pragma(msg, sym.stringof);
}
}

Output:

int
"literal"
main()

See also: Static Foreach.

Foreach Ref Parameters

ref can be used to update the original elements:

void test()
{
static uint[2] a = [7, 8];

foreach (ref uint u; a)
{
u++;
}
foreach (uint u; a)
{
writefln("%d", u);
}
}

which would print:

8
9

ref can not be applied to the index values.

If not specified, the Types in the ForeachType can be inferred from the type of the ForeachAggregate.

Foreach Restrictions

The aggregate itself must not be resized, reallocated, free'd, reassigned or destructed while the foreach is iterating over the elements.

int[] a;
int[] b;
foreach (int i; a)
{
a = null;       // error
a.length += 10; // error
a = b;          // error
}
a = null;         // ok

Foreach Range Statement

A foreach range statement loops over the specified range.

RangeForeach:
Foreach ( ForeachType ; LwrExpression .. UprExpression )

LwrExpression:
Expression

UprExpression:
Expression

ForeachRangeStatement:
RangeForeach ScopeStatement

ForeachType declares a variable with either an explicit type, or a type inferred from LwrExpression and UprExpression. The ScopeStatement is then executed n times, where n is the result of UprExpression - LwrExpression. If UprExpression is less than or equal to LwrExpression, the ScopeStatement is executed zero times. If Foreach is foreach, then the variable is set to LwrExpression, then incremented at the end of each iteration. If Foreach is foreach_reverse, then the variable is set to UprExpression, then decremented before each iteration. LwrExpression and UprExpression are each evaluated exactly once, regardless of how many times the ScopeStatement is executed.

import std.stdio;

int foo()
{
write("foo");
return 10;
}

void main()
{
foreach (i; 0 .. foo())
{
write(i);
}
}

prints:

foo0123456789

Break and Continue out of Foreach

A BreakStatement in the body of the foreach will exit the foreach, a ContinueStatement will immediately start the next iteration.

Switch Statement
A switch statement goes to one of a collection of case statements depending on the value of the switch expression.

SwitchStatement:
switch ( Expression ) ScopeStatement

CaseStatement:
case ArgumentList : ScopeStatementListopt

CaseRangeStatement:
case FirstExp : .. case LastExp : ScopeStatementListopt

FirstExp:
AssignExpression

LastExp:
AssignExpression

DefaultStatement:
default : ScopeStatementListopt

ScopeStatementList:
StatementListNoCaseNoDefault

StatementListNoCaseNoDefault:
StatementNoCaseNoDefault
StatementNoCaseNoDefault StatementListNoCaseNoDefault

StatementNoCaseNoDefault:
EmptyStatement
NonEmptyStatementNoCaseNoDefault
ScopeBlockStatement

Expression is evaluated. If the type of the Expression is enum, it is repeatedly converted to its EnumBaseType until it isn't. Then, if the type is integral, the Expression undergoes Integer Promotions. Then, the type of the Expression must be either integral or a static or dynamic array of char, wchar, or dchar.

The result is compared against each of the case expressions. If there is a match, the corresponding case statement is transferred to.

The case expressions, ArgumentList, are a comma separated list of expressions.

A CaseRangeStatement is a shorthand for listing a series of case statements from FirstExp to LastExp.

If none of the case expressions match, and there is a default statement, the default statement is transferred to.

A switch statement must have a default statement.

The case expressions must all evaluate to a constant value or array, or a runtime initialized const or immutable variable of integral type. They must be implicitly convertible to the type of the switch Expression.

Case expressions must all evaluate to distinct values. Const or immutable variables must all have different names. If they share a value, the first case statement with that value gets control. There must be exactly one default statement.

The ScopeStatementList introduces a new scope.

Case statements and default statements associated with the switch can be nested within block statements; they do not have to be in the outermost block. For example, this is allowed:

switch (i)
{
case 1:
{
case 2:
}
break;
}

A ScopeStatementList must either be empty, or be ended with a ContinueStatement, BreakStatement, ReturnStatement, GotoStatement, ThrowStatement or assert(0) expression unless this is the last case. This is to set apart with C's error-prone implicit fall-through behavior. goto case; could be used for explicit fall-through:

int number;
string message;
switch (number)
{
default:    // valid: ends with 'throw'
throw new Exception("unknown number");

case 3:     // valid: ends with 'break' (break out of the 'switch' only)
message ~= "three ";
break;

case 4:     // valid: ends with 'continue' (continue the enclosing loop)
message ~= "four ";
continue;

case 5:     // valid: ends with 'goto' (explicit fall-through to next case.)
message ~= "five ";
goto case;

case 6:     // ERROR: implicit fall-through
message ~= "six ";

case 1:     // valid: the body is empty
case 2:     // valid: this is the last case in the switch statement.
message = "one or two";
}

A break statement will exit the switch BlockStatement.

Strings can be used in switch expressions. For example:

string name;
...
switch (name)
{
case "fred":
case "sally":
...
}

For applications like command line switch processing, this can lead to much more straightforward code, being clearer and less error prone. char, wchar and dchar strings are allowed.

Implementation Note: The compiler's code generator may assume that the case statements are sorted by frequency of use, with the most frequent appearing first and the least frequent last. Although this is irrelevant as far as program correctness is concerned, it is of performance interest.

Final Switch Statement

FinalSwitchStatement:
final switch ( Expression ) ScopeStatement

A final switch statement is just like a switch statement, except that:

- No DefaultStatement is allowed.

- No CaseRangeStatements are allowed.

- If the switch Expression is of enum type, all the enum members must appear in the CaseStatements.

- The case expressions cannot evaluate to a run time initialized value.

Implementation Defined: If the Expression value does not match any of the CaseRangeStatements, whether that is diagnosed at compile time or at runtime.

Continue Statement

ContinueStatement:
continue Identifieropt ;

continue aborts the current iteration of its enclosing loop statement, and starts the next iteration.

continue executes the next iteration of its innermost enclosing while, for, foreach, or do loop. The increment clause is executed.

If continue is followed by Identifier, the Identifier must be the label of an enclosing while, for, or do loop, and the next iteration of that loop is executed. It is an error if there is no such statement.

Any intervening finally clauses are executed, and any intervening synchronization objects are released.

Note: If a finally clause executes a throw out of the finally clause, the continue target is never reached.

for (i = 0; i < 10; i++)
{
if (foo(i))
continue;
bar();
}

Break Statement

BreakStatement:
break Identifieropt ;

break exits the innermost enclosing while, for, foreach, do, or switch statement, resuming execution at the statement following it.

If break is followed by Identifier, the Identifier must be the label of an enclosing while, for, do or switch statement, and that statement is exited. It is an error if there is no such statement.

Any intervening finally clauses are executed, and any intervening synchronization objects are released.

Note: If a finally clause executes a throw out of the finally clause, the break target is never reached.

for (i = 0; i < 10; i++)
{
if (foo(i))
break;
}

Return Statement

ReturnStatement:
return Expressionopt ;

return exits the current function and supplies its return value.

Expression is required if the function specifies a return type that is not void. The Expression is implicitly converted to the function return type.

At least one return statement, throw statement, or assert(0) expression is required if the function specifies a return type that is not void, unless the function contains inline assembler code.

Before the function actually returns, any objects with scope storage duration are destroyed, any enclosing finally clauses are executed, any scope(exit) statements are executed, any scope(success) statements are executed, and any enclosing synchronization objects are released.

The function will not return if any enclosing finally clause does a return, goto or throw that exits the finally clause.

If there is an out postcondition (see Contract Programming), that postcondition is executed after the Expression is evaluated and before the function actually returns.

int foo(int x)
{
return x + 3;
}

Goto Statement

GotoStatement:
goto Identifier ;
goto default ;
goto case ;
goto case Expression ;

goto transfers to the statement labeled with Identifier.

if (foo)
goto L1;
x = 3;
L1:
x++;

The second form, goto default;, transfers to the innermost DefaultStatement of an enclosing SwitchStatement.

The third form, goto case;, transfers to the next CaseStatement of the innermost enclosing SwitchStatement.

The fourth form, goto case Expression;, transfers to the CaseStatement of the innermost enclosing SwitchStatement with a matching Expression.

switch (x)
{
case 3:
goto case;
case 4:
goto default;
case 5:
goto case 4;
default:
x = 4;
break;
}

Any intervening finally clauses are executed, along with releasing any intervening synchronization mutexes.

It is illegal for a GotoStatement to be used to skip initializations.

With Statement

The with statement is a way to simplify repeated references to the same object.

WithStatement:
with ( Expression ) ScopeStatement
with ( Symbol ) ScopeStatement
with ( TemplateInstance ) ScopeStatement

where Expression evaluates to a class reference or struct instance. Within the with body the referenced object is searched first for identifier symbols.
The WithStatement

with (expression)
{
...
ident;
}

is semantically equivalent to:

{
Object tmp;
tmp = expression;
...
tmp.ident;
}

Note that Expression only gets evaluated once and is not copied. The with statement does not change what this or super refer to.

For Symbol which is a scope or TemplateInstance, the corresponding scope is searched when looking up symbols. For example:

struct Foo
{
alias Y = int;
}
...
Y y;        // error, Y undefined
with (Foo)
{
Y y;    // same as Foo.Y y;
}

Use of with object symbols that shadow local symbols with the same identifier are not allowed. This is to reduce the risk of inadvertent breakage of with statements when new members are added to the object declaration.

struct S
{
float x;
}

void main()
{
int x;
S s;
with (s)
{
x++;  // error, shadows the int x declaration
}
}

In nested WithStatements, the inner-most scope takes precedence. If a symbol cannot be resolved at the inner-most scope, resolution is forwarded incrementally up the scope hierarchy.

import std.stdio;

struct Foo
{
void f() { writeln("Foo.f"); }
}

struct Bar
{
void f() { writeln("Bar.f"); }
}

struct Baz
{
// f() is not implemented
}

void f()
{
writeln("f");
}

void main()
{
Foo foo;
Bar bar;
Baz baz;

f();               // prints "f"

with(foo)
{
f();           // prints "Foo.f"

with(bar)
{
f();       // prints "Bar.f"

with(baz)
{
f();   // prints "Bar.f".  `Baz` does not implement `f()` so
// resolution is forwarded to `with(bar)`'s scope
}
}
with(baz)
{
f();       // prints "Foo.f".  `Baz` does not implement `f()` so
// resolution is forwarded to `with(foo)`'s scope
}
}
with(baz)
{
f();           // prints "f".  `Baz` does not implement `f()` so
// resolution is forwarded to `main`'s scope. `f()` is
// not implemented in `main`'s scope, so resolution is
// subsequently forward to module scope.
}
}

Synchronized Statement

The synchronized statement wraps a statement with a mutex to synchronize access among multiple threads.

SynchronizedStatement:
synchronized ScopeStatement
synchronized ( Expression ) ScopeStatement

Synchronized allows only one thread at a time to execute ScopeStatement by using a mutex.

What mutex is used is determined by the Expression. If there is no Expression, then a global mutex is created, one per such synchronized statement. Different synchronized statements will have different global mutexes.

If there is an Expression, it must evaluate to either an Object or an instance of an Interface, in which case it is cast to the Object instance that implemented that Interface. The mutex used is specific to that Object instance, and is shared by all synchronized statements referring to that instance.

The synchronization gets released even if ScopeStatement terminates with an exception, goto, or return.

Example:

synchronized { ... }

This implements a standard critical section.

Synchronized statements support recursive locking; that is, a function wrapped in synchronized is allowed to recursively call itself and the behavior will be as expected: The mutex will be locked and unlocked as many times as there is recursion.

Try Statement

Exception handling is done with the try-catch-finally statement.

TryStatement:
try ScopeStatement Catches
try ScopeStatement Catches FinallyStatement
try ScopeStatement FinallyStatement

Catches:
Catch
Catch Catches

Catch:
catch ( CatchParameter ) NoScopeNonEmptyStatement

CatchParameter:
BasicType Identifieropt

FinallyStatement:
finally NoScopeNonEmptyStatement

CatchParameter declares a variable v of type T, where T is Throwable or derived from Throwable. v is initialized by the throw expression if T is of the same type or a base class of the throw expression. The catch clause will be executed if the exception object is of type T or derived from T.

If just type T is given and no variable v, then the catch clause is still executed.

It is an error if any CatchParameter type T1 hides a subsequent Catch with type T2, i.e. it is an error if T1 is the same type as or a base class of T2.

The FinallyStatement is always executed, whether the try ScopeStatement exits with a goto, break, continue, return, exception, or fall-through.

If an exception is raised in the FinallyStatement and is not caught before the original exception is caught, it is chained to the previous exception via the next member of Throwable. Note that, in contrast to most other programming languages, the new exception does not replace the original exception. Instead, later exceptions are regarded as 'collateral damage' caused by the first exception. The original exception must be caught, and this results in the capture of the entire chain.

Thrown objects derived from the Error class are treated differently. They bypass the normal chaining mechanism, such that the chain can only be caught by catching the first Error. In addition to the list of subsequent exceptions, Error also contains a pointer that points to the original exception (the head of the chain) if a bypass occurred, so that the entire exception history is retained.

import std.stdio;

int main()
{
try
{
try
{
throw new Exception("first");
}
finally
{
writeln("finally");
throw new Exception("second");
}
}
catch (Exception e)
{
writefln("catch %s", e.msg);
}
writeln("done");
return 0;
}

prints:

finally
catch first
done

A FinallyStatement may not exit with a goto, break, continue, or return; nor may it be entered with a goto.

A FinallyStatement may not contain any Catches. This restriction may be relaxed in future versions.

Throw Statement

Throws an exception.

ThrowStatement:
throw Expression ;

Expression is evaluated and must be a Throwable reference. The Throwable reference is thrown as an exception.

throw new Exception("message");

Best Practices: Use Assert Expressions rather than Error to report program bugs and abort the program.

Scope Guard Statement

ScopeGuardStatement:
scope ( exit ) NonEmptyOrScopeBlockStatement
scope ( success ) NonEmptyOrScopeBlockStatement
scope ( failure ) NonEmptyOrScopeBlockStatement

The ScopeGuardStatement executes NonEmptyOrScopeBlockStatement at the close of the current scope, rather than at the point where the ScopeGuardStatement appears. scope(exit) executes NonEmptyOrScopeBlockStatement when the scope exits normally or when it exits due to exception unwinding. scope(failure) executes NonEmptyOrScopeBlockStatement when the scope exits due to exception unwinding. scope(success) executes NonEmptyOrScopeBlockStatement when the scope exits normally.

If there are multiple ScopeGuardStatements in a scope, they will be executed in the reverse lexical order in which they appear. If any scope instances are to be destroyed upon the close of the scope, their destructions will be interleaved with the ScopeGuardStatements in the reverse lexical order in which they appear.

write("1");
{
write("2");
scope(exit) write("3");
scope(exit) write("4");
write("5");
}
writeln();

writes:

12543

{
scope(exit) write("1");
scope(success) write("2");
scope(exit) write("3");
scope(success) write("4");
}
writeln();

writes:

4321

struct Foo
{
this(string s) { write(s); }
~this() { write("1"); }
}

try
{
scope(exit) write("2");
scope(success) write("3");
Foo f = Foo("0");
scope(failure) write("4");
throw new Exception("msg");
scope(exit) write("5");
scope(success) write("6");
scope(failure) write("7");
}
catch (Exception e)
{
}
writeln();

writes:

0412

A scope(exit) or scope(success) statement may not exit with a throw, goto, break, continue, or return; nor may it be entered with a goto.
Catching C++ Class Objects

On many platforms, catching C++ class objects is supported. Catching C++ objects and D objects cannot both be done in the same TryStatement. Upon exit from the Catch, any destructors for the C++ object will be run and the storage used for it reclaimed. C++ objects cannot be caught in @safe code.

Asm Statement

Inline assembler is supported with the asm statement:

AsmStatement:
asm FunctionAttributesopt { AsmInstructionListopt }

AsmInstructionList:
AsmInstruction ;
AsmInstruction ; AsmInstructionList

An asm statement enables the direct use of assembly language instructions. This makes it easy to obtain direct access to special CPU features without resorting to an external assembler. The D compiler will take care of the function calling conventions, stack setup, etc.

The format of the instructions is, of course, highly dependent on the native instruction set of the target CPU, and so is implementation defined. But, the format will follow the following conventions:

- It must use the same tokens as the D language uses.

- The comment form must match the D language comments.

- Asm instructions are terminated by a ;, not by an end of line.

These rules exist to ensure that D source code can be tokenized independently of syntactic or semantic analysis.
For example, for the Intel Pentium:

int x = 3;
asm
{
mov EAX,x; // load x and put it in register EAX
}

Inline assembler can be used to access hardware directly:

int gethardware()
{
asm
{
mov EAX, dword ptr 0x1234;
}
}

For some D implementations, such as a translator from D to C, an inline assembler makes no sense, and need not be implemented. The version statement can be used to account for this:

version (D_InlineAsm_X86)
{
asm
{
...
}
}
else
{
/* ... some workaround ... */
}

Semantically consecutive AsmStatements shall not have any other instructions (such as register save or restores) inserted between them by the compiler.

Pragma Statement

See PragmaStatement
.
Mixin Statement

MixinStatement:
mixin ( ArgumentList ) ;

Each AssignExpression in the ArgumentList is evaluated at compile time, and the result must be representable as a string. The resulting strings are concatenated to form a string. The text contents of the string must be compilable as a valid StatementList, and is compiled as such.

import std.stdio;

void main()
{
int j;
mixin("
int x = 3;
for (int i = 0; i < 3; i++)
writeln(x + i, ++j);
");    // ok

string s = "int y;";
mixin(s);  // ok
y = 4;     // ok, mixin declared y

string t = "y = 3;";
mixin(t);  // error, t is not evaluatable at compile time

mixin("y =") 4; // error, string must be complete statement

mixin("y =" ~ "4;");  // ok

mixin("y =", 2+2, ";");  // ok
}

Arrays

Kinds

There are four kinds of arrays:

Kinds of Arrays

Syntax

Description

type*

Pointers to data

type[integer]

Static arrays

type[]

Dynamic arrays

type[type]

Associative arrays

Pointers

int* p;

A pointer to type T has a value which is a reference (address) to another object of type T. It is commonly called a pointer to T.

If a pointer contains a null value, it is not pointing to a valid object.

When a pointer to T is dereferenced, it must either contain a null value, or point to a valid object of type T.

Implementation Defined:

1. The behavior when a null pointer is dereferenced. Typically the program will be aborted.

Undefined Behavior:

1. Dereferencing a pointer that is not null and does not point to a valid object of type T.

Best Practices: These are simple pointers to data. Pointers are provided for interfacing with C and for specialized systems work. There is no length associated with it, and so there is no way for the compiler or runtime to do bounds checking, etc., on it. Most conventional uses for pointers can be replaced with dynamic arrays, out and ref parameters, and reference types.

Static Arrays

int[3] s;

Static arrays have a length fixed at compile time.

The total size of a static array cannot exceed 16Mb.

A static array with a dimension of 0 is allowed, but no space is allocated for it.

Static arrays are value types. They are passed to and returned by functions by value.

Best Practices:

1. Use dynamic arrays for larger arrays.

2. Static arrays with 0 elements are useful as the last member of a variable length struct, or as the degenerate case of a template expansion.

3. Because static arrays are passed to functions by value, a larger array can consume a lot of stack space. Use dynamic arrays instead.

Dynamic Arrays

int[] a;

Dynamic arrays consist of a length and a pointer to the array data. Multiple dynamic arrays can share all or parts of the array data.

Best Practices:

1. Use dynamic arrays instead of pointers to arrays as much as practical. Indexing of dynamic arrays are bounds checked, avoiding buffer underflow and overflow problems.

Array Declarations

Declarations appear before the identifier being declared and read right to left, so:

int[] a;      // dynamic array of ints
int[4][3] b;  // array of 3 arrays of 4 ints each
int[][5] c;   // array of 5 dynamic arrays of ints.
int*[]*[3] d; // array of 3 pointers to dynamic arrays of pointers to ints
int[]* e;     // pointer to dynamic array of ints

Array Usage

There are two broad kinds of operations to do on an array - affecting the handle to the array, and affecting the contents of the array.

The handle to an array is specified by naming the array, as in p, s or a:

int* p;
int[3] s;
int[] a;

int[] b;

p = s.ptr; // p points to the first element of the array s.
p = a.ptr; // p points to the first element of the array a.

a = p;     // error, since the length of the array pointed
// to by p is unknown
a = s;     // a is initialized to point to the s array
a = b;     // a points to the same array as b does

Indexing

See also IndexExpression.

Slicing

Slicing an array means to specify a subarray of it. An array slice does not copy the data, it is only another reference to it. For example:

void foo(int value)
{
writeln("value=", value);
}

int[10] a;   // declare array of 10 ints
int[] b;

b = a[1..3]; // a[1..3] is a 2 element array consisting of
// a[1] and a[2]
foo(b[1]);   // equivalent to foo(0)
a[2] = 3;
foo(b[1]);   // equivalent to foo(3)

The [] is shorthand for a slice of the entire array. For example, the assignments to b:

int[10] a = [ 1,2,3,4,5,6,7,8,9,10 ];
int[] b1, b2, b3, b4;

b1 = a;
b2 = a[];
b3 = a[0 .. a.length];
b4 = a[0 .. $];
writeln(b1);
writeln(b2);
writeln(b3);
writeln(b4);

are all semantically equivalent.

Slicing is not only handy for referring to parts of other arrays, but for converting pointers into bounds-checked arrays:

int[10] a = [ 1,2,3,4,5,6,7,8,9,10 ];

int* p = &a[2];
int[] b = p[0..8];
writeln(b);
writeln(p[7]);      // 10
writeln(p[8]);      // undefined behaviour
writeln(b[7]);      // 10
//writeln(b[8]);    // range error

See also SliceExpression.

Array Copying

When the slice operator appears as the left-hand side of an assignment expression, it means that the contents of the array are the target of the assignment rather than a reference to the array. Array copying happens when the left-hand side is a slice, and the right-hand side is an array of or pointer to the same type.

int[3] s;
int[3] t;

s = t;             // the 3 elements of t are copied into s
s[] = t;           // the 3 elements of t are copied into s
s[] = t[];         // the 3 elements of t are copied into s
s[1..2] = t[0..1]; // same as s[1] = t[0]
s[0..2] = t[1..3]; // same as s[0] = t[1], s[1] = t[2]
s[0..4] = t[0..4]; // error, only 3 elements in s
s[0..2] = t;       // error, operands have different lengths

Overlapping Copying

Overlapping copies are an error:

s[0..2] = s[1..3]; // error, overlapping copy
s[1..3] = s[0..2]; // error, overlapping copy

Disallowing overlapping makes it possible for more aggressive parallel code optimizations than possible with the serial semantics of C.

If overlapping is required, use std.algorithm.mutation.copy:

import std.algorithm;
int[] s = [1, 2, 3, 4];

copy(s[1..3], s[0..2]);
assert(s == [2, 3, 3, 4]);

Array Setting

If a slice operator appears as the left-hand side of an assignment expression, and the type of the right-hand side is the same as the element type of the left-hand side, then the array contents of the left-hand side are set to the right-hand side.

int[3] s;
int* p;

s[] = 3;     // same as s[0] = 3, s[1] = 3, s[2] = 3
p[0..2] = 3; // same as p[0] = 3, p[1] = 3

Array Concatenation

The binary operator ~ is the cat operator. It is used to concatenate arrays:

int[] a;
int[] b;
int[] c;

a = b ~ c; // Create an array from the concatenation
// of the b and c arrays

Many languages overload the + operator to mean concatenation. This confusingly leads to, does:

"10" + 3 + 4

produce the number 17, the string "1034" or the string "107" as the result? It isn't obvious, and the language designers wind up carefully writing rules to disambiguate it - rules that get incorrectly implemented, overlooked, forgotten, and ignored. It's much better to have + mean addition, and a separate operator to be array concatenation.

Similarly, the ~= operator means append, as in:

a ~= b; // a becomes the concatenation of a and b

Concatenation always creates a copy of its operands, even if one of the operands is a 0 length array, so:

a = b;           // a refers to b
a = b ~ c[0..0]; // a refers to a copy of b

Appending does not always create a copy, see setting dynamic array length for details.

Array Operations

Many array operations, also known as vector operations, can be expressed at a high level rather than as a loop. For example, the loop:

T[] a, b;
...
for (size_t i = 0; i < a.length; i++)
a[i] = b[i] + 4;

assigns to the elements of a the elements of b with 4 added to each. This can also be expressed in vector notation as:

T[] a, b;
...
a[] = b[] + 4;

A vector operation is indicated by the slice operator appearing as the left-hand side of an assignment or an op-assignment expression. The right-hand side can be an expression consisting either of an array slice of the same length and type as the left-hand side or a scalar expression of the element type of the left-hand side, in any combination.

T[] a, b, c;
...
a[] -= (b[] + 4) * c[];

The slice on the left and any slices on the right must not overlap. All operands are evaluated exactly once, even if the array slice has zero elements in it.

The order in which the array elements are computed is implementation defined, and may even occur in parallel. An application must not depend on this order.

Implementation note: Many vector operations are expected to take advantage of any vector math instructions available on the target computer.

Pointer Arithmetic

int[3] abc;              // static array of 3 ints
int[] def = [ 1, 2, 3 ]; // dynamic array of 3 ints

void dibb(int* array)
{
array[2];     // means same thing as *(array + 2)
*(array + 2); // get 3rd element
}

void diss(int[] array)
{
array[2];     // ok
*(array + 2); // error, array is not a pointer
}

void ditt(int[3] array)
{
array[2];     // ok
*(array + 2); // error, array is not a pointer
}

Rectangular Arrays

Experienced FORTRAN numerics programmers know that multidimensional "rectangular" arrays for things like matrix operations are much faster than trying to access them via pointers to pointers resulting from "array of pointers to array" semantics. For example, the D syntax:

double[][] matrix;

declares matrix as an array of pointers to arrays. (Dynamic arrays are implemented as pointers to the array data.) Since the arrays can have varying sizes (being dynamically sized), this is sometimes called "jagged" arrays. Even worse for optimizing the code, the array rows can sometimes point to each other! Fortunately, D static arrays, while using the same syntax, are implemented as a fixed rectangular layout in a contiguous block of memory:

double[6][3] matrix = 0; // Sets all elements to 0.
writeln(matrix); // [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0]]

Note that dimensions and indices appear in opposite orders. Dimensions in the declaration are read right to left whereas indices are read left to right:

double[6][3] matrix = 0;
matrix[2][5] = 3.14; // Assignment to bottom right element.
writeln(matrix); // [[0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 3.14]]

static assert(!__traits(compiles, matrix[5][2])); // Array index out of bounds.

Array Length

Within the [ ] of a static or a dynamic array, the symbol $ represents the length of the array.

int[4] foo;
int[]  bar = foo;
int*   p = &foo[0];

// These expressions are equivalent:
bar[]
bar[0 .. 4]
bar[0 .. $]
bar[0 .. bar.length]

p[0 .. $]      // '$' is not defined, since p is not an array
bar[0]+$            // '$' is not defined, out of scope of [ ]

bar[$-1] // retrieves last element of the array

Array Properties

Static array properties are:

Static Array Properties

Property

Description

.init

Returns an array literal with each element of the literal being the .init property of the array element type.

.sizeof

Returns the array length multiplied by the number of bytes per array element.

.length

Returns the number of elements in the array. This is a fixed quantity for static arrays. It is of type size_t.

.ptr

Returns a pointer to the first element of the array.

.dup

Create a dynamic array of the same size and copy the contents of the array into it. The copy will have any immutability or const stripped. If this conversion is invalid the call will not compile.

.idup

Create a dynamic array of the same size and copy the contents of the array into it. The copy is typed as being immutable. If this conversion is invalid the call will not compile.

Dynamic array properties are:

Dynamic Array Properties

Property

Description

.init

Returns null.

.sizeof

Returns the size of the dynamic array reference, which is 8 in 32-bit builds and 16 on 64-bit builds.

.length

Get/set number of elements in the array. It is of type size_t.

.ptr

Returns a pointer to the first element of the array.

.dup

Create a dynamic array of the same size and copy the contents of the array into it. The copy will have any immutability or const stripped. If this conversion is invalid the call will not compile.

.idup

Create a dynamic array of the same size and copy the contents of the array into it. The copy is typed as being immutable. If this conversion is invalid the call will not compile.

Examples:

int* p;
int[3] s;
int[] a;

p.length; // error, length not known for pointer
s.length; // compile time constant 3
a.length; // runtime value

p.dup;    // error, length not known
s.dup;    // creates an array of 3 elements, copies
// elements s into it
a.dup;    // creates an array of a.length elements, copies
// elements of a into it

Setting Dynamic Array Length

The .length property of a dynamic array can be set as the left-hand side of an = operator:

array.length = 7;

This causes the array to be reallocated in place, and the existing contents copied over to the new array. If the new array length is shorter, the array is not reallocated, and no data is copied. It is equivalent to slicing the array:

array = array[0..7];

If the new array length is longer, the remainder is filled out with the default initializer.

To maximize efficiency, the runtime always tries to resize the array in place to avoid extra copying. It will always do a copy if the new size is larger and the array was not allocated via the new operator or resizing in place would overwrite valid data in the array. For example:

char[] a = new char[20];
char[] b = a[0..10];
char[] c = a[10..20];
char[] d = a;

b.length = 15; // always reallocates because extending in place would
// overwrite other data in a.
b[11] = 'x';   // a[11] and c[1] are not affected

d.length = 1;
d.length = 20; // also reallocates, because doing this will overwrite a and
// c

c.length = 12; // may reallocate in place if space allows, because nothing
// was allocated after c.
c[5] = 'y';    // may affect contents of a, but not b or d because those
// were reallocated.

a.length = 25; // This always reallocates because if c extended in place,
// then extending a would overwrite c.  If c didn't
// reallocate in place, it means there was not enough space,
// which will still be true for a.
a[15] = 'z';   // does not affect c, because either a or c has reallocated.

To guarantee copying behavior, use the .dup property to ensure a unique array that can be resized. Also, one may use the .capacity property to determine how many elements can be appended to the array without reallocating.

These issues also apply to appending arrays with the ~= operator. Concatenation using the ~ operator is not affected since it always reallocates.

Resizing a dynamic array is a relatively expensive operation. So, while the following method of filling an array:

int[] array;
while (1)
{
import core.stdc.stdio : getchar;
auto c = getchar;
if (!c)
break;
++array.length;
array[array.length - 1] = c;
}

will work, it will be inefficient. A more practical approach would be to minimize the number of resizes:

int[] array;
array.length = 100;        // guess
int i;
for (i = 0; ; i++)
{
import core.stdc.stdio : getchar;
auto c = getchar;
if (!c)
break;
if (i == array.length)
array.length *= 2;
array[i] = c;
}
array.length = i;

Base selection of the initial size on expected common use cases, which can be determined by instrumenting the code, or simply using good judgement. For example, when gathering user input from the console - it's unlikely to be longer than 80.

The reserve function expands an array's capacity for use by the append operator.

int[] array;
size_t cap = array.reserve(10); // request
array ~= [1, 2, 3, 4, 5];
assert(cap >= 10); // allocated may be more than request
assert(cap == array.capacity);

Functions as Array Properties

See Uniform Function Call Syntax (UFCS).

Array Bounds Checking

It is an error to index an array with an index that is less than 0 or greater than or equal to the array length. If an index is out of bounds, a RangeError exception is raised if detected at runtime, and an error if detected at compile time. A program may not rely on array bounds checking happening, for example, the following program is incorrect:

import core.exception;
try
{
auto array = [1, 2];
for (auto i = 0; ; i++)
{
array[i] = 5;
}
}
catch (RangeError)
{
// terminate loop
}

The loop is correctly written:

auto array = [1, 2];
for (auto i = 0; i < array.length; i++)
{
array[i] = 5;
}

Implementation Note: Compilers should attempt to detect array bounds errors at compile time, for example:

int[3] foo;
int x = foo[3]; // error, out of bounds

Insertion of array bounds checking code at runtime should be turned on and off with a compile time switch.

Undefined Behavior: An out of bounds memory access will cause undefined behavior, therefore array bounds check is normally enabled in @safe functions. The runtime behavior is part of the language semantics.

See also Safe Functions.

Disabling Array Bounds Checking

Insertion of array bounds checking code at runtime may be turned off with a compiler switch -boundscheck.

If the bounds check in @system or @trusted code is disabled, the code correctness must still be guaranteed by the code author.

On the other hand, disabling the bounds check in @safe code will break the guaranteed memory safety by compiler. It's not recommended unless motivated by speed measurements.

Array Initialization

Default Initialization

- Pointers are initialized to null.

- Static array contents are initialized to the default initializer for the array element type.

- Dynamic arrays are initialized to having 0 elements.

- Associative arrays are initialized to having 0 elements.

Void Initialization

Void initialization happens when the Initializer for an array is void. What it means is that no initialization is done, i.e. the contents of the array will be undefined. This is most useful as an efficiency optimization. Void initializations are an advanced technique and should only be used when profiling indicates that it matters.

to void initialise the elements of dynamic array use std.array.uninitializedArray.

Static Initialization of Statically Allocated Arrays

Static initalizations are supplied by a list of array element values enclosed in [ ]. The values can be optionally preceded by an index and a :. If an index is not supplied, it is set to the previous index plus 1, or 0 if it is the first value.

int[3] a = [ 1:2, 3 ]; // a[0] = 0, a[1] = 2, a[2] = 3

This is most handy when the array indices are given by enums:

enum Color { red, blue, green }

int[Color.max + 1] value =
[ Color.blue :6,
Color.green:2,
Color.red  :5 ];

These arrays are statically allocated when they appear in global scope. Otherwise, they need to be marked with const or static storage classes to make them statically allocated arrays.

Special Array Types

Strings

A string is an array of characters. String literals are just an easy way to write character arrays. String literals are immutable (read only).

char[] str1 = "abc";                // error, "abc" is not mutable
char[] str2 = "abc".dup;            // ok, make mutable copy
immutable(char)[] str3 = "abc";     // ok
immutable(char)[] str4 = str1;      // error, str4 is not mutable
immutable(char)[] str5 = str1.idup; // ok, make immutable copy

The name string is aliased to immutable(char)[], so the above declarations could be equivalently written as:

char[] str1 = "abc";     // error, "abc" is not mutable
char[] str2 = "abc".dup; // ok, make mutable copy
string str3 = "abc";     // ok
string str4 = str1;      // error, str4 is not mutable
string str5 = str1.idup; // ok, make immutable copy

char[] strings are in UTF-8 format. wchar[] strings are in UTF-16 format. dchar[] strings are in UTF-32 format.

Strings can be copied, compared, concatenated, and appended:

str1 = str2;
if (str1 < str3) { ... }
func(str3 ~ str4);
str4 ~= str1;

with the obvious semantics. Any generated temporaries get cleaned up by the garbage collector (or by using alloca()). Not only that, this works with any array not just a special String array.

A pointer to a char can be generated:

char* p = &str[3]; // pointer to 4th element
char* p = str;     // pointer to 1st element

Since strings, however, are not 0 terminated in D, when transferring a pointer to a string to C, add a terminating 0:

str ~= "\0";

or use the function std.string.toStringz.
The type of a string is determined by the semantic phase of compilation. The type is one of: char[], wchar[], dchar[], and is determined by implicit conversion rules. If there are two equally applicable implicit conversions, the result is an error. To disambiguate these cases, a cast or a postfix of c, w or d can be used:

cast(immutable(wchar) [])"abc" // this is an array of wchar characters
"abc"w                         // so is this

String literals that do not have a postfix character and that have not been cast can be implicitly converted between string, wstring, and dstring as necessary.

char c;
wchar w;
dchar d;

c = 'b';     // c is assigned the character 'b'
w = 'b';     // w is assigned the wchar character 'b'
//w = 'bc';  // error - only one wchar character at a time
w = "b"[0];  // w is assigned the wchar character 'b'
w = "\r"[0]; // w is assigned the carriage return wchar character
d = 'd';     // d is assigned the character 'd'

Strings and Unicode

Note that built-in comparison operators operate on a code unit basis. The end result for valid strings is the same as that of code point for code point comparison as long as both strings are in the same normalization form. Since normalization is a costly operation not suitable for language primitives it's assumed to be enforced by the user.

The standard library lends a hand for comparing strings with mixed encodings (by transparently decoding, see std.algorithm.cmp), case-insensitive comparison and normalization.

Last but not least, a desired string sorting order differs by culture and language and is usually nothing like code point for code point comparison. The natural order of strings is obtained by applying the Unicode collation algorithm that should be implemented in the standard library.

C's printf() and Strings

printf() is a C function and is not part of D. printf() will print C strings, which are 0 terminated. There are two ways to use printf() with D strings. The first is to add a terminating 0, and cast the result to a char*:

str ~= "\0";
printf("the string is '%s'\n", str.ptr);

or:

import std.string;
printf("the string is '%s'\n", std.string.toStringz(str));

String literals already have a 0 appended to them, so can be used directly:

printf("the string is '%s'\n", "string literal".ptr);

So, why does the first string literal to printf not need the .ptr? The first parameter is prototyped as a const(char)*, and a string literal can be implicitly cast to a const(char)*. The rest of the arguments to printf, however, are variadic (specified by ...), and a string literal typed immutable(char)[] cannot pass to variadic parameters.

The second way is to use the precision specifier. The length comes first, followed by the pointer:

printf("the string is '%.*s'\n", str.length, str.ptr);

The best way is to use std.stdio.writefln, which can handle D strings:

import std.stdio;
writefln("the string is '%s'", str);

Void Arrays

There is a special type of array which acts as a wildcard that can hold arrays of any kind, declared as void[]. Void arrays are used for low-level operations where some kind of array data is being handled, but the exact type of the array elements are unimportant. The .length of a void array is the length of the data in bytes, rather than the number of elements in its original type. Array indices in indexing and slicing operations are interpreted as byte indices.

Arrays of any type can be implicitly converted to a void array; the compiler inserts the appropriate calculations so that the .length of the resulting array's size is in bytes rather than number of elements. Void arrays cannot be converted back to the original type without using a cast, and it is an error to convert to an array type whose element size does not evenly divide the length of the void array.

void main()
{
int[] data1 = [1,2,3];
long[] data2;

void[] arr = data1;            // OK, int[] implicit converts to void[].
assert(data1.length == 3);
assert(arr.length == 12);      // length is implicitly converted to bytes.

//data1 = arr;                 // Illegal: void[] does not implicitly
// convert to int[].
int[] data3 = cast(int[]) arr; // OK, can convert with explicit cast.
data2 = cast(long[]) arr;      // Runtime error: long.sizeof == 8, which
// does not divide arr.length, which is 12
// bytes.
}

Void arrays can also be static if their length is known at compile-time. The length is specified in bytes:

void main()
{
byte[2] x;
int[2] y;

void[2] a = x; // OK, lengths match
void[2] b = y; // Error: int[2] is 8 bytes long, doesn't fit in 2 bytes.
}

While it may seem that void arrays are just fancy syntax for ubyte[], there is a subtle distinction. The garbage collector generally will not scan ubyte[] arrays for pointers, ubyte[] being presumed to contain only pure byte data, not pointers. However, it will scan void[] arrays for pointers, since such an array may have been implicitly converted from an array of pointers or an array of elements that contain pointers. Allocating an array that contains pointers as ubyte[] may run the risk of the GC collecting live memory if these pointers are the only remaining references to their targets.

Implicit Conversions

A pointer T* can be implicitly converted to one of the following:

- void*

A static array T[dim] can be implicitly converted to one of the following:

- T[]

- const(U)[]

- const(U[])

- void[]

A dynamic array T[] can be implicitly converted to one of the following (U is a base class of T):

- const(U)[]

- const(U[])

- void[]

Associative Arrays

Associative arrays have an index that is not necessarily an integer, and can be sparsely populated. The index for an associative array is called the key, and its type is called the KeyType.

Associative arrays are declared by placing the KeyType within the [ ] of an array declaration:

int[string] aa;   // Associative array of ints that are
// indexed by string keys.
// The KeyType is string.
aa["hello"] = 3;  // set value associated with key "hello" to 3
int value = aa["hello"];  // lookup value from a key
assert(value == 3);

Neither the KeyTypes nor the element types of an associative array can be function types or void.

Implementation Defined: The built-in associative arrays do not preserve the order of the keys inserted into the array. In particular, in a foreach loop the order in which the elements are iterated is typically unspecified.

Removing Keys

Particular keys in an associative array can be removed with the remove function:

aa.remove("hello");

remove(key) does nothing if the given key does not exist and returns false. If the given key does exist, it removes it from the AA and returns true.

All keys can be removed by using the method clear.

Testing Membership

The InExpression yields a pointer to the value if the key is in the associative array, or null if not:

int* p;

p = ("hello" in aa);
if (p !is null)
{
*p = 4;  // update value associated with key
assert(aa["hello"] == 4);
}

Undefined Behavior: Adjusting the pointer to point before or after the element whose address is returned, and then dereferencing it.

Using Classes as the KeyType

Classes can be used as the KeyType. For this to work, the class definition must override the following member functions of class Object:

- size_t toHash() @trusted nothrow

- bool opEquals(Object)

Note that the parameter to opEquals is of type Object, not the type of the class in which it is defined.

For example:

class Foo
{
int a, b;

override size_t toHash() { return a + b; }

override bool opEquals(Object o)
{
Foo foo = cast(Foo) o;
return foo && a == foo.a && b == foo.b;
}
}

Implementation Defined: opCmp is not used to check for equality by the associative array. However, since the actual opEquals or opCmp called is not decided until runtime, the compiler cannot always detect mismatched functions. Because of legacy issues, the compiler may reject an associative array key type that overrides opCmp but not opEquals. This restriction may be removed in future versions.

Undefined Behavior:

1. If toHash must consistently be the same value when opEquals returns true. In other words, two objects that are considered equal should always have the same hash value. Otherwise, undefined behavior will result.

Best Practices:

1. Use the attributes @safe, @nogc, pure, const, and scope as much as possible on the toHash and opEquals overrides.

Using Structs or Unions as the KeyType

If the KeyType is a struct or union type, a default mechanism is used to compute the hash and comparisons of it based on the fields of the struct value. A custom mechanism can be used by providing the following functions as struct members:

size_t toHash() const @safe pure nothrow;
bool opEquals(ref const typeof(this) s) const @safe pure nothrow;

For example:

import std.string;

struct MyString
{
string str;

size_t toHash() const @safe pure nothrow
{
size_t hash;
foreach (char c; str)
hash = (hash * 9) + c;
return hash;
}

bool opEquals(ref const MyString s) const @safe pure nothrow
{
return std.string.cmp(this.str, s.str) == 0;
}
}

The functions can use @trusted instead of @safe.

Implementation Defined: opCmp is not used to check for equality by the associative array. For this reason, and for legacy reasons, an associative array key is not allowed to define a specialized opCmp, but omit a specialized opEquals. This restriction may be removed in future versions of D.

Undefined Behavior:

1. If toHash must consistently be the same value when opEquals returns true. In other words, two structs that are considered equal should always have the same hash value. Otherwise, undefined behavior will result.

Best Practices:

1. Use the attributes @nogc as much as possible on the toHash and opEquals overrides.

Construction or Assignment on Setting AA Entries

When an AA indexing access appears on the left side of an assignment operator, it is specially handled for setting an AA entry associated with the key.

string[int] aa;
string s;
s = aa[1];          // throws RangeError in runtime
aa[1] = "hello";    // handled for setting AA entry
s = aa[1];          // succeeds to lookup
assert(s == "hello");

If the assigned value type is equivalent with the AA element type:

1. If the indexing key does not yet exist in AA, a new AA entry will be allocated, and it will be initialized with the assigned value.

2. If the indexing key already exists in the AA, the setting runs normal assignment.

struct S
{
int val;
void opAssign(S rhs) { this.val = rhs.val * 2; }
}
S[int] aa;
aa[1] = S(10);  // first setting initializes the entry aa[1]
assert(aa[1].val == 10);
aa[1] = S(10);  // second setting invokes normal assignment, and
// operator-overloading rewrites it to member opAssign function.
assert(aa[1].val == 20);

If the assigned value type is not equivalent with the AA element type, the expression could invoke operator overloading with normal indexing access:

struct S
{
int val;
void opAssign(int v) { this.val = v * 2; }
}
S[int] aa;
aa[1] = 10;     // is rewritten to: aa[1].opAssign(10), and
// throws RangeError before opAssign is called

However, if the AA element type is a struct which supports an implicit constructor call from the assigned value, implicit construction is used for setting the AA entry:

struct S
{
int val;
this(int v) { this.val = v; }
void opAssign(int v) { this.val = v * 2; }
}
S s = 1;    // OK, rewritten to: S s = S(1);
s = 1;    // OK, rewritten to: s.opAssign(1);

S[int] aa;
aa[1] = 10; // first setting is rewritten to: aa[1] = S(10);
assert(aa[1].val == 10);
aa[1] = 10; // second setting is rewritten to: aa[1].opAssign(10);
assert(aa[1].val == 20);

This is designed for efficient memory reuse with some value-semantics structs, eg. std.bigint.BigInt.

import std.bigint;
BigInt[string] aa;
aa["a"] = 10;   // construct BigInt(10) and move it in AA
aa["a"] = 20;   // call aa["a"].opAssign(20)

Inserting if not present

When AA access requires that there must be a value corresponding to the key, a value must be constructed and inserted if not present. The require function provides a means to construct a new value via a lazy argument. The lazy argument is evaluated when the key is not present. The require operation avoids the need to perform multiple key lookups.

class C{}
C[string] aa;

auto a = aa.require("a", new C);   // lookup "a", construct if not present

Sometimes it is necessary to know whether the value was constructed or already exists. The require function doesn't provide a boolean parameter to indicate whether the value was constructed but instead allows the construction via a function or delegate. This allows the use of any mechanism as demonstrated below.

class C{}
C[string] aa;

bool constructed;
auto a = aa.require("a", { constructed=true; return new C;}());
assert(constructed == true);

C newc;
auto b = aa.require("b", { newc = new C; return newc;}());
assert(b is newc);

Advanced updating

Typically updating a value in an associative array is simply done with an assign statement.

int[string] aa;

aa["a"] = 3;  // set value associated with key "a" to 3

Sometimes it is necessary to perform different operations depending on whether a value already exists or needs to be constructed. The update function provides a means to construct a new value via the create delegate or update an existing value via the update delegate. The update operation avoids the need to perform multiple key lookups.

class C{}
C[string] aa;

C older;
C newer;
aa.update("a",
{
newer = new C;
return newer;
},
(ref C c)
{
older = c;
newer = new C;
return newer;
});

Static Initialization of AAs
NOTE: Not yet implemented.

immutable long[string] aa = [
"foo": 5,
"bar": 10,
"baz": 2000
];

unittest
{
assert(aa["foo"] == 5);
assert(aa["bar"] == 10);
assert(aa["baz"] == 2000);
}

Runtime Initialization of Immutable AAs

Immutable associative arrays are often desirable, but sometimes initialization must be done at runtime. This can be achieved with a constructor (static constructor depending on scope), a buffer associative array and assumeUnique:

immutable long[string] aa;

shared static this()
{
import std.exception : assumeUnique;
import std.conv : to;

long[string] temp; // mutable buffer
foreach(i; 0 .. 10)
{
temp[to!string(i)] = i;
}
temp.rehash; // for faster lookups

aa = assumeUnique(temp);
}

unittest
{
assert(aa["1"] == 1);
assert(aa["5"] == 5);
assert(aa["9"] == 9);
}

Construction and Reference Semantics

An Associative Array defaults to null, and is constructed upon assigning the first key/value pair. However, once constructed, an associative array has reference semantics, meaning that assigning one array to another does not copy the data. This is especially important when attempting to create multiple references to the same array.

int[int] aa;             // defaults to null
int[int] aa2 = aa;       // copies the null reference
aa[1] = 1;
assert(aa2.length == 0); // aa2 still is null
aa2 = aa;
aa2[2] = 2;
assert(aa[2] == 2);      // now both refer to the same instance

Properties

Properties for associative arrays are:

Associative Array Properties

Property

Description

.sizeof

Returns the size of the reference to the associative array; it is 4 in 32-bit builds and 8 on 64-bit builds.

.length

Returns number of values in the associative array. Unlike for dynamic arrays, it is read-only.

.dup

Create a new associative array of the same size and copy the contents of the associative array into it.

.keys

Returns dynamic array, the elements of which are the keys in the associative array.

.values

Returns dynamic array, the elements of which are the values in the associative array.

.rehash

Reorganizes the associative array in place so that lookups are more efficient. rehash is effective when, for example, the program is done loading up a symbol table and now needs fast lookups in it. Returns a reference to the reorganized array.

.clear

Removes all remaining keys and values from an associative array. The array is not rehashed after removal, to allow for the existing storage to be reused. This will affect all references to the same instance and is not equivalent to destroy(aa) which only sets the current reference to null

.byKey()

Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over the keys of the associative array.

.byValue()

Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over the values of the associative array.

.byKeyValue()

Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over key-value pairs of the associative array. The returned pairs are represented by an opaque type with .key and .value properties for accessing the key and value of the pair, respectively. Note that this is a low-level interface to iterating over the associative array and is not compatible with the Tuple type in Phobos. For compatibility with Tuple, use std.array.byPair instead.

.get(Key key, lazy Value defVal)

Looks up key; if it exists returns corresponding value else evaluates and returns defVal.

.require(Key key, lazy Value value)

Looks up key; if it exists returns corresponding value else evaluates value, adds it to the associative array and returns it.

.update(Key key, Value delegate() create, Value delegate(Value) update)

Looks up key; if it exists applies the update delegate else evaluates the create delegate and adds it to the associative array

Associative Array Example: word count

Let's consider the file is ASCII encoded with LF EOL. In general case we should use dchar c for iteration over code points and functions from std.uni.

import std.file;         // D file I/O
import std.stdio;
import std.ascii;

void main (string[] args)
{
ulong totalWords, totalLines, totalChars;
ulong[string] dictionary;

writeln("   lines   words   bytes file");
foreach (arg; args[1 .. $]) // for each argument except the first one
{
ulong wordCount, lineCount, charCount;

foreach(line; File(arg).byLine())
{
bool inWord;
size_t wordStart;

void tryFinishWord(size_t wordEnd)
{
if (inWord)
{
auto word = line[wordStart .. wordEnd];
++dictionary[word.idup];   // increment count for word
inWord = false;
}
}

foreach (i, char c; line)
{
if (std.ascii.isDigit(c))
{
// c is a digit (0..9)
}
else if (std.ascii.isAlpha(c))
{
// c is an ASCII letter (A..Z, a..z)
if (!inWord)
{
wordStart = i;
inWord = true;
++wordCount;
}
}
else
tryFinishWord(i);
++charCount;
}
tryFinishWord(line.length);
++lineCount;
}

writefln("%8s%8s%8s %s", lineCount, wordCount, charCount, arg);
totalWords += wordCount;
totalLines += lineCount;
totalChars += charCount;
}

if (args.length > 2)
{
writefln("-------------------------------------\n%8s%8s%8s total",
totalLines, totalWords, totalChars);
}

writeln("-------------------------------------");
foreach (word; dictionary.keys.sort)
{
writefln("%3s %s", dictionary[word], word);
}
}

Associative Array Example: counting Tuples

An Associative Array can be iterated in key/value fashion using a foreach statement. As an example, the number of occurrences of all possible substrings of length 2 (aka 2-mers) in a string will be counted:

import std.range : dropOne, only, save, zip;
import std.stdio : writefln;
import std.typecons : Tuple;
import std.utf : byCodeUnit; // avoids UTF-8 auto-decoding

int[Tuple!(immutable char, immutable char)] aa;

// The string `arr` has a limited alphabet: {A, C, G, T}
// Thus, for better performance, iteration can be done _without_ decoding
auto arr = "AGATAGA".byCodeUnit;

// iterate over all pairs in the string and observe each pair
// ('A', 'G'), ('G', 'A'), ('A', 'T'), ...
foreach (window; arr.zip(arr.save.dropOne))
aa[window]++;

// iterate over all key/value pairs of the Associative Array
foreach (key, value; aa)
{
// the second parameter uses tuple-expansion
writefln("key: %s [%s], value: %d", key, key.expand.only, value);
}

% rdmd count.d
key: Tuple!(immutable(char), immutable(char))('A', 'G') [AG], value: 2
key: Tuple!(immutable(char), immutable(char))('G', 'A') [GA], value: 2
key: Tuple!(immutable(char), immutable(char))('A', 'T') [AT], value: 1
key: Tuple!(immutable(char), immutable(char))('T', 'A') [TA], value: 1

Structs and Unions

Whereas classes are reference types, structs are value types. Structs and unions are simple aggregations of data and their associated operations on that data.

AggregateDeclaration:
ClassDeclaration
InterfaceDeclaration
StructDeclaration
UnionDeclaration

StructDeclaration:
struct Identifier ;
struct Identifier AggregateBody
StructTemplateDeclaration
AnonStructDeclaration

AnonStructDeclaration:
struct AggregateBody

UnionDeclaration:
union Identifier ;
union Identifier AggregateBody
UnionTemplateDeclaration
AnonUnionDeclaration

AnonUnionDeclaration:
union AggregateBody

AggregateBody:
{ DeclDefsopt }

A struct is defined to not have an identity; that is, the implementation is free to make bit copies of the struct as convenient.

Structs and unions may not contain an instance of themselves, however, they may contain a pointer to the same type.

Best Practices:

1. Bit fields are supported with the bitfields template.

Struct Layout

The non-static data members of a struct are called fields. Fields are laid out in lexical order. Fields are aligned according to the Align Attribute in effect. Unnamed padding is inserted between fields to align fields. There is no padding between the first field and the start of the object.

Structs with no fields of non-zero size (aka Empty Structs) have a size of one byte.

Non-static function-nested D structs, which access the context of their enclosing scope, have an extra field.

Implementation Defined:

1. The default layout of the fields of a struct is an exact match with the associated C compiler.

2. g++ and clang++ differ in how empty structs are handled. Both return 1 from sizeof, however, clang++ does not push them onto the parameter stack while g++ does. This is a binary incompatibility between g++ and clang++. dmd follows clang++ behavior for OSX and FreeBSD, and g++ behavior for Linux and other Posix platforms.

3. clang and gcc both return 0 from sizeof for empty structs. Using extern "C++" in clang++ and g++ does not cause them to conform to the behavior of their respective C compilers.

Undefined Behavior:

1. The padding data can be accessed, but its contents are undefined.

2. Do not pass or return structs with no fields of non-zero size to extern (C) functions. According to C11 6.7.2.1p8 this is undefined behavior.

Best Practices:

1. When laying out a struct to match an externally defined layout, use align attributes to describe an exact match. Using a Static Assert to ensure the result is as expected.

2. Although the contents of the padding are often zero, do not rely on that.

3. Avoid using empty structs when interfacing with C and C++ code.

4. Avoid using empty structs as parameters or arguments to variadic functions.

Plain Old Data

A struct or union is Plain Old Data (POD) if it meets the following criteria:

1. it is not nested

2. it has no postblits, copy constructors, destructors, or assignment operators

3. it has no ref fields or fields that are themselves non-POD

Best Practices: Structs or unions that interface with C code should be POD.

Opaque Structs and Unions

Opaque struct and union declarations do not have a AggregateBody:

struct S;
union U;
struct V(T);
union W(T);

The members are completely hidden to the user, and so the only operations on those types are ones that do not require any knowledge of the contents of those types. For example:

struct S;
S.sizeof; // error, size is not known
S s;      // error, cannot initialize unknown contents
S* p;     // ok, knowledge of members is not necessary

Best Practices: They can be used to implement the PIMPL idiom.

Default Initialization of Structs

Struct fields are by default initialized to whatever the Initializer for the field is, and if none is supplied, to the default initializer for the field's type.

struct S { int a = 4; int b; }
S x; // x.a is set to 4, x.b to 0

The default initializers are evaluated at compile time.

The default initializers may not contain references to mutable data.

Static Initialization of Structs

If a StructInitializer is supplied, the fields are initialized by the StructMemberInitializer syntax. StructMemberInitializers with the Identifier : NonVoidInitializer syntax may be appear in any order, where Identifier is the field identifier. StructMemberInitializers with the NonVoidInitializer syntax appear in the lexical order of the fields in the StructDeclaration.

Fields not specified in the StructInitializer are default initialized.

struct S { int a, b, c, d = 7; }
S r;                          // r.a = 0, r.b = 0, r.c = 0, r.d = 7
S s = { a:1, b:2 };           // s.a = 1, s.b = 2, s.c = 0, s.d = 7
S t = { c:4, b:5, a:2, d:5 }; // t.a = 2, t.b = 5, t.c = 4, t.d = 5
S u = { 1, 2 };               // u.a = 1, u.b = 2, u.c = 0, u.d = 7
S v = { 1, d:3 };             // v.a = 1, v.b = 0, v.c = 0, v.d = 3
S w = { b:1, 3 };             // w.a = 0, w.b = 1, w.c = 3, w.d = 7

Initializing a field more than once is an error:

S x = { 1, a:2 };  // error: duplicate initializer for field `a`

Default Initialization of Unions

Unions are by default initialized to whatever the Initializer for the first field is, and if none is supplied, to the default initializer for the first field's type.

If the union is larger than the first field, the remaining bits are set to 0.

union U { int a = 4; long b; }
U x; // x.a is set to 4, x.b to an implementation-defined value

union V { int a; long b = 4; }     // error: union field `b` with default initialization `4` must be before field `a`
union W { int a = 4; long b = 5; } // error: overlapping default initialization for `a` and `b`

The default initializer is evaluated at compile time.

Implementation Defined: The values the fields other than the default initialized field are set to.

Static Initialization of Unions

Unions are initialized similarly to structs, except that only one initializer is allowed.

union U { int a; double b; }
U u = { 2 };       // u.a = 2
U v = { b : 5.0 }; // v.b = 5.0

U w = { 2, 3 };    // error: overlapping initialization for field `a` and `b`

If the union is larger than the initialized field, the remaining bits are set to 0.

Implementation Defined: The values the fields other than the initialized field are set to.

Dynamic Initialization of Structs

The static initializer syntax can also be used to initialize non-static variables. The initializer need not be evaluable at compile time.

struct S { int a, b, c, d = 7; }

void test(int i)
{
S q = { 1, b:i }; // q.a = 1, q.b = i, q.c = 0, q.d = 7
}

Structs can be dynamically initialized from another value of the same type:

struct S { int a; }
S t;      // default initialized
t.a = 3;
S s = t;  // s.a is set to 3

If the struct has a constructor, and the struct is initialized with a value that is of a different type, then the constructor is called:

struct S
{
int a;

this(int v)
{
this.a = v;
}
}

S s = 3; // sets s.a to 3 using S's constructor

If the struct does not have a constructor but opCall is overridden for the struct, and the struct is initialized with a value that is of a different type, then the opCall operator is called:

struct S
{
int a;

static S opCall(int v)
{
S s;
s.a = v;
return s;
}

static S opCall(S v)
{
assert(0);
}
}

S s = 3; // sets s.a to 3 using S.opCall(int)
S t = s; // sets t.a to 3, S.opCall(S) is not called

Struct Literals

Struct literals consist of the name of the struct followed by a parenthesized argument list:

struct S { int x; float y; }

int foo(S s) { return s.x; }

foo( S(1, 2) ); // set field x to 1, field y to 2

Struct literals are syntactically like function calls. If a struct has a member function named opCall, then struct literals for that struct are not possible. See also opCall operator overloading for the issue workaround. It is an error if there are more arguments than fields of the struct. If there are fewer arguments than fields, the remaining fields are initialized with their respective default initializers. If there are anonymous unions in the struct, only the first member of the anonymous union can be initialized with a struct literal, and all subsequent non-overlapping fields are default initialized.

Struct Properties

Struct Properties

Name

Description

.sizeof

Size in bytes of struct

.alignof

Size boundary struct needs to be aligned on

Struct Instance Properties

Struct Instance Properties

Name

Description

.tupleof

An expression sequence of all struct fields - see Class Properties for a class-based example.

Struct Field Properties

Struct Field Properties

Name

Description

.offsetof

Offset in bytes of field from beginning of struct

Const, Immutable and Shared Structs

A struct declaration can have a storage class of const, immutable or shared. It has an equivalent effect as declaring each member of the struct as const, immutable or shared.

const struct S { int a; int b = 2; }
void main()
{
S s = S(3); // initializes s.a to 3
S t;        // initializes t.a to 0
t = s;      // error, t.a and t.b are const, so cannot modify them.
t.a = 4;    // error, t.a is const
}

Union Constructors

Unions are constructed in the same way as structs.

Struct Constructors

Struct constructors are used to initialize an instance of a struct when a more complex construction is needed than is allowed by static initialization or a struct literal.

Constructors are defined with a function name of this and have no return value. The grammar is the same as for the class Constructor.

A struct constructor is called by the name of the struct followed by Parameters.

If the ParameterList is empty, the struct instance is default initialized.

struct S
{
int x, y = 4, z = 6;
this(int a, int b)
{
x = a;
y = b;
}
}

void main()
{
S a = S(4, 5); // calls S.this(4, 5):  a.x = 4, a.y = 5, a.z = 6
S b = S();  // default initialized:    b.x = 0, b.y = 4, b.z = 6
S c = S(1); // error, matching this(int) not found
}

A default constructor (i.e. one with an empty ParameterList) is not allowed.

struct S
{
int x;
this() { } // error, struct default constructor not allowed
}

Delegating Constructors

A constructor can call another constructor for the same struct in order to share common initializations. This is called a delegating constructor:

struct S
{
int j = 1;
long k = 2;
this(long k)
{
this.k = k;
}
this(int i)
{
// At this point: j=1, k=2
this(6); // delegating constructor call
// At this point: j=1, k=6
j = i;
// At this point: j=i, k=6
}
}

The following restrictions apply:

1. If a constructor's code contains a delegating constructor call, all possible execution paths through the constructor must make exactly one delegating constructor call:

struct S
{
int a;
this(int i) { }

this(char c)
{
c || this(1);  // error, not on all paths
}

this(wchar w)
{
(w) ? this(1) : this('c');  // ok
}

this(byte b)
{
foreach (i; 0 .. b)
{
this(1);  // error, inside loop
}
}
}

2. It is illegal to refer to this implicitly or explicitly prior to making a delegating constructor call.

3. Once the delegating constructor returns, all fields are considered constructed.

4. Delegating constructor calls cannot appear after labels.

See also: delegating class constructors.

Struct Instantiation

When an instance of a struct is created, the following steps happen:

1. The raw data is statically initialized using the values provided in the struct definition. This operation is equivalent to doing a memory copy of a static version of the object onto the newly allocated one.

2. If there is a constructor defined for the struct, the constructor matching the argument list is called.

3. If struct invariant checking is turned on, the struct invariant is called at the end of the constructor.

Constructor Attributes

A constructor qualifier (const, immutable or shared) constructs the object instance with that specific qualifier.

struct S1
{
int[] a;
this(int n) { a = new int[](n); }
}
struct S2
{
int[] a;
this(int n) immutable { a = new int[](n); }
}
void main()
{
// Mutable constructor creates mutable object.
S1 m1 = S1(1);

// Constructed mutable object is implicitly convertible to const.
const S1 c1 = S1(1);

// Constructed mutable object is not implicitly convertible to immutable.
immutable i1 = S1(1); // error

// Mutable constructor cannot construct immutable object.
auto x1 = immutable S1(1); // error

// Immutable constructor creates immutable object.
immutable i2 = immutable S2(1);

// Immutable constructor cannot construct mutable object.
auto x2 = S2(1); // error

// Constructed immutable object is not implicitly convertible to mutable.
S2 m2 = immutable S2(1); // error

// Constructed immutable object is implicitly convertible to const.
const S2 c2 = immutable S2(1);
}

Constructors can be overloaded with different attributes.

struct S
{
this(int);           // non-shared mutable constructor
this(int) shared;    // shared mutable constructor
this(int) immutable; // immutable constructor
}

S m = S(1);
shared s = shared S(2);
immutable i = immutable S(3);

Pure Constructors

If the constructor can create a unique object (i.e. if it is pure), the object is implicitly convertible to any qualifiers.

struct S
{
this(int) pure;
// Based on the definition, this creates a mutable object. But the
// created object cannot contain any mutable global data.
// Therefore the created object is unique.

this(int[] arr) immutable pure;
// Based on the definition, this creates an immutable object. But
// the argument int[] never appears in the created object so it
// isn't implicitly convertible to immutable. Also, it cannot store
// any immutable global data.
// Therefore the created object is unique.
}

immutable i = immutable S(1); // this(int) pure is called
shared s = shared S(1);       // this(int) pure is called
S m = S([1,2,3]);             // this(int[]) immutable pure is called

Disabling Default Struct Construction

If a struct constructor is annotated with @disable and has an empty ParameterList, the struct has disabled default construction. The only way it can be constructed is via a call to another constructor with a non-empty ParameterList.

A struct with a disabled default constructor, and no other constructors, cannot be instantiated other than via a VoidInitializer.

A disabled default constructor may not have a FunctionBody.

If any fields have disabled default construction, struct default construction is also disabled.

struct S
{
int x;

// Disables default construction
@disable this();

this(int v) { x = v; }
}
struct T
{
int y;
S s;
}
void main()
{
S s;          // error: default construction is disabled
S t = S();    // error: also disabled
S u = S(1);   // constructed by calling `S.this(1)`
S v = void;   // not initialized, but allowed
S w = { 1 };  // error: cannot use { } since constructor exists
S[3] a;       // error: default construction is disabled
S[3] b = [S(1), S(20), S(-2)]; // ok
T t;          // error: default construction is disabled
}

Best Practices: Disabling default construction is useful when the default value, such as null, is not acceptable.

Field initialization inside a constructor

In a constructor body, if a delegating constructor is called, all field assignments are considered assignments. Otherwise, the first instance of field assignment is its initialization, and assignments of the form field = expression are treated as equivalent to typeof(field)(expression). The values of fields may be read before initialization or construction with a delegating constructor.

struct S
{
int num;
int ber;
this(int i)
{
num = i + 1;   // initialization
num = i + 2;   // assignment
ber = ber + 1; // ok to read before initialization
}
this(int i, int j)
{
this(i);
num = i + 1;  // assignment
}
}

If the field type has an opAssign method, it will not be used for initialization.

struct A
{
this(int n) {}
void opAssign(A rhs) {}
}
struct S
{
A val;
this(int i)
{
val = A(i);  // val is initialized to the value of A(i)
val = A(2);  // rewritten to val.opAssign(A(2))
}
}

If the field type is not mutable, multiple initialization will be rejected.

struct S
{
immutable int num;
this(int)
{
num = 1;  // OK
num = 2;  // Error: assignment to immutable
}
}

If the field is initialized on one path, it must be initialized on all paths.

struct S
{
immutable int num;
immutable int ber;
this(int i)
{
if (i)
num = 3;   // initialization
else
num = 4;   // initialization
}
this(long j)
{
j ? (num = 3) : (num = 4); // ok
j || (ber = 3);  // Error: initialized on only one path
j && (ber = 3);  // Error: initialized on only one path
}
}

A field initialization may not appear in a loop or after a label.

struct S
{
immutable int num;
immutable string str;
this(int j)
{
foreach (i; 0..j)
{
num = 1;    // Error: field initialization not allowed in loops
}
size_t i = 0;
Label:
str = "hello";  // Error: field initialization not allowed after labels
if (i++ < 2)
goto Label;
}
this(int j, int k)
{
switch (j)
{
case 1: ++j; break;
default: break;
}
num = j;        // Error: `case` and `default` are also labels
}
}

If a field's type has disabled default construction, then it must be initialized in the constructor.

struct S { int y; @disable this(); }

struct T
{
S s;
this(S t) { s = t; }       // ok
this(int i) { this('c'); } // ok
this(char) { }             // Error: s not initialized
}

Struct Copy Constructors

Copy constructors are used to initialize a struct instance from another struct of the same type.

A constructor declaration is a copy constructor declaration if and only if it is a constructor declaration that takes only one non-default parameter by reference that is of the same type as typeof(this), followed by any number of default parameters:

struct A
{
this(ref return scope A rhs) {}                        // copy constructor
this(ref return scope const A rhs, int b = 7) {}       // copy constructor with default parameter
}

The copy constructor is type checked as a normal constructor.

If a copy constructor is defined, implicit calls to it will be inserted in the following situations:

1. When a variable is explicitly initialized:

struct A
{
this(ref return scope A rhs) {}
}

void main()
{
A a;
A b = a; // copy constructor gets called
}

2. When a parameter is passed by value to a function:

struct A
{
this(ref return scope A another) {}
}

void fun(A a) {}

void main()
{
A a;
fun(a);    // copy constructor gets called
}

3. When a parameter is returned by value from a function and Named Returned Value Optiomization (NRVO) cannot be performed:

struct A
{
this(ref return scope A another) {}
}

A fun()
{
A a;
return a;       // NRVO, no copy constructor call
}

A a;
A gun()
{
return a;       // cannot perform NRVO, rewrite to: return (A __tmp; __tmp.copyCtor(a));
}

void main()
{
A a = fun();
A b = gun();
}

When a copy constructor is defined for a struct (or marked @disable), the compiler no longer implicitly generates default copy/blitting constructors for that struct:

struct A
{
int[] a;
this(ref return scope A rhs) {}
}

void fun(immutable A) {}

void main()
{
immutable A a;
fun(a);        // error: copy constructor cannot be called with types (immutable) immutable
}

If a union S has fields that define a copy constructor, whenever an object of type S is initialized by copy, an error will be issued. The same rule applies to overlapped fields (anonymous unions).

A struct that defines a copy constructor is not a POD.

Copy Constructor Attributes

The copy constructor can be overloaded with different qualifiers applied to the parameter (copying from a qualified source) or to the copy constructor itself (copying to a qualified destination):

struct A
{
this(ref return scope A another) {}                        // 1 - mutable source, mutable destination
this(ref return scope immutable A another) {}              // 2 - immutable source, mutable destination
this(ref return scope A another) immutable {}              // 3 - mutable source, immutable destination
this(ref return scope immutable A another) immutable {}    // 4 - immutable source, immutable destination
}

void main()
{
A a;
immutable A ia;

A a2 = a;      // calls 1
A a3 = ia;     // calls 2
immutable A a4 = a;     // calls 3
immutable A a5 = ia;    // calls 4
}

The inout qualifier may be applied to the copy constructor parameter in order to specify that mutable, const, or immutable types are treated the same:

struct A
{
this(ref return scope inout A rhs) immutable {}
}

void main()
{
A r1;
const(A) r2;
immutable(A) r3;

// All call the same copy constructor because `inout` acts like a wildcard
immutable(A) a = r1;
immutable(A) b = r2;
immutable(A) c = r3;
}

Implicit Copy Constructors

A copy constructor is generated implicitly by the compiler for a struct S if all of the following conditions are met:

1. S does not explicitly declare any copy constructors;

2. S defines at least one direct member that has a copy constructor, and that member is not overlapped (by means of union) with any other member.

If the restrictions above are met, the following copy constructor is generated:

this(ref return scope inout(S) src) inout
{
foreach (i, ref inout field; src.tupleof)
this.tupleof[i] = field;
}

If the generated copy constructor fails to type check, it will receive the @disable attribute.

Struct Postblits

Postblit:
this ( this ) MemberFunctionAttributesopt ;
this ( this ) MemberFunctionAttributesopt FunctionBody

WARNING: The postblit is considered legacy and is not recommended for new code. Code should use copy constructors defined in the previous section. For backward compatibility reasons, a struct that explicitly defines both a copy constructor and a postblit will only use the postblit for implicit copying. However, if the postblit is disabled, the copy constructor will be used. If a struct defines a copy constructor (user-defined or generated) and has fields that define postblits, a deprecation will be issued, informing that the postblit will have priority over the copy constructor.

Copy construction is defined as initializing a struct instance from another struct of the same type. Copy construction is divided into two parts:

1. blitting the fields, i.e. copying the bits

2. running postblit on the result

The first part is done automatically by the language, the second part is done if a postblit function is defined for the struct. The postblit has access only to the destination struct object, not the source. Its job is to ‘fix up’ the destination as necessary, such as making copies of referenced data, incrementing reference counts, etc. For example:

struct S
{
int[] a;    // array is privately owned by this instance
this(this)
{
a = a.dup;
}
}

Disabling struct postblit makes the object not copyable.

struct T
{
@disable this(this);  // disabling makes T not copyable
}
struct S
{
T t;   // uncopyable member makes S also not copyable
}

void main()
{
S s;
S t = s; // error, S is not copyable
}

Depending on the struct layout, the compiler may generate the following internal postblit functions:

1. void __postblit(). The compiler assigns this name to the explicitly defined postblit this(this) so that it can be treated exactly as a normal function. Note that if a struct defines a postblit, it cannot define a function named __postblit - no matter the signature - as this would result in a compilation error due to the name conflict.

2. void __fieldPostblit(). If a struct X has at least one struct member that in turn defines (explicitly or implicitly) a postblit, then a field postblit is generated for X that calls all the underlying postblits of the struct fields in declaration order.

3. void __aggrPostblit(). If a struct has an explicitly defined postblit and at least 1 struct member that has a postblit (explicit or implicit) an aggregated postblit is generated which calls __fieldPostblit first and then __postblit.

4. void __xpostblit(). The field and aggregated postblits, although generated for a struct, are not actual struct members. In order to be able to call them, the compiler internally creates an alias, called __xpostblit which is a member of the struct and which points to the generated postblit that is the most inclusive.

// struct with alias __xpostblit = __postblit
struct X
{
this(this) {}
}

// struct with alias __xpostblit = __fieldPostblit
// which contains a call to X.__xpostblit
struct Y
{
X a;
}

// struct with alias __xpostblit = __aggrPostblit which contains
// a call to Y.__xpostblit and a call to Z.__postblit
struct Z
{
Y a;
this(this) {}
}

void main()
{
// X has __postblit and __xpostblit (pointing to __postblit)
static assert(__traits(hasMember, X, "__postblit"));
static assert(__traits(hasMember, X, "__xpostblit"));

// Y does not have __postblit, but has __xpostblit (pointing to __fieldPostblit)
static assert(!__traits(hasMember, Y, "__postblit"));
static assert(__traits(hasMember, Y, "__xpostblit"));
// __fieldPostblit is not a member of the struct
static assert(!__traits(hasMember, Y, "__fieldPostblit"));

// Z has  __postblit and __xpostblit (pointing to __aggrPostblit)
static assert(__traits(hasMember, Z, "__postblit"));
static assert(__traits(hasMember, Z, "__xpostblit"));
// __aggrPostblit is not a member of the struct
static assert(!__traits(hasMember, Z, "__aggrPostblit"));
}

Neither of the above postblits is defined for structs that don't define this(this) and don't have fields that transitively define it. If a struct does not define a postblit (implicit or explicit) but defines functions that use the same name/signature as the internally generated postblits, the compiler is able to identify that the functions are not actual postblits and does not insert calls to them when the struct is copied. Example:

struct X
{}

int a;

struct Y
{
int a;
X b;
void __fieldPostPostblit()
{
a = 42;
}
}

void main()
{
static assert(!__traits(hasMember, X, "__postblit"));
static assert(!__traits(hasMember, X, "__xpostblit"));

static assert(!__traits(hasMember, Y, "__postblit"));
static assert(!__traits(hasMember, Y, "__xpostblit"));

Y y;
auto y2 = y;
assert(a == 0); // __fieldPostBlit does not get called
}

Postblits cannot be overloaded. If two or more postblits are defined, even if the signatures differ, the compiler assigns the __postblit name to both and later issues a conflicting function name error:

struct X
{
this(this) {}
this(this) const {} // error: function X.__postblit conflicts with function X.__postblit
}

The following describes the behavior of the qualified postblit definitions:

1. const. When a postblit is qualified with const as in this(this) const; or const this(this); then the postblit is successfully called on mutable (unqualified), const, and immutable objects, but the postblit cannot modify the object because it regards it as const; hence const postblits are of limited usefulness. Example:

struct S
{
int n;
this(this) const
{
import std.stdio : writeln;
writeln("postblit called");
//++n; // error: cannot modify this.n in `const` function
}
}

void main()
{
S s1;
auto s2 = s1;
const S s3;
auto s4 = s3;
immutable S s5;
auto s6 = s5;
}

2. immutable. When a postblit is qualified with immutable as in this(this) immutable or immutable this(this) the code is ill-formed. The immutable postblit passes the compilation phase but cannot be invoked. Example:

struct Y
{
// not invoked anywhere, no error is issued
this(this) immutable
{ }
}

struct S
{
this(this) immutable
{ }
}

void main()
{
S s1;
auto s2 = s1;    // error: immutable method `__postblit` is not callable using a mutable object
const S s3;
auto s4 = s3;    // error: immutable method `__postblit` is not callable using a mutable object
immutable S s5;
auto s6 = s5;    // error: immutable method `__postblit` is not callable using a mutable object
}

3. shared. When a postblit is qualified with shared as in this(this) shared or shared this(this) solely shared objects may invoke the postblit; attempts of postbliting unshared objects will result in compile time errors:

struct S
{
this(this) shared
{ }
}

void main()
{
S s1;
auto s2 = s1;    // error: shared method `__postblit` is not callable using a non-shared object
const S s3;
auto s4 = s3;    // error: shared method `__postblit` is not callable using a non-shared object
immutable S s5;
auto s6 = s5;    // error: shared method `__postblit` is not callable using a non-shared object

// calling the shared postblit on a shared object is accepted
shared S s7;
auto s8 = s7;
}

An unqualified postblit will get called even if the struct is instantiated as immutable or const, but the compiler issues an error if the struct is instantiated as shared:

struct S
{
int n;
this(this) { ++n; }
}

void main()
{
immutable S a;      // shared S a; => error : non-shared method is not callable using a shared object
auto a2 = a;
import std.stdio: writeln;
writeln(a2.n);     // prints 1
}

From a postblit perspective, qualifiying the struct definition yields the same result as explicitly qualifying the postblit.

The following table lists all the possibilities of grouping qualifiers for a postblit associated with the type of object that needs to be used in order to successfully invoke the postblit:

Qualifier Groups

object type to be invoked on

const

immutable

shared

any object type

✔

uncallable

✔

shared object

✔

uncallable

✔

✔

shared object

✔

✔

uncallable

✔

✔

uncallable

✔

✔

✔

Note that when const and immutable are used to explicitly qualify a postblit as in this(this) const immutable; or const immutable this(this); - the order in which the qualifiers are declared does not matter - the compiler generates a conflicting attribute error, however declaring the struct as const/immutable and the postblit as immutable/const achieves the effect of applying both qualifiers to the postblit. In both cases the postblit is qualified with the more restrictive qualifier, which is immutable.

The postblits __fieldPostblit and __aggrPostblit are generated without any implicit qualifiers and are not considered struct members. This leads to the situation where qualifying an entire struct declaration with const or immutable does not have any impact on the above-mentioned postblits. However, since __xpostblit is a member of the struct and an alias of one of the other postblits, the qualifiers applied to the struct will affect the aliased postblit.

struct S
{
this(this)
{ }
}

// `__xpostblit` aliases the aggregated postblit so the `const` applies to it.
// However, the aggregated postblit calls the field postblit which does not have
// any qualifier applied, resulting in a qualifier mismatch error
const struct B
{
S a;        // error : mutable method B.__fieldPostblit is not callable using a const object
this(this)
{ }
}

// `__xpostblit` aliases the field postblit; no error
const struct B2
{
S a;
}

// Similar to B
immutable struct C
{
S a;        // error : mutable method C.__fieldPostblit is not callable using a immutable object
this(this)
{ }
}

// Similar to B2, compiles
immutable struct C2
{
S a;
}

In the above situations the errors do not contain line numbers because the errors are regarding generated code.

Qualifying an entire struct as shared correctly propagates the attribute to the generated postblits:

shared struct A
{
this(this)
{
import std.stdio : writeln;
writeln("the shared postblit was called");
}
}

struct B
{
A a;
}

void main()
{
shared B b1;
auto b2 = b1;
}

Unions may have fields that have postblits. However, a union itself never has a postblit. Copying a union does not result in postblit calls for any fields. If those calls are desired, they must be inserted explicitly by the programmer:

struct S
{
int count;
this(this)
{
++count;
}
}

union U
{
S s;
}

void main()
{
U a = U.init;
U b = a;
assert(b.s.count == 0);
b.s.__postblit;
assert(b.s.count == 1);
}

Struct Destructors

Destructors are called when an object goes out of scope. Their purpose is to free up resources owned by the struct object.

Unions may have fields that have destructors. However, a union itself never has a destructor. When a union goes out of scope, destructors for it's fields are not called. If those calls are desired, they must be inserted explicitly by the programmer:

struct S
{
~this()
{
import std.stdio;
writeln("S is being destructed");
}
}

union U
{
S s;
}

void main()
{
import std.stdio;
{
writeln("entering first scope");
U u = U.init;
scope (exit) writeln("exiting first scope");
}
{
writeln("entering second scope");
U u = U.init;
scope (exit)
{
writeln("exiting second scope");
destroy(u.s);
}
}
}

Struct Invariants

Invariant:
invariant ( ) BlockStatement
invariant BlockStatement
invariant ( AssertArguments ) ;

Struct Invariants specify the relationships among the members of a struct instance. Those relationships must hold for any interactions with the instance from its public interface.

The invariant is in the form of a const member function. The invariant is defined to hold if all the AssertExpressions within the invariant that are executed succeed.

If the invariant does not hold, then the program enters an invalid state.

Any invariants for fields are applied before the struct invariant.

There may be multiple invariants in a struct. They are applied in lexical order.

Struct Invariants must hold at the exit of the struct constructor (if any), and at the entry of the struct destructor (if any).

Struct Invariants must hold at the entry and exit of all public or exported non-static member functions. The order of application of invariants is:

1. preconditions

2. invariant

3. function body

4. invariant

5. postconditions

The invariant need not hold if the struct instance is implicitly constructed using the default .init value.

struct Date
{
this(int d, int h)
{
day = d;    // days are 1..31
hour = h;   // hours are 0..23
}

invariant
{
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}

private:
int day;
int hour;
}

Public or exported non-static member functions cannot be called from within an invariant.

struct Foo
{
public void f() { }
private void g() { }

invariant
{
f();  // error, cannot call public member function from invariant
g();  // ok, g() is not public
}
}

Undefined Behavior: happens if the invariant does not hold and execution continues.

Implementation Defined:

1. Whether the struct Invariant is executed at runtime or not. This is typically controlled with a compiler switch.

2. The behavior when the invariant does not hold is typically the same as for when AssertExpressions fail.

Best Practices:

1. Do not indirectly call exported or public member functions within a struct invariant, as this can result in infinite recursion.

2. Avoid reliance on side effects in the invariant. as the invariant may or may not be executed.

3. Avoid having mutable public fields of structs with invariants, as then the invariant cannot verify the public interface.

Identity Assignment Overload

While copy construction takes care of initializing an object from another object of the same type, assignment is defined as copying the contents of a source object over those of a destination object, calling the destination object's destructor if it has one in the process:

struct S { ... }  // S has postblit or destructor
S s;      // default construction of s
S t = s;  // t is copy-constructed from s
t = s;    // t is assigned from s

Struct assignment t=s is defined to be semantically equivalent to:

t.opAssign(s);

where opAssign is a member function of S:

ref S opAssign(ref S s)
{
S tmp = this;   // bitcopy this into tmp
this = s;       // bitcopy s into this
tmp.__dtor();   // call destructor on tmp
return this;
}

An identity assignment overload is required for a struct if one or more of these conditions hold:

- it has a destructor

- it has a postblit

- it has a field with an identity assignment overload

If an identity assignment overload is required and does not exist, an identity assignment overload function of the type ref S opAssign(ref S) will be automatically generated.

A user-defined one can implement the equivalent semantics, but can be more efficient.

One reason a custom opAssign might be more efficient is if the struct has a reference to a local buffer:

struct S
{
int[] buf;
int a;

ref S opAssign(ref const S s) return
{
a = s.a;
return this;
}

this(this)
{
buf = buf.dup;
}
}

Here, S has a temporary workspace buf[]. The normal postblit will pointlessly free and reallocate it. The custom opAssign will reuse the existing storage.

Nested Structs

A struct is a nested struct if

1. it is declared inside the scope of a function, or

2. it is a templated struct with one or more template arguments that alias local functions.

A nested struct can have member functions. It has access to the context of its enclosing scope via a hidden field.

void foo()
{
int i = 7;
struct SS
{
int x,y;
int bar() { return x + i + 1; }
}
SS s;
s.x = 3;
s.bar(); // returns 11
}

The static attribute will prevent a struct from being nested. As such, the struct will not have access to its enclosing scope.

void foo()
{
int i = 7;
static struct SS
{
int x, y;
int bar()
{
return i; // error, SS is not a nested struct
}
}
}

Unions and Special Member Functions

Unions may not have postblits, destructors, or invariants.

Classes

The object-oriented features of D all come from classes. The class hierarchy has as its root the class Object. Object defines a minimum level of functionality that each derived class has, and a default implementation for that functionality.

Classes are programmer defined types. Support for classes are what make D an object oriented language, giving it encapsulation, inheritance, and polymorphism. D classes support the single inheritance paradigm, extended by adding support for interfaces. Class objects are instantiated by reference only.

A class can be exported, which means its name and all its non-private members are exposed externally to the DLL or EXE.

A class declaration is defined:

ClassDeclaration:
class Identifier ;
class Identifier BaseClassListopt AggregateBody
ClassTemplateDeclaration

BaseClassList:
: SuperClassOrInterface
: SuperClassOrInterface , Interfaces

SuperClassOrInterface:
BasicType

Interfaces:
Interface
Interface , Interfaces

Interface:
BasicType

Classes consist of:

- a super class

- interfaces

- a nested anonymous metaclass for classes declared with extern (Objective-C)

- dynamic fields

- static fields

- types

- an optional synchronized attribute

- member functions

- static member functions

- Virtual Functions

- Constructors

- Destructors

- Static Constructors

- Static Destructors

- SharedStaticConstructors

- SharedStaticDestructors

- Class Invariants

- Unit Tests

- Class Allocators

- Alias This

A class is defined:

class Foo
{
... members ...
}

Note that there is no trailing ; after the closing } of the class definition. It is also not possible to declare a variable var like:

class Foo { } var;

Instead:

class Foo { }
Foo var;

Access Control

Access to class members is controlled using visibility attributes. The default visibility attribute is public.

Fields

Class members are always accessed with the . operator.

Members of a base class can be accessed by prepending the name of the base class followed by a dot:

class A { int a; int a2;}
class B : A { int a; }

void foo(B b)
{
b.a = 3;   // accesses field B.a
b.a2 = 4;  // accesses field A.a2
b.A.a = 5; // accesses field A.a
}

The D compiler is free to rearrange the order of fields in a class to optimally pack them in an implementation-defined manner. Consider the fields much like the local variables in a function - the compiler assigns some to registers and shuffles others around all to get the optimal stack frame layout. This frees the code designer to organize the fields in a manner that makes the code more readable rather than being forced to organize it according to machine optimization rules. Explicit control of field layout is provided by struct/union types, not classes.

Fields of extern(Objective-C) classes have a dynamic offset. That means that the base class can change (add or remove instance variables) without the subclasses needing to recompile or relink.

Field Properties

The .offsetof property gives the offset in bytes of the field from the beginning of the class instantiation. .offsetof is not available for fields of extern(Objective-C) classes due to their fields having a dynamic offset. .offsetof can only be applied to expressions which produce the type of the field itself, not the class type:

class Foo
{
int x;
}
...
void test(Foo foo)
{
size_t o;

o = Foo.x.offsetof; // error, Foo.x needs a 'this' reference
o = foo.x.offsetof; // ok
}

Class Properties

The .tupleof property is an expression sequence of all the fields in the class, excluding the hidden fields and the fields in the base class. .tupleof is not available for extern(Objective-C) classes due to their fields having a dynamic offset.

class Foo { int x; long y; }

void test(Foo foo)
{
import std.stdio;
static assert(typeof(foo.tupleof).stringof == `(int, long)`);

foo.tupleof[0] = 1; // set foo.x to 1
foo.tupleof[1] = 2; // set foo.y to 2
foreach (x; foo.tupleof)
write(x);       // prints 12
}

The properties .__vptr and .__monitor give access to the class object's vtbl[] and monitor, respectively, but should not be used in user code.

Super Class
All classes inherit from a super class. If one is not specified, it inherits from Object. Object forms the root of the D class inheritance hierarchy.
Member Functions (a.k.a. Methods)

Non-static member functions have an extra hidden parameter called this through which the class object's other members can be accessed.

Non-static member functions can have, in addition to the usual FunctionAttributes, the attributes const, immutable, shared, or inout. These attributes apply to the hidden this parameter.

class C
{
int a;
const void foo()
{
a = 3; // error, 'this' is const
}
void foo() immutable
{
a = 3; // error, 'this' is immutable
}
}

Objective-C linkage

Static member functions with Objective-C linkage also have an extra hidden parameter called this through which the class object's other members can be accessed.

Member functions with Objective-C linkage have an additional hidden, anonymous, parameter which is the selector the function was called with.

Static member functions with Objective-C linkage are placed in the hidden nested metaclass as non-static member functions.

Synchronized Classes

All member functions of synchronized classes are synchronized. A static member function is synchronized on the classinfo object for the class, which means that one monitor is used for all static member functions for that synchronized class. For non-static functions of a synchronized class, the monitor used is part of the class object. For example:

synchronized class Foo
{
void bar() { ...statements... }
}

is equivalent to (as far as the monitors go):

synchronized class Foo
{
void bar()
{
synchronized (this) { ...statements... }
}
}

Member functions of non-synchronized classes can be individually marked as synchronized.

class Foo
{
synchronized void foo() { }  // foo is synchronized
}

Member fields of a synchronized class cannot be public:

synchronized class Foo
{
int foo;  // disallowed: public field
}

synchronized class Bar
{
private int bar;  // ok
}

The synchronized attribute can only be applied to classes, structs cannot be marked to be synchronized.

Constructors

Constructor:
this Parameters MemberFunctionAttributesopt FunctionBody
ConstructorTemplate

Fields are by default initialized to the default initializer for their type (usually 0 for integer types and NAN for floating point types). If the field declaration has an optional Initializer that will be used instead of the default.

class Abc
{
int a;      // default initializer for a is 0
long b = 7; // default initializer for b is 7
float f;    // default initializer for f is NAN
}

The Initializer is evaluated at compile time.

This initialization is done before any constructors are called.

Constructors are defined with a function name of this and have no return value:

class Foo
{
this(int x)  // declare constructor for Foo
{   ...
}
this()
{   ...
}
}

Base class construction is done by calling the base class constructor by the name super:

class A { this(int y) { } }

class B : A
{
int j;
this()
{
...
super(3);  // call base constructor A.this(3)
...
}
}

Delegating Constructors

A constructor can call another constructor for the same class in order to share common initializations. This is called a delegating constructor:

class C
{
int j;
this()
{
...
}
this(int i)
{
this(); // delegating constructor call
j = i;
}
}

The following restrictions apply:

1. It is illegal for constructors to mutually call each other.

this() { this(1); }
this(int i) { this(); } // illegal, cyclic constructor calls

Implementation Defined: The compiler is not required to detect cyclic constructor calls.

Undefined Behavior: If the program executes with cyclic constructor calls.

2. If a constructor's code contains a delegating constructor call, all possible execution paths through the constructor must make exactly one delegating constructor call:

this() { a || super(); }       // illegal

this() { (a) ? this(1) : super(); }     // ok

this()
{
for (...)
{
super();  // illegal, inside loop
}
}

3. It is illegal to refer to this implicitly or explicitly prior to making a delegating constructor call.

4. Delegating constructor calls cannot appear after labels.

Implicit Base Class Construction

If there is no constructor for a class, but there is a constructor for the base class, a default constructor is implicitly generated with the form:

this() { }

If no calls to a delegating constructor or super appear in a constructor, and the base class has a nullary constructor, a call to super() is inserted at the beginning of the constructor. If that base class has a constructor that requires arguments and no nullary constructor, a matching call to super is required.

Class Instantiation

Instances of class objects are created with a NewExpression:

A a = new A(3);

The following steps happen:

1. Storage is allocated for the object. If this fails, rather than return null, an OutOfMemoryError is thrown. Thus, tedious checks for null references are unnecessary.

2. The raw data is statically initialized using the values provided in the class definition. The pointer to the vtbl[] (the array of pointers to virtual functions) is assigned. Constructors are passed fully formed objects for which virtual functions can be called. This operation is equivalent to doing a memory copy of a static version of the object onto the newly allocated one.

3. If there is a constructor defined for the class, the constructor matching the argument list is called.

4. If a delegating constructor is not called, a call to the base class's default constructor is issued.

5. The body of the constructor is executed.

6. If class invariant checking is turned on, the class invariant is called at the end of the constructor.

Constructor Attributes

Constructors can have one of these member function attributes: const, immutable, and shared. Construction of qualified objects will then be restricted to the implemented qualified constructors.

class C
{
this();   // non-shared mutable constructor
}

// create mutable object
C m = new C();

// create const object using mutable constructor
const C c2 = new const C();

// a mutable constructor cannot create an immutable object
// immutable C i = new immutable C();

// a mutable constructor cannot create a shared object
// shared C s = new shared C();

Constructors can be overloaded with different attributes.

class C
{
this();               // non-shared mutable constructor
this() shared;        // shared mutable constructor
this() immutable;     // immutable constructor
}

C m = new C();
shared s = new shared C();
immutable i = new immutable C();

Pure Constructors

If the constructor can create a unique object (e.g. if it is pure), the object can be implicitly convertible to any qualifiers.

class C
{
this() pure;
// Based on the definition, this creates a mutable object. But the
// created object cannot contain any mutable global data.
// Therefore the created object is unique.

this(int[] arr) immutable pure;
// Based on the definition, this creates an immutable object. But
// the argument int[] never appears in the created object so it
// isn't implicitly convertible to immutable. Also, it cannot store
// any immutable global data.
// Therefore the created object is unique.
}

immutable i = new immutable C();           // this() pure is called
shared s = new shared C();                 // this() pure is called
C m = new C([1,2,3]);       // this(int[]) immutable pure is called

Field initialization inside a constructor

In a constructor body, the first instance of field assignment is its initialization.

class C
{
int num;
this()
{
num = 1;  // initialization
num = 2;  // assignment
}
}

If the field type has an opAssign method, it will not be used for initialization.

struct A
{
this(int n) {}
void opAssign(A rhs) {}
}
class C
{
A val;
this()
{
val = A(1);  // val is initialized to the value of A(1)
val = A(2);  // rewritten to val.opAssign(A(2))
}
}

If the field type is not mutable, multiple initialization will be rejected.

class C
{
immutable int num;
this()
{
num = 1;  // OK
num = 2;  // Error: multiple field initialization
}
}

If the field is initialized on one path, it must be initialized on all paths.

class C
{
immutable int num;
immutable int ber;
this(int i)
{
if (i)
num = 3;   // initialization
else
num = 4;   // initialization
}
this(long j)
{
j ? (num = 3) : (num = 4); // ok
j || (ber = 3);  // error, intialized on only one path
j && (ber = 3);  // error, intialized on only one path
}
}

A field initialization may not appear in a loop or after a label.

class C
{
immutable int num;
immutable string str;
this()
{
foreach (i; 0..2)
{
num = 1;    // Error: field initialization not allowed in loops
}
size_t i = 0;
Label:
str = "hello";  // Error: field initialization not allowed after labels
if (i++ < 2)
goto Label;
}
}

If a field's type has disabled default construction, then it must be initialized in the constructor.

struct S { int y; @disable this(); }

class C
{
S s;
this(S t) { s = t; }    // ok
this(int i) { this(); } // ok
this() { }              // error, s not initialized
}

Destructors

Destructor:
~ this ( ) MemberFunctionAttributesopt FunctionBody

The garbage collector calls the destructor function when the object is deleted. The syntax is:

class Foo
{
~this() // destructor for Foo
{
}
}

There can be only one destructor per class, the destructor does not have any parameters, and has no attributes. It is always virtual.

The destructor is expected to release any resources held by the object.

The program can explicitly inform the garbage collector that an object is no longer referred to with destroy, and then the garbage collector calls the destructor immediately. The destructor is guaranteed to never be called twice.

The destructor for the super class automatically gets called when the destructor ends. There is no way to call the super destructor explicitly.

The garbage collector is not guaranteed to run the destructor for all unreferenced objects. Furthermore, the order in which the garbage collector calls destructors for unreferenced objects is not specified. This means that when the garbage collector calls a destructor for an object of a class that has members which are references to garbage collected objects, those references may no longer be valid. This means that destructors cannot reference sub objects. This rule does not apply to auto objects or objects destructed with destroy, as the destructor is not being run by the garbage collector, meaning all references are valid.

Objects referenced from the data segment never get collected by the gc.

Static Constructors

StaticConstructor:
static this ( ) MemberFunctionAttributesopt ;
static this ( ) MemberFunctionAttributesopt FunctionBody

A static constructor is a function that performs initializations of thread local data before the main() function gets control for the main thread, and upon thread startup.

Static constructors are used to initialize static class members with values that cannot be computed at compile time.

Static constructors in other languages are built implicitly by using member initializers that can't be computed at compile time. The trouble with this stems from not having good control over exactly when the code is executed, for example:

class Foo
{
static int a = b + 1;
static int b = a * 2;
}

What values do a and b end up with, what order are the initializations executed in, what are the values of a and b before the initializations are run, is this a compile error, or is this a runtime error? Additional confusion comes from it not being obvious if an initializer is static or dynamic.
D makes this simple. All member initializations must be determinable by the compiler at compile time, hence there is no order-of-evaluation dependency for member initializations, and it is not possible to read a value that has not been initialized. Dynamic initialization is performed by a static constructor, defined with a special syntax static this().

class Foo
{
static int a;         // default initialized to 0
static int b = 1;
static int c = b + a; // error, not a constant initializer

static this()    // static constructor
{
a = b + 1;          // a is set to 2
b = a * 2;          // b is set to 4
}
}

If main() or the thread returns normally, (does not throw an exception), the static destructor is added to the list of functions to be called on thread termination.

Static constructors have empty parameter lists.

Static constructors within a module are executed in the lexical order in which they appear. All the static constructors for modules that are directly or indirectly imported are executed before the static constructors for the importer.

The static in the static constructor declaration is not an attribute, it must appear immediately before the this:

class Foo
{
static this() { ... } // a static constructor
static private this() { ... } // not a static constructor
static
{
this() { ... }      // not a static constructor
}
static:
this() { ... }      // not a static constructor
}

Static Destructors

StaticDestructor:
static ~ this ( ) MemberFunctionAttributesopt ;
static ~ this ( ) MemberFunctionAttributesopt FunctionBody

A static destructor is defined as a special static function with the syntax static ~this().

class Foo
{
static ~this() // static destructor
{
}
}

A static destructor gets called on thread termination, but only if the static constructor completed successfully. Static destructors have empty parameter lists. Static destructors get called in the reverse order that the static constructors were called in.

The static in the static destructor declaration is not an attribute, it must appear immediately before the ~this:

class Foo
{
static ~this() { ... }  // a static destructor
static private ~this() { ... } // not a static destructor
static
{
~this() { ... }  // not a static destructor
}
static:
~this() { ... }  // not a static destructor
}

Shared Static Constructors

SharedStaticConstructor:
shared static this ( ) MemberFunctionAttributesopt ;
shared static this ( ) MemberFunctionAttributesopt FunctionBody

Shared static constructors are executed before any StaticConstructors, and are intended for initializing any shared global data.

Shared Static Destructors

SharedStaticDestructor:
shared static ~ this ( ) MemberFunctionAttributesopt ;
shared static ~ this ( ) MemberFunctionAttributesopt FunctionBody

Shared static destructors are executed at program termination in the reverse order that SharedStaticConstructors were executed.

Class Invariants

Invariant:
invariant ( ) BlockStatement
invariant BlockStatement
invariant ( AssertArguments ) ;

Class Invariants specify the relationships among the members of a class instance. Those relationships must hold for any interactions with the instance from its public interface.

The invariant is in the form of a const member function. The invariant is defined to hold if all the AssertExpressions within the invariant that are executed succeed.

If the invariant does not hold, then the program enters an invalid state.

Any class invariants for base classes are applied before the class invariant for the derived class.

There may be multiple invariants in a class. They are applied in lexical order.

Class Invariants must hold at the exit of the class constructor (if any), at the entry of the class destructor (if any).

Class Invariants must hold at the entry and exit of all public or exported non-static member functions. The order of application of invariants is:

1. preconditions

2. invariant

3. function body

4. invariant

5. postconditions

The invariant need not hold if the class instance is implicitly constructed using the default .init value.

class Date
{
this(int d, int h)
{
day = d;    // days are 1..31
hour = h;   // hours are 0..23
}

invariant
{
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}

private:
int day;
int hour;
}

Public or exported non-static member functions cannot be called from within an invariant.

class Foo
{
public void f() { }
private void g() { }

invariant
{
f();  // error, cannot call public member function from invariant
g();  // ok, g() is not public
}
}

Undefined Behavior: happens if the invariant does not hold and execution continues.

Implementation Defined:

1. Whether the class Invariant is executed at runtime or not. This is typically controlled with a compiler switch.

2. The behavior when the invariant does not hold is typically the same as for when AssertExpressions fail.

Best Practices:

1. Do not indirectly call exported or public member functions within a class invariant, as this can result in infinite recursion.

2. Avoid reliance on side effects in the invariant. as the invariant may or may not be executed.

3. Avoid having mutable public fields of classes with invariants, as then the invariant cannot verify the public interface.

Class Allocators
Note: Class allocators are deprecated in D2.

Allocator:
new Parameters FunctionBody

A class member function of the form:

new(size_t size)
{
...
}

is called a class allocator. The class allocator can have any number of parameters, provided the first one is of type size_t. Any number can be defined for a class, the correct one is determined by the usual function overloading rules. When a new expression:

new Foo;

is executed, and Foo is a class that has an allocator, the allocator is called with the first argument set to the size in bytes of the memory to be allocated for the instance. The allocator must allocate the memory and return it as a void*. If the allocator fails, it must not return a null, but must throw an exception. If there is more than one parameter to the allocator, the additional arguments are specified within parentheses after the new in the NewExpression:

class Foo
{
this(string a) { ... }

new(size_t size, int x, int y)
{
...
}
}

...

new(1,2) Foo(a);        // calls new(Foo.sizeof,1,2)

Derived classes inherit any allocator from their base class, if one is not specified.

The class allocator is not called if the instance is created on the stack.

See also Explicit Class Instance Allocation.

Alias This

AliasThis:
alias Identifier this ;

An AliasThis declaration names a member to subtype. The Identifier names that member.

A class or struct can be implicitly converted to the AliasThis member.

struct S
{
int x;
alias x this;
}

int foo(int i) { return i * 2; }

void test()
{
S s;
s.x = 7;
int i = -s;  // i == -7
i = s + 8;   // i == 15
i = s + s;   // i == 14
i = 9 + s;   // i == 16
i = foo(s);  // implicit conversion to int
}

If the member is a class or struct, undefined lookups will be forwarded to the AliasThis member.

struct Foo
{
int baz = 4;
int get() { return 7; }
}

class Bar
{
Foo foo;
alias foo this;
}

void test()
{
auto bar = new Bar;
int i = bar.baz; // i == 4
i = bar.get(); // i == 7
}

If the Identifier refers to a property member function with no parameters, conversions and undefined lookups are forwarded to the return value of the function.

struct S
{
int x;
@property int get()
{
return x * 2;
}
alias get this;
}

void test()
{
S s;
s.x = 2;
int i = s; // i == 4
}

If an aggregate declaration defines an opCmp or opEquals method, it will take precedence to that of the aliased this member. Note that, unlike an opCmp method, an opEquals method is implicitly defined for a struct declaration if a user defined one isn't provided; this means that if the aliased this member opEquals is preferred it should be explicitly defined:

struct S
{
int a;
bool opEquals(S rhs) const
{
return this.a == rhs.a;
}
}

struct T
{
int b;
S s;
alias s this;
}

struct U
{
int a;
bool opCmp(U rhs) const
{
return this.a < rhs.a;
}
}

struct V
{
int b;
U u;
alias u this;
}

void main()
{
S s1, s2;
T t1, t2;
U u1, u2;
V v1, v2;

assert(s1 == s2);      // calls S.opEquals
assert(t1 == t2);      // calls compiler generated T.opEquals that implements member-wise equality
assert(!(u1 < u2));    // calls U.opCmp
assert(!(v1 < v2));    // calls U.opCmp because V does not define an opCmp method
// so the alias this of v1 is employed; U.opCmp expects a
// paramter of type U, so alias this of v2 is used

assert(s1 == t1);      // calls s1.opEquals(t1.s);
assert(t1 == s1);      // calls t1.s.opEquals(s1);
assert(!(u1 < v1));    // calls u1.opCmp(v1.u);
assert(!(v1 < u1));    // calls v1.u.opCmp(v1);
}

Attributes are ignored for AliasThis.

Multiple AliasThis are currently not allowed.

Scope Classes
Note: Scope classes have been recommended for deprecation.
A scope class is a class with the scope attribute, as in:

scope class Foo { ... }

The scope characteristic is inherited, so any classes derived from a scope class are also scope.

A scope class reference can only appear as a function local variable. It must be declared as being scope:

scope class Foo { ... }

void func()
{
Foo f;    // error, reference to scope class must be scope
scope Foo g = new Foo(); // correct
}

When a scope class reference goes out of scope, the destructor (if any) for it is automatically called. This holds true even if the scope was exited via a thrown exception.

Final Classes

Final classes cannot be subclassed:

final class A { }
class B : A { }  // error, class A is final

Methods of a final class are final by default.

Nested Classes

A nested class is a class that is declared inside the scope of a function or another class. A nested class has access to the variables and other symbols of the classes and functions it is nested inside:

class Outer
{
int m;

class Inner
{
int foo()
{
return m;   // Ok to access member of Outer
}
}
}

void func()
{
int m;

class Inner
{
int foo()
{
return m; // Ok to access local variable m of func()
}
}
}

Static Nested Classes

If a nested class has the static attribute, then it can not access variables of the enclosing scope that are local to the stack or need a this reference:

class Outer
{
int m;
static int n;

static class Inner
{
int foo()
{
return m;   // Error, Inner is static and m needs a this
return n;   // Ok, n is static
}
}
}

void func()
{
int m;
static int n;

static class Inner
{
int foo()
{
return m;   // Error, Inner is static and m is local to the stack
return n;   // Ok, n is static
}
}
}

Context Pointer

Non-static nested classes work by containing an extra hidden member (called the context pointer) that is the frame pointer of the enclosing function if it is nested inside a function, or the this reference of the enclosing class's instance if it is nested inside a class.

When a non-static nested class is instantiated, the context pointer is assigned before the class's constructor is called, therefore the constructor has full access to the enclosing variables. A non-static nested class can only be instantiated when the necessary context pointer information is available:

class Outer
{
class Inner { }

static class SInner { }
}

void func()
{
class Nested { }

Outer o = new Outer;        // Ok
Outer.Inner oi = new Outer.Inner;   // Error, no 'this' for Outer
Outer.SInner os = new Outer.SInner; // Ok

Nested n = new Nested;      // Ok
}

Explicit Instantiation

A this reference can be supplied to the creation of an inner class instance by prefixing it to the NewExpression:

class Outer
{
int a;

class Inner
{
int foo()
{
return a;
}
}
}

int bar()
{
Outer o = new Outer;
o.a = 3;
Outer.Inner oi = o.new Inner;
return oi.foo();    // returns 3
}

Here o supplies the this reference to the inner class instance of Outer.

outer Property

For a nested class instance, the .outer property is the this reference for the enclosing class's instance. If there is no enclosing class context, .outer would be a void* to the enclosing function frame.

class Outer
{
class Inner1
{
Outer getOuter()
{
return this.outer;
}
}

void foo()
{
Inner1 i = new Inner1;
assert(i.getOuter() is this);
}

void bar()
{
// x is referenced from nested scope, so
// bar makes a closure environment.
int x = 1;

class Inner2
{
Outer getOuter()
{
x = 2;
// The Inner2 instance has access to the function frame
// of bar as a static frame pointer, but .outer returns
// the enclosing Outer class instance property.
return this.outer;
}
}

Inner2 i = new Inner2;
assert(i.getOuter() is this);
}

// baz cannot access an instance of Outer
static void baz()
{
// make a closure environment
int x = 1;

class Inner3
{
void* getOuter()
{
x = 2;
// There's no accessible enclosing class instance, so the
// .outer property returns the function frame of baz.
return this.outer;
}
}

Inner3 i = new Inner3;
assert(i.getOuter() !is null);
}
}

Anonymous Nested Classes

An anonymous nested class is both defined and instantiated with a NewAnonClassExpression:

NewAnonClassExpression:
new AllocatorArgumentsopt class ConstructorArgsopt SuperClassOrInterfaceopt Interfacesopt AggregateBody

ConstructorArgs:
( ArgumentListopt )

which is equivalent to:

class Identifier : SuperClassOrInterface Interfaces AggregateBody
// ...
new AllocatorArguments Identifier ConstructorArgs

where Identifier is the name generated for the anonymous nested class.

interface I
{
void foo();
}

auto obj = new class I
{
void foo()
{
writeln("foo");
}
};
obj.foo();

Const, Immutable and Shared Classes

If a ClassDeclaration has a const, immutable or shared storage class, then it is as if each member of the class was declared with that storage class. If a base class is const, immutable or shared, then all classes derived from it are also const, immutable or shared.

Interfaces

An Interface describes a list of functions that a class which inherits from the interface must implement.

InterfaceDeclaration:
interface Identifier ;
interface Identifier BaseInterfaceListopt AggregateBody
InterfaceTemplateDeclaration

BaseInterfaceList:
: Interfaces

Implementation Defined:

Specialized interfaces may be supported:

1. COM Interfaces are binary compatible with COM/OLE/ActiveX objects for Windows.

2. C++ Interfaces are binary compatible with C++ abstract classes.

3. Objective-C Interfaces are binary compatible with Objective-C protocols.

A class that implements an interface can be implicitly converted to a reference to that interface.

Interfaces cannot derive from classes; only from other interfaces. Classes cannot derive from an interface multiple times.

interface D
{
void foo();
}

class A : D, D  // error, duplicate interface
{
}

An instance of an interface cannot be created.

interface D
{
void foo();
}

...

D d = new D();  // error, cannot create instance of interface

Virtual interface member functions do not have implementations. Interfaces are expected to implement static or final functions.

interface D
{
void bar() { }  // error, implementation not allowed
static void foo() { } // ok
final void abc() { } // ok
}

Interfaces can have function templates in the members. All instantiated functions are implicitly final.

interface D
{
void foo(T)() { }  // ok, it's implicitly final
}

Classes that inherit from an interface may not override final or static interface member functions.

interface D
{
void bar();
static void foo() { }
final void abc() { }
}

class C : D
{
void bar() { } // ok
void foo() { } // error, cannot override static D.foo()
void abc() { } // error, cannot override final D.abc()
}

All interface functions must be defined in a class that inherits from that interface:

interface D
{
void foo();
}

class A : D
{
void foo() { }  // ok, provides implementation
}

class B : D
{
int foo() { }   // error, no void foo() implementation
}

Interfaces can be inherited and functions overridden:

interface D
{
int foo();
}

class A : D
{
int foo() { return 1; }
}

class B : A
{
int foo() { return 2; }
}

...

B b = new B();
b.foo();            // returns 2
D d = cast(D) b;    // ok since B inherits A's D implementation
d.foo();            // returns 2;

Interfaces can be reimplemented in derived classes:

interface D
{
int foo();
}

class A : D
{
int foo() { return 1; }
}

class B : A, D
{
int foo() { return 2; }
}

...

B b = new B();
b.foo();            // returns 2
D d = cast(D) b;
d.foo();            // returns 2
A a = cast(A) b;
D d2 = cast(D) a;
d2.foo();           // returns 2, even though it is A's D, not B's D

A reimplemented interface must implement all the interface functions, it does not inherit them from a super class:

interface D
{
int foo();
}

class A : D
{
int foo() { return 1; }
}

class B : A, D
{
}       // error, no foo() for interface D

Interfaces with Contracts

Interface member functions can have contracts even though there is no body for the function. The contracts are inherited by any class member function that implements that interface member function.

interface I
{
int foo(int i)
in { assert(i > 7); }
out (result) { assert(result & 1); }

void bar();
}

Const and Immutable Interfaces

If an interface has const or immutable storage class, then all members of the interface are const or immutable. This storage class is not inherited.

COM Interfaces

A variant on interfaces is the COM interface. A COM interface is designed to map directly onto a Windows COM object. Any COM object can be represented by a COM interface, and any D object with a COM interface can be used by external COM clients.

A COM interface is defined as one that derives from the interface core.stdc.windows.com.IUnknown. A COM interface differs from a regular D interface in that:

- It derives from the interface core.stdc.windows.com.IUnknown.

- It cannot be the argument to destroy.

- References cannot be upcast to the enclosing class object, nor can they be downcast to a derived interface. Implement QueryInterface() for that interface in standard COM fashion to convert to another COM interface.

- Classes derived from COM interfaces are COM classes.

- The default linkage for member functions of COM classes is extern(System).
Note: To implement or override any base-class methods of D interfaces or classes (ones which do not inherit from IUnknown), explicitly mark them as having the extern(D) linkage.

import core.sys.windows.windows;
import core.stdc.windows.com;

interface IText
{
void write();
}

abstract class Printer : IText
{
void print() { }
}

class C : Printer, IUnknown
{
// Implements the IText `write` class method.
extern(D) void write() { }

// Overrides the Printer `print` class method.
extern(D) override void print() { }

// Overrides the Object base class `toString` method.
extern(D) override string toString() { return "Class C"; }

// Methods of class implementing the IUnknown interface have
// the extern(System) calling convention by default.
HRESULT QueryInterface(const(IID)*, void**);
uint AddRef();
uint Release();
}

The same applies to other Object methods such as opCmp, toHash, etc.

- The first member of the COM vtbl[] is not the pointer to the InterfaceInfo, but the first virtual function pointer.

See also Modern COM Programming in D

C++ Interfaces

C++ interfaces are interfaces declared with C++ linkage:

extern (C++) interface Ifoo
{
void foo();
void bar();
}

which is meant to correspond with the following C++ declaration:

class Ifoo
{
virtual void foo();
virtual void bar();
};

Any interface that derives from a C++ interface is also a C++ interface. A C++ interface differs from a D interface in that:

- It cannot be the argument to destroy.

- References cannot be upcast to the enclosing class object, nor can they be downcast to a derived interface.

- The C++ calling convention is the default convention for its member functions, rather than the D calling convention.

- The first member of the vtbl[] is not the pointer to the Interface, but the first virtual function pointer.

Enums

EnumDeclaration:
enum Identifier EnumBody
enum Identifier : EnumBaseType EnumBody
AnonymousEnumDeclaration

EnumBaseType:
Type

EnumBody:
{ EnumMembers }
;

EnumMembers:
EnumMember
EnumMember ,
EnumMember , EnumMembers

EnumMemberAttributes:
EnumMemberAttribute
EnumMemberAttribute EnumMemberAttributes

EnumMemberAttribute:
DeprecatedAttribute
UserDefinedAttribute
@disable

EnumMember:
EnumMemberAttributesopt Identifier
EnumMemberAttributesopt Identifier = AssignExpression

AnonymousEnumDeclaration:
enum : EnumBaseType { EnumMembers }
enum { EnumMembers }
enum { AnonymousEnumMembers }

AnonymousEnumMembers:
AnonymousEnumMember
AnonymousEnumMember ,
AnonymousEnumMember , AnonymousEnumMembers

AnonymousEnumMember:
EnumMember
Type Identifier = AssignExpression

Enum declarations are used to define a group of constants.

Named Enums

Named enums are used to declare related constants and group them by giving them a unique type. The EnumMembers are declared in the scope of the named enum. The named enum declares a new type, and all the EnumMembers have that type.

This defines a new type X which has values X.A=0, X.B=1, X.C=2:

enum X { A, B, C }  // named enum

If the EnumBaseType is not explicitly set, and the first EnumMember has an AssignExpression, it is set to the type of that AssignExpression. Otherwise, it defaults to type int.

Named enum members may not have individual Types.

A named enum member can be implicitly cast to its EnumBaseType, but EnumBaseType types cannot be implicitly cast to an enum type.

The value of an EnumMember is given by its AssignExpression. If there is no AssignExpression and it is the first EnumMember, its value is EnumBaseType.init.

If there is no AssignExpression and it is not the first EnumMember, it is given the value of the previous EnumMember+1. If the value of the previous EnumMember is EnumBaseType.max, it is an error. If the value of the previous EnumMember+1 is the same as the value of the previous EnumMember, it is an error. (This can happen with floating point types.)

All EnumMembers are in scope for the AssignExpressions.

enum A = 3;
enum B
{
A = A // error, circular reference
}
enum C
{
A = B,  // A = 4
B = D,  // B = 4
C = 3,  // C = 3
D       // D = 4
}
enum E : C
{
E1 = C.D,
E2      // error, C.D is C.max
}

An empty enum body (For example enum E;) signifies an opaque enum - the enum members are unknown.

Enum Default Initializer

The .init property of an enum type is the value of the first member of that enum. This is also the default initializer for the enum type.

enum X { A=3, B, C }
X x;      // x is initialized to 3

Enum Properties

Enum properties only exist for named enums.

Named Enum Properties

.init

First enum member value

.min

Smallest enum member value

.max

Largest enum member value

.sizeof

Size of storage for an enumerated value

For example:

enum X { A=3, B=1, C=4, D, E=2 }
X.init   // is X.A
X.min    // is X.B
X.max    // is X.D
X.sizeof // is same as int.sizeof

The EnumBaseType of named enums must support comparison in order to compute the .max and .min properties.

Anonymous Enums

If the enum Identifier is not present, then the enum is an anonymous enum, and the EnumMembers are declared in the scope the EnumDeclaration appears in. No new type is created.

The EnumMembers can have different types. Those types are given by the first of:

1. The Type, if present. Types are not permitted when an EnumBaseType is present.

2. The EnumBaseType, if present.

3. The type of the AssignExpression, if present.

4. The type of the previous EnumMember, if present.

5. int

enum { A, B, C }  // anonymous enum

Defines the constants A=0, B=1, C=2, all of type int.

Enums must have at least one member.

The value of an EnumMember is given by its AssignExpression. If there is no AssignExpression and it is the first EnumMember, its value is the .init property of the EnumMember's type.

If there is no AssignExpression and it is not the first EnumMember, it is given the value of the previous EnumMember+1. If the value of the previous EnumMember is the .max property if the previous EnumMember's type, it is an error. If the value of the previous EnumMember+1 is the same as the value of the previous EnumMember, it is an error. (This can happen with floating point types.)

All EnumMembers are in scope for the AssignExpressions.

enum { A, B = 5+7, C, D = 8+C, E }

Sets A=0, B=12, C=13, D=21, and E=22, all of type int.

enum : long { A = 3, B }

Sets A=3, B=4 all of type long.

enum : string
{
A = "hello",
B = "betty",
C     // error, cannot add 1 to "betty"
}

enum
{
A = 1.2f,  // A is 1.2f of type float
B,         // B is 2.2f of type float
int C = 3, // C is 3 of type int
D          // D is 4 of type int
}

Manifest Constants

If there is only one member of an anonymous enum, the { } can be omitted. Gramatically speaking, this is an AutoDeclaration.

enum i = 4;      // i is 4 of type int
enum long l = 3; // l is 3 of type long

Manifest constants are not lvalues, meaning their address cannot be taken. They exist only in the memory of the compiler.

enum size = __traits(classInstanceSize, Foo);  // evaluated at compile-time

The initializer for a manifest constant is evaluated using compile time function evaluation.

template Foo(T)
{
// Not bad, but the 'size' variable will be located in the executable.
const size_t size = T.sizeof;       // evaluated at compile-time

// ... use of 'size' at compile time ...
}

template Bar(T)
{
// Better, the manifest constant has no runtime location in the executable.
enum size_t size = T.sizeof;        // evaluated at compile-time

// ... use of 'size' at compile time ...

// Taking the address of Foo!T.size also causes it to go into the exe file.
auto p = &Foo!T.size;
}

Type Qualifiers

Type qualifiers modify a type by applying a TypeCtor. TypeCtors are: const, immutable, shared, and inout. Each applies transitively to all subtypes.

Const and Immutable

When examining a data structure or interface, it is very helpful to be able to easily tell which data can be expected to not change, which data might change, and who may change that data. This is done with the aid of the language typing system. Data can be marked as const or immutable, with the default being changeable (or mutable).

immutable applies to data that cannot change. Immutable data values, once constructed, remain the same for the duration of the program's execution. Immutable data can be placed in ROM (Read Only Memory) or in memory pages marked by the hardware as read only. Since immutable data does not change, it enables many opportunities for program optimization, and has applications in functional style programming.

const applies to data that cannot be changed by the const reference to that data. It may, however, be changed by another reference to that same data. Const finds applications in passing data through interfaces that promise not to modify them.

Both immutable and const are transitive, which means that any data reachable through an immutable reference is also immutable, and likewise for const.

Immutable Storage Class

The simplest immutable declarations use it as a storage class. It can be used to declare manifest constants.

immutable int x = 3;  // x is set to 3
x = 4;        // error, x is immutable
char[x] s;    // s is an array of 3 chars

The type can be inferred from the initializer:

immutable y = 4; // y is of type int
y = 5;           // error, y is immutable

If the initializer is not present, the immutable can be initialized from the corresponding constructor:

immutable int z;
void test()
{
z = 3; // error, z is immutable
}
static this()
{
z = 3; // ok, can set immutable that doesn't
// have static initializer
}

The initializer for a non-local immutable declaration must be evaluatable at compile time:

int foo(int f) { return f * 3; }
int i = 5;
immutable x = 3 * 4;      // ok, 12
immutable y = i + 1;      // error, cannot evaluate at compile time
immutable z = foo(2) + 1; // ok, foo(2) can be evaluated at compile time, 7

The initializer for a non-static local immutable declaration is evaluated at run time:

int foo(int f)
{
immutable x = f + 1;  // evaluated at run time
x = 3;                // error, x is immutable
}

Because immutable is transitive, data referred to by an immutable is also immutable:

immutable char[] s = "foo";
s[0] = 'a';  // error, s refers to immutable data
s = "bar";   // error, s is immutable

Immutable declarations can appear as lvalues, i.e. they can have their address taken, and occupy storage.

Const Storage Class

A const declaration is exactly like an immutable declaration, with the following differences:

- Any data referenced by the const declaration cannot be changed from the const declaration, but it might be changed by other references to the same data.

- The type of a const declaration is itself const.

Immutable Type

Data that will never change its value can be typed as immutable. The immutable keyword can be used as a type qualifier:

immutable(char)[] s = "hello";

The immutable applies to the type within the following parentheses. So, while s can be assigned new values, the contents of s[] cannot be:

s[0] = 'b';  // error, s[] is immutable
s = null;    // ok, s itself is not immutable

Immutability is transitive, meaning it applies to anything that can be referenced from the immutable type:

immutable(char*)** p = ...;
p = ...;        // ok, p is not immutable
*p = ...;       // ok, *p is not immutable
**p = ...;      // error, **p is immutable
***p = ...;     // error, ***p is immutable

Immutable used as a storage class is equivalent to using immutable as a type qualifier for the entire type of a declaration:

immutable int x = 3;   // x is typed as immutable(int)
immutable(int) y = 3;  // y is immutable

Creating Immutable Data

The first way is to use a literal that is already immutable, such as string literals. String literals are always immutable.

auto s = "hello";   // s is immutable(char)[5]
char[] p = "world"; // error, cannot implicitly convert immutable
// to mutable

The second way is to cast data to immutable. When doing so, it is up to the programmer to ensure that any mutable references to the same data are not used to modify the data after the cast.

char[] s = ['a'];
s[0] = 'b'; // ok
immutable(char)[] p = cast(immutable)s; // ok, if data is not mutated
// through s anymore
s[0] = 'c'; // undefined behavior
immutable(char)[] q = cast(immutable)s.dup; // always ok, unique reference

char[][] s2 = [['a', 'b'], ['c', 'd']];
immutable(char[][]) p2 = cast(immutable)s2.dup; // dangerous, only the first
// level of elements is unique
s2[0] = ['x', 'y']; // ok, doesn't affect p2
s2[1][0] = 'z'; // undefined behavior
immutable(char[][]) q2 = [s2[0].dup, s2[1].dup]; // always ok, unique references

The .idup property is a convenient way to create an immutable copy of an array:

auto p = s.idup;
p[0] = ...;       // error, p[] is immutable

Removing Immutable or Const with a Cast

An immutable or const type qualifier can be removed with a cast:

immutable int* p = ...;
int* q = cast(int*)p;

This does not mean, however, that one can change the data:

*q = 3; // allowed by compiler, but result is undefined behavior

The ability to cast away immutable-correctness is necessary in some cases where the static typing is incorrect and not fixable, such as when referencing code in a library one cannot change. Casting is, as always, a blunt and effective instrument, and when using it to cast away immutable-correctness, one must assume the responsibility to ensure the immutability of the data, as the compiler will no longer be able to statically do so.

Note that casting away a const qualifier and then mutating is undefined behavior, too, even when the referenced data is mutable. This is so that compilers and programmers can make assumptions based on const alone. For example, here it may be assumed that f does not alter x:

void f(const int* a);
void main()
{
int x = 1;
f(&x);
assert(x == 1); // guaranteed to hold
}

Immutable Member Functions

Immutable member functions are guaranteed that the object and anything referred to by the this reference is immutable. They are declared as:

struct S
{
int x;

void foo() immutable
{
x = 4;      // error, x is immutable
this.x = 4; // error, x is immutable
}
}

Note that using immutable on the left hand side of a method does not apply to the return type:

struct S
{
immutable int[] bar()  // bar is still immutable, return type is not!
{
}
}

To make the return type immutable, surround it with parentheses:

struct S
{
immutable(int[]) bar()  // bar is now mutable, return type is immutable.
{
}
}

To make both the return type and the method immutable, write:

struct S
{
immutable(int[]) bar() immutable
{
}
}

Const Type

Const types are like immutable types, except that const forms a read-only view of data. Other aliases to that same data may change it at any time.

Const Member Functions

Const member functions are functions that are not allowed to change any part of the object through the member function's this reference.

Combining Qualifiers

More than one qualifier may apply to a type. The order of application is irrelevant, for example given an unqualified type T, const shared T and shared const T are the same type. For that reason, this document depicts qualifier combinations without parentheses unless necessary and in alphabetic order.

Applying a qualifier to a type that already has that qualifier is legal but has no effect, e.g. given an unqualified type T, shared(const shared T) yields the type const shared T.

Applying the immutable qualifier to any type (qualified or not) results in immutable T. Applying any qualifier to immutable T results in immutable T. This makes immutable a fixed point of qualifier combinations and makes types such as const(immutable(shared T)) impossible to create.

Assuming T is an unqualified type, the graph below illustrates how qualifiers combine (combinations with immutable are omitted). For each node, applying the qualifier labeling the edge leads to the resulting type.

Implicit Qualifier Conversions

Values that have no mutable indirections (including structs that don't contain any field with mutable indirections) can be implicitly converted across mutable, const, immutable, const shared, inout and inout shared.

References to qualified objects can be implicitly converted according to the following rules:

In the graph above, any directed path is a legal implicit conversion. No other qualifier combinations than the ones shown is valid. If a directed path exists between two sets of qualifiers, the types thus qualified are called qualifier-convertible. The same information is shown below in tabular format:

Implicit Conversion of Reference Types

from/to

mutable

const

shared

const shared

inout

const inout

inout shared

const inout shared

immutable

mutable

✔

✔

const

✔

const inout

✔

✔

const shared

✔

const inout shared

✔

✔

immutable

✔

✔

✔

✔

✔

inout

✔

✔

✔

shared

✔

✔

inout shared

✔

✔

✔

If an implicit conversion is disallowed by the table, an Expression may be converted if:

An expression may be converted from mutable or shared to immutable if the expression is unique and all expressions it transitively refers to are either unique or immutable.

An expression may be converted from mutable to shared if the expression is unique and all expressions it transitively refers to are either unique, immutable, or shared.

An expression may be converted from immutable to mutable if the expression is unique.

An expression may be converted from shared to mutable if the expression is unique.

A Unique Expression is one for which there are no other references to the value of the expression and all expressions it transitively refers to are either also unique or are immutable. For example:

void main()
{
immutable int** p = new int*(null); // ok, unique

int x;
immutable int** q = new int*(&x); // error, there may be other references to x

immutable int y;
immutable int** r = new immutable(int)*(&y); // ok, y is immutable
}

Otherwise, a CastExpression can be used to force a conversion when an implicit version is disallowed, but this cannot be done in @safe code, and the correctness of it must be verified by the user.

Functions

Grammar

Function declaration

FuncDeclaration:
StorageClassesopt BasicType FuncDeclarator FunctionBody
AutoFuncDeclaration

AutoFuncDeclaration:
StorageClasses Identifier FuncDeclaratorSuffix FunctionBody

FuncDeclarator:
TypeSuffixesopt Identifier FuncDeclaratorSuffix

FuncDeclaratorSuffix:
Parameters MemberFunctionAttributesopt
TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt

Parameters

Parameters:
( ParameterListopt )

ParameterList:
Parameter
Parameter ,
Parameter , ParameterList
VariadicArgumentsAttributesopt ...

Parameter:
ParameterAttributesopt BasicType Declarator
ParameterAttributesopt BasicType Declarator ...
ParameterAttributesopt BasicType Declarator = AssignExpression
ParameterAttributesopt Type
ParameterAttributesopt Type ...
ParameterAttributesopt Type = AssignExpression

ParameterAttributes:
InOut
UserDefinedAttribute
ParameterAttributes InOut
ParameterAttributes UserDefinedAttribute

InOut:
auto
TypeCtor
final
in
lazy
out
ref
return
scope

VariadicArgumentsAttributes:
VariadicArgumentsAttribute
VariadicArgumentsAttribute VariadicArgumentsAttributes

VariadicArgumentsAttribute:
const
immutable
return
scope
shared

Function attributes

FunctionAttributes:
FunctionAttribute
FunctionAttribute FunctionAttributes

FunctionAttribute:
FunctionAttributeKwd
Property
AtAttribute

MemberFunctionAttributes:
MemberFunctionAttribute
MemberFunctionAttribute MemberFunctionAttributes

MemberFunctionAttribute:
const
immutable
inout
return
scope
shared
FunctionAttribute

Function body

FunctionBody:
SpecifiedFunctionBody
MissingFunctionBody
ShortenedFunctionBody

FunctionLiteralBody:
SpecifiedFunctionBody

SpecifiedFunctionBody:
doopt BlockStatement
FunctionContractsopt InOutContractExpression doopt BlockStatement
FunctionContractsopt InOutStatement do BlockStatement

MissingFunctionBody:
;
FunctionContractsopt InOutContractExpression ;
FunctionContractsopt InOutStatement

ShortenedFunctionBody:
=> AssignExpression ;

Examples:

int hasSpecifiedBody() { return 1; }
int hasMissingBody();
int hasShortenedBody() => 1;

Note: The ShortenedFunctionBody form requires the -preview=shortenedMethods command-line switch, which is available starting in v2.096.0.

Function Contracts

FunctionContracts:
FunctionContract
FunctionContract FunctionContracts

FunctionContract:
InOutContractExpression
InOutStatement

InOutContractExpression:
InContractExpression
OutContractExpression

InOutStatement:
InStatement
OutStatement

InContractExpression:
in ( AssertArguments )

OutContractExpression:
out ( ; AssertArguments )
out ( Identifier ; AssertArguments )

InStatement:
in BlockStatement

OutStatement:
out BlockStatement
out ( Identifier ) BlockStatement

Function Contracts specify the preconditions and postconditions of a function. They are used in Contract Programming.

Preconditions and postconditions do not affect the type of the function.

Preconditions

An InContractExpression is a precondition.

The first AssignExpression of the AssertArguments must evaluate to true. If it does not, the precondition has failed.

The second AssignExpression, if present, must be implicitly convertible to type const(char)[].

An InStatement is also a precondition. Any AssertExpression appearing in an InStatement will be an InContractExpression.

Preconditions must semantically be satisfied before the function starts executing. If it is not, the program enters an Invalid State.

Implementation Defined: Whether the preconditions are actually run or not is implementation defined. This is usually selectable with a compiler switch. Its behavior upon precondition failure is also usually selectable with a compiler switch. One option is to throw an AssertError with a message consisting of the optional second AssignExpression.

Best Practices: Use preconditions to validate that input arguments have values that are expected by the function.

Best Practices: Since preconditions may or may not be actually checked at runtime, avoid using preconditions that have side effects.

The expression form is:

in (expression)
in (expression, "failure string")
{
...function body...
}

The block statement form is:

in
{
...contract preconditions...
}
do
{
...function body...
}

Postconditions

An OutContractExpression is a postcondition.

The first AssignExpression of the AssertArguments must evaluate to true. If it does not, the postcondition has failed.

The second AssignExpression, if present, must be implicitly convertible to type const(char)[].

An OutStatement is also a postcondition. Any AssertExpression appearing in an OutStatement will be an OutContractExpression.

Postconditions must semantically be satisfied after the function finishes executing. If it is not, the program enters an Invalid State.

Implementation Defined: Whether the postconditions are actually run or not is implementation defined. This is usually selectable with a compiler switch. Its behavior upon postcondition failure is also usually selectable with a compiler switch. One option is to throw an AssertError with a message consisting of the optional second AssignExpression.

Best Practices: Use postconditions to validate that the input arguments and return value have values that are expected by the function.

Best Practices: Since postconditions may or may not be actually checked at runtime, avoid using postconditions that have side effects.

The expression form is:

out (identifier; expression)
out (identifier; expression, "failure string")
out (; expression)
out (; expression, "failure string")
{
...function body...
}

The block statement form is:

out
{
...contract postconditions...
}
out (identifier)
{
...contract postconditions...
}
do
{
...function body...
}

The optional identifier in either type of postcondition is set to the return value of the function, and can be accessed from within the postcondition.

Example

int fun(ref int a, int b)
in (a > 0)
in (b >= 0, "b cannot be negative!")
out (r; r > 0, "return must be positive")
out (; a != 0)
{
// function body
}

int fun(ref int a, int b)
in
{
assert(a > 0);
assert(b >= 0, "b cannot be negative!");
}
out (r)
{
assert(r > 0, "return must be positive");
assert(a != 0);
}
do
{
// function body
}

The two functions are identical semantically.

In, Out and Inheritance

If a function in a derived class overrides a function from its super class, then only the preconditions of one of the function and its overridden functions must be satisfied. Overriding functions then becomes a process of loosening the preconditions.

A function without preconditions means its precondition is always satisfied. Therefore if any function in an inheritance hierarchy has no preconditions, then any preconditions on functions overriding it have no meaningful effect.

Conversely, all of the postconditions of the function and its overridden functions must to be satisfied. Adding overriding functions then becomes a processes of tightening the postconditions.

Function Return Values

Function return values not marked as ref are considered to be rvalues. This means they cannot be passed by reference to other functions.

Functions Without Bodies

Functions without bodies:

int foo();

that are not declared as abstract are expected to have their implementations elsewhere, and that implementation will be provided at the link step. This enables an implementation of a function to be completely hidden from the user of it, and the implementation may be in another language such as C, assembler, etc.

Pure Functions

Pure functions are annotated with the pure attribute.

Pure functions cannot directly access global or static mutable state.

Pure functions can only call pure functions.

A pure function can override an impure function, but cannot be overridden by an impure function. I.e. it is covariant with an impure function.

A weakly pure function has parameters with mutable indirections. Program state can be modified transitively through the matching argument.

pure int foo(int[] arr) { arr[] += 1; return arr.length; }
int[] a = [1, 2, 3];
foo(a);
assert(a == [2, 3, 4]);

A strongly pure function has no parameters with mutable indirections and cannot modify any program state external to the function.

struct S { double x; }
pure int foo(immutable(int)[] arr, int num, S val)
{
//arr[num] = 1; // compile error
num = 2;        // has no side effect to the caller side
val.x = 3.14;   // ditto
return arr.length;
}

A strongly pure function can call a weakly pure function.

Pure functions can modify the local state of the function.

A pure function can:

- read and write the floating point exception flags

- read and write the floating point mode flags, as long as those flags are restored to their initial state upon function entry

Undefined Behavior: occurs if these flags are not restored to their initial state upon function exit. It is the programmer's responsibility to ensure this.

A pure function can perform impure operations in statements that are in a ConditionalStatement controlled by a DebugCondition.

Best Practices: this relaxation of purity checks in DebugConditions is intended solely to make debugging programs easier.

A pure function can throw exceptions.

import std.stdio;
int x;
immutable int y;
const int* pz;

pure int foo(int i,
char* p,
const char* q,
immutable int* s)
{
debug writeln("in foo()"); // ok, impure code allowed in debug statement
x = i;   // error, modifying global state
i = x;   // error, reading mutable global state
i = y;   // ok, reading immutable global state
i = *pz; // error, reading const global state
return i;
}

Nested functions inside a pure function are implicitly marked as pure.

pure int foo(int x, immutable int y)
{
int bar()
// implicitly marked as pure, to be "weakly pure"
// since hidden context pointer to foo stack context is mutable
{
x = 10;     // can access states in enclosing scope
// through the mutable context pointer
return x;
}
pragma(msg, typeof(&bar));  // int delegate() pure

int baz() immutable
// qualifies hidden context pointer with immutable,
// and has no other parameters, therefore "strongly pure"
{
//return x; // error, cannot access mutable data
// through the immutable context pointer
return y;   // ok
}

// can call pure nested functions
return bar() + baz();
}

A pure factory function is a strongly pure function that returns a result that has mutable indirections. All mutable memory returned by the call may not be referenced by any other part of the program, i.e. it is newly allocated by the function. Nor may the mutable references of the result refer to any object that existed before the function call. For example:

struct List { int payload; List* next; }
pure List* make(int a, int b)
{
auto result = new List(a, null);
result.next = new List(b, result);
return result;
}

All references in make's result refer to other List objects created by make, and no other part of the program refers to any of these objects. This restriction does not apply to any Exception or Error thrown from the function.

Pure destructors do not benefit of special elision.

Implementation Defined: An implementation may assume that a strongly pure function that returns a result without mutable indirections will have the same effect for all invocations with equivalent arguments. It is allowed to memoize the result of the function under the assumption that equivalent parameters always produce equivalent results. A strongly pure function may still have behavior inconsistent with memoization by e.g. using casts or by changing behavior depending on the address of its parameters. An implementation is currently not required to enforce validity of memoization in all cases. If a strongly pure function throws an Exception or an Error, the assumptions related to memoization do not carry to the thrown exception.

Nothrow Functions

Nothrow functions can only throw exceptions derived from class Error.

Nothrow functions are covariant with throwing ones.

Ref Functions

Ref functions allow functions to return by reference, meaning that the return value must be an lvalue, and the lvalue is returned, not the rvalue.

ref int foo()
{
auto p = new int(2);
return *p;
}
...
int i = foo(); // i is set to 2
foo() = 3;     // reference returns can be lvalues

Returning a reference to an expired function context is not allowed. This includes local variables, temporaries and parameters that are part of an expired function context.

ref int sun()
{
int i;
return i;  // error, escaping a reference to local variable i
}

A ref parameter may not be returned by ref.

ref int moon(ref int i)
{
return i; // error
}

Auto Functions

Auto functions have their return type inferred from any ReturnStatements in the function body.

An auto function is declared without a return type. If it does not already have a storage class, use the auto storage class.

If there are multiple ReturnStatements, the types of them must be implicitly convertible to a common type. If there are no ReturnStatements, the return type is inferred to be void.

auto foo(int x) { return x + 3; }          // inferred to be int
auto bar(int x) { return x; return 2.5; }  // inferred to be double

Auto Ref Functions

Auto ref functions infer their return type just as auto functions do. In addition, they become ref functions if all return expressions are lvalues, and it would not be a reference to a local or a parameter.

auto ref f1(int x)     { return x; }  // value return
auto ref f2()          { return 3; }  // value return
auto ref f3(ref int x) { return x; }  // ref return
auto ref f4(out int x) { return x; }  // ref return
auto ref f5() { static int x; return x; }  // ref return

The ref-ness of a function is determined from all ReturnStatements in the function body:

auto ref f1(ref int x) { return 3; return x; }  // ok, value return
auto ref f2(ref int x) { return x; return 3; }  // ok, value return
auto ref f3(ref int x, ref double y)
{
return x; return y;
// The return type is deduced to double, but cast(double)x is not an lvalue,
// then become a value return.
}

Auto ref function can have explicit return type.

auto ref int (ref int x) { return x; }  // ok, ref return
auto ref int foo(double x) { return x; }   // error, cannot convert double to int

Inout Functions

Functions that differ only in whether the parameters are mutable, const or immutable, and have corresponding mutable, const or immutable return types, can be combined into one function using the inout type constructor. Consider the following overload set:

int[] slice(int[] a, int x, int y) { return a[x .. y]; }

const(int)[] slice(const(int)[] a, int x, int y) { return a[x .. y]; }

immutable(int)[] slice(immutable(int)[] a, int x, int y) { return a[x .. y]; }

The code generated by each of these functions is identical. The inout type constructor can combine them into one function:

inout(int)[] slice(inout(int)[] a, int x, int y) { return a[x .. y]; }

The inout keyword forms a wildcard that stands in for mutable, const, immutable, inout, or inout const. When calling the function, the inout state of the return type is changed to match that of the argument type passed to the inout parameter.

inout can also be used as a type constructor inside a function that has a parameter declared with inout. The inout state of a type declared with inout is changed to match that of the argument type passed to the inout parameter:

inout(int)[] asymmetric(inout(int)[] input_data)
{
inout(int)[] r = input_data;
while (r.length > 1 && r[0] == r[$-1])
r = r[1..$-1];
return r;
}

Inout types can be implicitly converted to const or inout const, but to nothing else. Other types cannot be implicitly converted to inout. Casting to or from inout is not allowed in @safe functions.

void f(inout int* ptr)
{
const int* p = ptr;
int* q = ptr; // error
immutable int* r = ptr; // error
}

Matching an inout Parameter

A set of arguments to a function with inout parameters is considered a match if any inout argument types match exactly, or:

1. No argument types are composed of inout types.

2. A mutable, const or immutable argument type can be matched against each corresponding parameter inout type.

If such a match occurs, inout is considered the common qualifier of the matched qualifiers. If more than two parameters exist, the common qualifier calculation is recursively applied.

Common qualifier of the two type qualifiers

mutable

const

immutable

inout

inout const

mutable (= m)

m

c

c

c

c

const (= c)

c

c

c

c

c

immutable (= i)

c

c

i

wc

wc

inout (= w)

c

c

wc

w

wc

inout const (= wc)

c

c

wc

wc

wc

The inout in the return type is then rewritten to match the inout qualifiers:

int[] ma;
const(int)[] ca;
immutable(int)[] ia;

inout(int)[] foo(inout(int)[] a) { return a; }
void test1()
{
// inout matches to mutable, so inout(int)[] is
// rewritten to int[]
int[] x = foo(ma);

// inout matches to const, so inout(int)[] is
// rewritten to const(int)[]
const(int)[] y = foo(ca);

// inout matches to immutable, so inout(int)[] is
// rewritten to immutable(int)[]
immutable(int)[] z = foo(ia);
}

inout(const(int))[] bar(inout(int)[] a) { return a; }
void test2()
{
// inout matches to mutable, so inout(const(int))[] is
// rewritten to const(int)[]
const(int)[] x = bar(ma);

// inout matches to const, so inout(const(int))[] is
// rewritten to const(int)[]
const(int)[] y = bar(ca);

// inout matches to immutable, so inout(int)[] is
// rewritten to immutable(int)[]
immutable(int)[] z = bar(ia);
}

Note: Shared types cannot be matched with inout.

Optional Parentheses

If a function call passes no explicit argument, i.e. it would syntactically use (), then these parentheses may be omitted, similar to a getter invocation of a property function.

void foo() {}   // no arguments
void fun(int x = 10) { }
void bar(int[] arr) {}  // for UFCS

void main()
{
foo();      // OK
foo;        // also OK
fun;        // OK

int[] arr;
arr.bar();  // OK
arr.bar;    // also OK
}

Optional parentheses are not applied to delegates or function pointers.

void main()
{
int function() fp;

assert(fp == 6);    // Error, incompatible types int function() and int
assert(*fp == 6);   // Error, incompatible types int() and int

int delegate() dg;
assert(dg == 6);    // Error, incompatible types int delegate() and int
}

If a function returns a delegate or function pointer, the parentheses are required if the returned value is to be called.

struct S {
int function() callfp() { return &numfp; }
int delegate() calldg() return { return &numdg; }
int numdg() { return 6; }
}

int numfp() { return 6; }

void main()
{
S s;
int function() fp;

fp = s.callfp;
assert(fp() == 6);

fp = s.callfp();
assert(fp() == 6);

int x = s.callfp()();
assert(x == 6);

int delegate() dg;

dg = s.calldg;
assert(dg() == 6);

dg = s.calldg();
assert(dg() == 6);

int y = s.calldg()();
assert(y == 6);
}

Property Functions

WARNING: The definition and usefulness of property functions is being reviewed, and the implementation is currently incomplete. Using property functions is not recommended until the definition is more certain and implementation more mature.

Properties are functions that can be syntactically treated as if they were fields or variables. Properties can be read from or written to. A property is read by calling a method or function with no arguments; a property is written by calling a method or function with its argument being the value it is set to.

Simple getter and setter properties can be written using UFCS. These can be enhanced with the additon of the @property attribute to the function, which adds the following behaviors:

- @property functions cannot be overloaded with non-@property functions with the same name.

- @property functions can only have zero, one or two parameters.

- @property functions cannot have variadic parameters.

- For the expression typeof(exp) where exp is an @property function, the type is the return type of the function, rather than the type of the function.

- For the expression __traits(compiles, exp) where exp is an @property function, a further check is made to see if the function can be called.

- @property are mangled differently, meaning that @property must be consistently used across different compilation units.

- The ObjectiveC interface recognizes @property setter functions as special and modifies them accordingly.

A simple property would be:

struct Foo
{
@property int data() { return m_data; } // read property

@property int data(int value) { return m_data = value; } // write property

private:
int m_data;
}

To use it:

int test()
{
Foo f;

f.data = 3;        // same as f.data(3);
return f.data + 3; // same as return f.data() + 3;
}

The absence of a read method means that the property is write-only. The absence of a write method means that the property is read-only. Multiple write methods can exist; the correct one is selected using the usual function overloading rules.

In all the other respects, these methods are like any other methods. They can be static, have different linkages, have their address taken, etc.

The built in properties .sizeof, .alignof, and .mangleof may not be declared as fields or methods in structs, unions, classes or enums.

If a property function has no parameters, it works as a getter. If has exactly one parameter, it works as a setter.

Virtual Functions

Virtual functions are class member functions that are called indirectly through a function pointer table, called a vtbl[], rather than directly.

Member functions that are virtual can be overridden, unless they are final.

Struct and union member functions are not virtual.

Static member functions are not virtual.

Member functions which are private or package are not virtual.

Member template functions are not virtual.

Member functions with Objective-C linkage are virtual even if marked with final or static.

class A
{
int def() { ... }
final int foo() { ... }
final private int bar() { ... }
private int abc() { ... }
}

class B : A
{
override int def() { ... }  // ok, overrides A.def
override int foo() { ... }  // error, A.foo is final
int bar() { ... }  // ok, A.bar is final private, but not virtual
int abc() { ... }  // ok, A.abc is not virtual, B.abc is virtual
}

void test(A a)
{
a.def();    // calls B.def
a.foo();    // calls A.foo
a.bar();    // calls A.bar
a.abc();    // calls A.abc
}

void func()
{
B b = new B();
test(b);
}

The overriding function may be covariant with the overridden function. A covariant function has a type that is implicitly convertible to the type of the overridden function.

class A { }
class B : A { }

class Foo
{
A test() { return null; }
}

class Bar : Foo
{
// overrides and is covariant with Foo.test()
override B test() { return null; }
}

Virtual functions all have a hidden parameter called the this reference, which refers to the class object instance for which the function is called.

Functions with Objective-C linkage have an additional hidden, unnamed, parameter which is the selector it was called with.

To directly call a base member function, insert base class name before the member function name. For example:

class B
{
int foo() { return 1; }
}
class C : B
{
override int foo() { return 2; }

void test()
{
assert(B.foo() == 1);  // translated to this.B.foo(), and
// calls B.foo statically.
assert(C.foo() == 2);  // calls C.foo statically, even if
// the actual instance of 'this' is D.
}
}
class D : C
{
override int foo() { return 3; }
}
void main()
{
auto d = new D();
assert(d.foo() == 3);    // calls D.foo
assert(d.B.foo() == 1);  // calls B.foo
assert(d.C.foo() == 2);  // calls C.foo
d.test();
}

Implementation Defined: Normally calling a virtual function implies getting the address of the function at runtime by indexing into the class's vtbl[]. If the implementation can determine that the called virtual function will be statically known, such as if it is final, it can use a direct call instead.

Function Inheritance and Overriding

A function in a derived class with the same name and covariant with a function in a base class overrides that function:

class A
{
int foo(int x) { ... }
}

class B : A
{
override int foo(int x) { ... }
}

void test()
{
B b = new B();
bar(b);
}

void bar(A a)
{
a.foo(1);   // calls B.foo(int)
}

When doing overload resolution, the functions in the base class are not considered, as they are not in the same Overload Set:

class A
{
int foo(int x) { ... }
int foo(long y) { ... }
}

class B : A
{
override int foo(long x) { ... }
}

void test()
{
B b = new B();
b.foo(1);  // calls B.foo(long), since A.foo(int) not considered
A a = b;

a.foo(1);  // issues runtime error (instead of calling A.foo(int))
}

To include the base class's functions in the overload resolution process, use an AliasDeclaration:

class A
{
int foo(int x) { ... }
int foo(long y) { ... }
}

class B : A
{
alias foo = A.foo;
override int foo(long x) { ... }
}

void test()
{
B b = new B();
bar(b);
}

void bar(A a)
{
a.foo(1);      // calls A.foo(int)
B b = new B();
b.foo(1);      // calls A.foo(int)
}

If such an AliasDeclaration is not used, the derived class's functions completely override all the functions of the same name in the base class, even if the types of the parameters in the base class functions are different. It is illegal if, through implicit conversions to the base class, those other functions do get called:

class A
{
void set(long i) { }
void set(int i)  { }
}
class B : A
{
void set(long i) { }
}

void foo(A a)
{
int i;
a.set(3);   // error, use of A.set(int) is hidden by B
// use 'alias set = A.set;' to introduce base class overload set
assert(i == 1);
}

void main()
{
foo(new B);
}

A function parameter's default value is not inherited:

class A
{
void foo(int x = 5) { ... }
}

class B : A
{
void foo(int x = 7) { ... }
}

class C : B
{
void foo(int x) { ... }
}

void test()
{
A a = new A();
a.foo();       // calls A.foo(5)

B b = new B();
b.foo();       // calls B.foo(7)

C c = new C();
c.foo();       // error, need an argument for C.foo
}

An overriding function inherits any unspecified FunctionAttributes from the attributes of the overridden function.

class B
{
void foo() pure nothrow @safe {}
}
class D : B
{
override void foo() {}
}
void main()
{
auto d = new D();
pragma(msg, typeof(&d.foo));
// prints "void delegate() pure nothrow @safe" in compile time
}

The attributes @disable and deprecated are not allowed on overriding functions.

Rationale: To stop the compilation or to output the deprecation message, the implementation must be able to determine the target of the call, which can't be guaranteed when it is virtual.

class B
{
void foo() {}
}

class D : B
{
@disable override void foo() {}  // error, can't apply @disable to overriding function
}

Static functions with Objective-C linkage are overridable.

Inline Functions

The compiler makes the decision whether to inline a function or not. This decision may be controlled by pragma(inline).

Implementation Defined: Whether a function is inlined or not is implementation defined, though any FunctionLiteral should be inlined when used in its declaration scope.

Function Overloading

Function overloading occurs when two or more functions in the same scope have the same name. The function selected is the one that is the best match to the arguments. The matching levels are:

1. no match

2. match with implicit conversions

3. match with qualifier conversion (if the argument type is qualifier-convertible to the parameter type)

4. exact match

Each argument (including any this reference) is compared against the function's corresponding parameter to determine the match level for that argument. The match level for a function is the worst match level of each of its arguments.

Literals do not match ref or out parameters.

If two or more functions have the same match level, then partial ordering is used to disambiguate to find the best match. Partial ordering finds the most specialized function. If neither function is more specialized than the other, then it is an ambiguity error. Partial ordering is determined for functions f and g by taking the parameter types of f, constructing a list of arguments by taking the default values of those types, and attempting to match them against g. If it succeeds, then g is at least as specialized as f. For example:

class A { }
class B : A { }
class C : B { }
void foo(A);
void foo(B);

void test()
{
C c;
/* Both foo(A) and foo(B) match with implicit conversions (level 2).
* Applying partial ordering rules,
* foo(B) cannot be called with an A, and foo(A) can be called
* with a B. Therefore, foo(B) is more specialized, and is selected.
*/
foo(c); // calls foo(B)
}

A function with a variadic argument is considered less specialized than a function without.

A static member function can be overloaded with a member function. The struct, class or union of the static member function is inferred from the type of the this argument.

struct S {
void eggs(int);
static void eggs(long);
}
S s;
s.eggs(0);  // calls void eggs(int);
S.eggs(0);  // error: need `this`
s.eggs(0L); // calls static void eggs(long);
S.eggs(0L); // calls static void eggs(long);

struct T {
void bacon(int);
static void bacon(int);
}
T t;
t.bacon(0);  // error: ambiguous
T.bacon(0);  // error: ambiguous

Rationale: A static member function that doesn't need the this parameter does not need to pass it.

Overload Sets

Functions declared at the same scope overload against each other, and are called an Overload Set. An example of an overload set are functions defined at module level:

module A;
void foo() { }
void foo(long i) { }

A.foo() and A.foo(long) form an overload set. A different module can also define another overload set of functions with the same name:

module B;
class C { }
void foo(C) { }
void foo(int i) { }

and A and B can be imported by a third module, C. Both overload sets, the A.foo overload set and the B.foo overload set, are found when searching for symbol foo. An instance of foo is selected based on it matching in exactly one overload set:

import A;
import B;

void bar(C c , long i)
{
foo();    // calls A.foo()
foo(i);  // calls A.foo(long)
foo(c);   // calls B.foo(C)
foo(1,2); // error, does not match any foo
foo(1);   // error, matches A.foo(long) and B.foo(int)
A.foo(1); // calls A.foo(long)
}

Even though B.foo(int) is a better match than A.foo(long) for foo(1), it is an error because the two matches are in different overload sets.

Overload sets can be merged with an alias declaration:

import A;
import B;

alias foo = A.foo;
alias foo = B.foo;

void bar(C c)
{
foo();    // calls A.foo()
foo(1L);  // calls A.foo(long)
foo(c);   // calls B.foo(C)
foo(1,2); // error, does not match any foo
foo(1);   // calls B.foo(int)
A.foo(1); // calls A.foo(long)
}

Function Parameters

Parameter Storage Classes

Parameter storage classes are in, out, ref, lazy, return and scope. Parameters can also take the type constructors const, immutable, shared and inout.

in, out, ref and lazy are mutually exclusive. The first three are used to denote input, input/output, and output parameters, respectively. For example:

int read(in char[] input, ref size_t count, out int errno);

void main()
{
size_t a = 42;
int b;
int r = read("Hello World", a, b);
}

read has three parameters. input will only be read and no reference to it will be retained. count may be read and written to, and errno will be set to a value from within the function.

The argument "Hello World" gets bound to parameter input, a gets bound to count and b to errno.

Parameter Storage Class and Type Constructor Overview

Storage Class

Description

none

The parameter will be a mutable copy of its argument.

in

The parameter is an input to the function.

out

The argument must be an lvalue, which will be passed by reference and initialized upon function entry with the default value (T.init) of its type.

ref

The parameter is an input/output parameter, passed by reference.

scope

The parameter must not escape the function call (e.g. by being assigned to a global variable). Ignored for any parameter that is not a reference type.

return

Parameter may be returned or copied to the first parameter, but otherwise does not escape from the function. Such copies are required not to outlive the argument(s) they were derived from. Ignored for parameters with no references. See Scope Parameters.

lazy

argument is evaluated by the called function and not by the caller

Type Constructor

Description

const

argument is implicitly converted to a const type

immutable

argument is implicitly converted to an immutable type

shared

argument is implicitly converted to a shared type

inout

argument is implicitly converted to an inout type

In Parameters
Note: The following requires the -preview=in switch, available in v2.094.0 or higher. When not in use, in is equivalent to const.
The parameter is an input to the function. Input parameters behave as if they have the const scope storage classes. Input parameters may also be passed by reference by the compiler.

Unlike ref parameters, in parameters can bind to both lvalues and rvalues (such as literals).

Types that would trigger a side effect if passed by value (such as types with copy constructor, postblit, or destructor), and types which cannot be copied (e.g. if their copy constructor is marked as @disable) will always be passed by reference. Dynamic arrays, classes, associative arrays, function pointers, and delegates will always be passed by value.

Implementation Defined: If the type of the parameter does not fall in one of those categories, whether or not it is passed by reference is implementation defined, and the backend is free to choose the method that will best fit the ABI of the platform.

Ref and Out Parameters

By default, parameters take rvalue arguments. A ref parameter takes an lvalue argument, so changes to its value will operate on the caller's argument.

void inc(ref int x)
{
x += 1;
}

void seattle()
{
int z = 3;
inc(z);
assert(z == 4);
}

A ref parameter can also be returned by reference, see Return Ref Parameters.

An out parameter is similar to a ref parameter, except it is initialized with x.init upon function invocation.

void zero(out int x)
{
assert(x == 0);
}

void two(out int x)
{
x = 2;
}

void tacoma()
{
int a = 3;
zero(a);
assert(a == 0);

int y = 3;
two(y);
assert(y == 2);
}

For dynamic array and class object parameters, which are always passed by reference, out and ref apply only to the reference and not the contents.

Lazy Parameters

An argument to a lazy parameter is not evaluated before the function is called. The argument is only evaluated if/when the parameter is evaluated within the function. Hence, a lazy argument can be executed 0 or more times.

import std.stdio : writeln;

void main()
{
int x;
3.times(writeln(x++));
writeln("-");
writeln(x);
}

void times(int n, lazy void exp)
{
while (n--)
exp();
}

prints to the console:

0
1
2
−
3

A lazy parameter cannot be an lvalue.

The underlying delegate of the lazy parameter may be extracted by using the & operator:

void test(lazy int dg)
{
int delegate() dg_ = &dg;
assert(dg_() == 7);
assert(dg == dg_());
}

void main()
{
int a = 7;
test(a);
}

A lazy parameter of type void can accept an argument of any type.

See Also: Lazy Variadic Functions

Function Default Arguments

Function parameter declarations can have default values:

void foo(int x, int y = 3)
{
...
}
...
foo(4);   // same as foo(4, 3);

Default parameters are resolved and semantically checked in the context of the function declaration.

module m;
private immutable int b;
pure void g(int a=b){}

import m;
int b;
pure void f()
{
g();  // ok, uses m.b
}

The attributes of the AssignExpression are applied where the default expression is used.

module m;
int b;
pure void g(int a=b){}

import m;
enum int b = 3;
pure void f()
{
g();  // error, cannot access mutable global `m.b` in pure function
}

If the default value for a parameter is given, all following parameters must also have default values.

Return Ref Parameters

Return ref parameters are used with ref functions to ensure that the returned reference will not outlive the matching argument's lifetime.

ref int identity(return ref int x) {
return x; // pass-through function that does nothing
}

ref int fun() {
int x;
return identity(x); // Error: escaping reference to local variable x
}

ref int gun(return ref int x) {
return identity(x); // OK
}

Struct non-static methods marked with the return attribute ensure the returned reference will not outlive the struct instance.

struct S
{
private int x;
ref int get() return { return x; }
}

ref int escape()
{
S s;
return s.get(); // Error: escaping reference to local variable s
}

Returning the address of a ref variable is also checked.

int* pluto(ref int i)
{
return &i; // error: returning &i escapes a reference to parameter i
}

int* mars(return ref int i)
{
return &i;  // ok
}

If the function returns void, and the first parameter is ref or out, then all subsequent return ref parameters are considered as being assigned to the first parameter for lifetime checking. The this reference parameter to a struct non-static member function is considered the first parameter.

If there are multiple return ref parameters, the lifetime of the return value is the smallest lifetime of the corresponding arguments.

Neither the type of the return ref parameter(s) nor the type of the return value is considered when determining the lifetime of the return value.

It is not an error if the return type does not contain any indirections.

int mercury(return ref int i)
{
return i; // ok
}

Template functions, auto functions, nested functions and lambdas can deduce the return attribute.

ref int templateFunction()(ref int i)
{
return i; // ok
}

ref auto autoFunction(ref int i)
{
return i; // ok
}

void uranus()
{
ref int nestedFunction(ref int i)
{
return i; // ok
}
}

void venus()
{
auto lambdaFunction =
(ref int i)
{
return &i; // ok
};
}

inout ref parameters imply the return attribute.

inout(int)* neptune(inout ref int i)
{
return &i;  // ok
}

Scope Parameters

A scope parameter of reference type must not escape the function call (e.g. by being assigned to a global variable). It has no effect for non-reference types. scope escape analysis is only done for @safe functions. For other functions scope semantics must be manually enforced.

Note: scope escape analysis is currently only done by the dmd compiler when the -dip1000 switch is passed.

@safe:

int* gp;
void thorin(scope int*);
void gloin(int*);
int* balin(scope int* q, int* r)
{
gp = q; // error, q escapes to global gp
gp = r; // ok

thorin(q); // ok, q does not escape thorin()
thorin(r); // ok

gloin(q); // error, gloin() escapes q
gloin(r); // ok that gloin() escapes r

return q; // error, cannot return 'scope' q
return r; // ok
}

As a scope parameter must not escape, the compiler can potentially avoid heap-allocating a unique argument to a scope parameter. Due to this, passing an array literal, delegate literal or a NewExpression to a scope parameter may be allowed in a @nogc context, depending on the compiler implementation.

Return Scope Parameters

Parameters marked as return scope that contain indirections can only escape those indirections via the function's return value.

@safe:

int* gp;
void thorin(scope int*);
void gloin(int*);
int* balin(return scope int* p)
{
gp = p; // error, p escapes to global gp
thorin(p); // ok, p does not escape thorin()
gloin(p); // error, gloin() escapes p
return p; // ok
}

Class references are considered pointers that are subject to scope.

@safe:

class C { }
C gp;
void thorin(scope C);
void gloin(C);
C balin(return scope C p, scope C q, C r)
{
gp = p; // error, p escapes to global gp
gp = q; // error, q escapes to global gp
gp = r; // ok

thorin(p); // ok, p does not escape thorin()
thorin(q); // ok
thorin(r); // ok

gloin(p); // error, gloin() escapes p
gloin(q); // error, gloin() escapes q
gloin(r); // ok that gloin() escapes r

return p; // ok
return q; // error, cannot return 'scope' q
return r; // ok
}

return scope can be applied to the this of class and interface member functions.

class C
{
C bofur() return scope { return this; }
}

Template functions, auto functions, nested functions and lambdas can deduce the return scope attribute.

Ref Return Scope Parameters

Parameters marked as ref return scope come in two forms:

U xerxes(ref return scope V v);      // (1) ref and return scope
ref U sargon(ref return scope V v);  // (2) return ref and scope

The first form attaches the return to the scope, and has return scope parameter semantics for the value of the ref parameter.

The second form attaches the return to the ref, and has return ref parameter semantics with additional scope parameter semantics.

Although a struct constructor returns a reference to the instance being constructed, it is treated as form (1).

The lexical order of the attributes ref, return, and scope is not significant.

It is not possible to have both return ref and return scope semantics for the same parameter.

@safe:

struct S
{
this(return scope ref int* p) { ptr = p; }

int  val;
int* ptr;
}

int* foo1(ref return scope S s);
int  foo2(ref return scope S s);

ref int* foo3(ref return scope S s);
ref int  foo4(ref return scope S s);

int* test1(scope S s)
{
return foo1(s);  // Error: scope variable `s` may not be returned
return foo3(s);  // Error: scope variable `s` may not be returned
}

int test2(S s)
{
return foo2(s);
return foo4(s);
}

ref int* test3(S s)
{
return foo3(s);  // Error: returning `foo3(s)` escapes a reference to parameter `s`
}

ref int test4(S s)
{
return foo4(s);  // Error: returning `foo4(s)` escapes a reference to parameter `s`
}

S test5(ref scope int* p)
{
return S(p); // Error: scope variable `p` may not be returned
}

S test6(ref return scope int* p)
{
return S(p);
}

User-Defined Attributes for Parameters
See also: User-Defined Attributes
Variadic Functions

Variadic Functions take a variable number of arguments. There are three forms:

1. C-style variadic functions

2. Variadic functions with type info

3. Typesafe variadic functions

C-style Variadic Functions

A C-style variadic function is declared with a parameter ... as the last function parameter. It has non-D linkage, such as extern (C).

To access the variadic arguments, import the standard library module core.stdc.stdarg.

import core.stdc.stdarg;

extern (C) void dry(int x, int y, ...); // C-style Variadic Function

void spin()
{
dry(3, 4);      // ok, no variadic arguments
dry(3, 4, 6.8); // ok, one variadic argument
dry(2);         // error, no argument for parameter y
}

There must be at least one non-variadic parameter declared.

extern (C) int def(...); // error, must have at least one parameter

C-style variadic functions match the C calling convention for variadic functions, and can call C Standard library functions like printf.

extern (C) int printf(const(char)*, ...);

void main()
{
printf("hello world\n");
}

C-style variadic functions cannot be marked as @safe.

void wash()
{
rinse(3, 4, 5);   // first variadic argument is 5
}

import core.stdc.stdarg;
extern (C) void rinse(int x, int y, ...)
{
va_list args;
va_start(args, y); // y is the last named parameter
int z;
va_arg(args, z);   // z is set to 5
va_end(args);
}

D-style Variadic Functions

D-style variadic functions have D linkage and ... as the last parameter.

... can be the only parameter.

If there are parameters preceding the ... parameter, there must be a comma separating them from the ....

Note: If the comma is ommitted, it is a TypeSafe Variadic Function.

int abc(char c, ...);   // one required parameter: c
int def(...);           // no required parameters
int ghi(int i ...);     // a typesafe variadic function
//int boo(, ...);       // error

Two hidden arguments are passed to the function:

- void* _argptr

- TypeInfo[] _arguments

_argptr is a reference to the first of the variadic arguments. To access the variadic arguments, import core.vararg. Use _argptr in conjunction with core.va_arg:

import core.vararg;

void test()
{
foo(3, 4, 5);   // first variadic argument is 5
}

@system void foo(int x, int y, ...)
{
int z = va_arg!int(_argptr); // z is set to 5 and _argptr is advanced
// to the next argument
}

_arguments gives the number of arguments and the typeid of each, enabling type safety to be checked at run time.

import std.stdio;

void main()
{
Foo f = new Foo();
Bar b = new Bar();

writefln("%s", f);
printargs(1, 2, 3L, 4.5, f, b);
}

class Foo { int x = 3; }
class Bar { long y = 4; }

import core.vararg;

@system void printargs(int x, ...)
{
writefln("%d arguments", _arguments.length);
for (int i = 0; i < _arguments.length; i++)
{
writeln(_arguments[i]);

if (_arguments[i] == typeid(int))
{
int j = va_arg!(int)(_argptr);
writefln("\t%d", j);
}
else if (_arguments[i] == typeid(long))
{
long j = va_arg!(long)(_argptr);
writefln("\t%d", j);
}
else if (_arguments[i] == typeid(double))
{
double d = va_arg!(double)(_argptr);
writefln("\t%g", d);
}
else if (_arguments[i] == typeid(Foo))
{
Foo f = va_arg!(Foo)(_argptr);
writefln("\t%s", f);
}
else if (_arguments[i] == typeid(Bar))
{
Bar b = va_arg!(Bar)(_argptr);
writefln("\t%s", b);
}
else
assert(0);
}
}

which prints:

0x00870FE0
5 arguments
int
2
long
3
double
4.5
Foo
0x00870FE0
Bar
0x00870FD0

D-style variadic functions cannot be marked as @safe.

Typesafe Variadic Functions

A typesafe variadic function has D linkage and a variadic parameter declared as either an array or a class. The array or class is constructed from the arguments, and is passed as an array or class object.

For dynamic arrays:

int sum(int[] ar ...) // typesafe variadic function
{
int s;
foreach (int x; ar)
s += x;
return s;
}

import std.stdio;

void main()
{
writeln(stan());  // 6
writeln(ollie()); // 15
}

int stan()
{
return sum(1, 2, 3) + sum(); // returns 6+0
}

int ollie()
{
int[3] ii = [4, 5, 6];
return sum(ii);             // returns 15
}

For static arrays, the number of arguments must match the array dimension.

int sum(int[3] ar ...) // typesafe variadic function
{
int s;
foreach (int x; ar)
s += x;
return s;
}

int frank()
{
return sum(2, 3);    // error, need 3 values for array
return sum(1, 2, 3); // returns 6
}

int dave()
{
int[3] ii = [4, 5, 6];
int[] jj = ii;
return sum(ii); // returns 15
return sum(jj); // error, type mismatch
}

For class objects:

int tesla(int x, C c ...)
{
return x + c.x;
}

class C
{
int x;
string s;

this(int x, string s)
{
this.x = x;
this.s = s;
}
}

void edison()
{
C g = new C(3, "abc");
tesla(1, c);         // ok, since c is an instance of C
tesla(1, 4, "def");  // ok
tesla(1, 5);         // error, no matching constructor for C
}

The lifetime of the variadic class object or array instance ends at the end of the function.

C orville(C c ...)
{
return c;   // error, c instance contents invalid after return
}

int[] wilbur(int[] a ...)
{
return a;       // error, array contents invalid after return
return a[0..1]; // error, array contents invalid after return
return a.dup;   // ok, since copy is made
}

Implementation Defined: the variadic object or array instance may be constructed on the stack.

For other types, the argument is passed by value.

int neil(int i ...)
{
return i;
}

void buzz()
{
neil(3);    // returns 3
neil(3, 4); // error, too many arguments
int[] x;
neil(x);    // error, type mismatch
}

Lazy Variadic Functions

If the variadic parameter of a function is an array of delegates with no parameters, then each of the arguments whose type does not match that of the delegate is converted to a delegate of that type.

void hal(scope int delegate()[] dgs ...);

void dave()
{
int delegate() dg;
hal(1, 3+x, dg, cast(int delegate())null);   // (1)
hal( { return 1; }, { return 3+x; }, dg, null ); // same as (1)
}

The variadic delegate array differs from using a lazy variadic array. With the former each array element access would evaluate every array element. With the latter, only the element being accessed would be evaluated.

import std.stdio;

void main()
{
int x;
ming(++x, ++x);

int y;
flash(++y, ++y);
}

// lazy variadic array
void ming(lazy int[] arr...)
{
writeln(arr[0]); // 1
writeln(arr[1]); // 4
}

// variadic delegate array
void flash(scope int delegate()[] arr ...)
{
writeln(arr[0]); // 1
writeln(arr[1]); // 2
}

Best Practices: Use scope when declaring the array of delegates parameter. This will prevent a closure being generated for the delegate, as scope means the delegate will not escape the function.

Local Variables

Local variables are declared within the scope of a function. Function parameters are included.

A local variable cannot be read without first assigning it a value.

Implementation Defined: The implementation may not always be able to detect these cases.

The address of or reference to a local non-static variable cannot be returned from the function.

A local variable and a label in the same function cannot have the same name.

A local variable cannot hide another local variable in the same function.

Rationale: whenever this is done it often is a bug or at least looks like a bug.

ref double func(int x)
{
int x;       // error, hides previous definition of x
double y;
{
char y;  // error, hides previous definition of y
int z;
}
{
wchar z; // Ok, previous z is out of scope
}
z:             // error, z is a local variable and a label
return y;    // error, returning ref to local
}

Local Static Variables

Local variables in functions declared as static, shared static or __gshared are statically allocated rather than being allocated on the stack. The lifetime of __gshared and shared static variables begins when the function is first executed and ends when the program ends. The lifetime of static variables begins when the function is first executed within the thread and ends when that thread terminates.

void foo()
{
static int n;
if (++n == 100)
writeln("called 100 times");
}

The initializer for a static variable must be evaluatable at compile time. There are no static constructors or static destructors for static local variables.

Although static variable name visibility follows the usual scoping rules, the names of them must be unique within a particular function.

void main()
{
{ static int x; }
{ static int x; } // error
{ int i; }
{ int i; } // ok
}

Nested Functions

Functions may be nested within other functions:

int bar(int a)
{
int foo(int b)
{
int abc() { return 1; }

return b + abc();
}
return foo(a);
}

void test()
{
int i = bar(3); // i is assigned 4
}

Nested functions can be accessed only if the name is in scope.

void foo()
{
void A()
{
B(); // error, B() is forward referenced
C(); // error, C undefined
}
void B()
{
A(); // ok, in scope
void C()
{
void D()
{
A();      // ok
B();      // ok
C();      // ok
D();      // ok
}
}
}
A(); // ok
B(); // ok
C(); // error, C undefined
}

and:

int bar(int a)
{
int foo(int b) { return b + 1; }
int abc(int b) { return foo(b); }   // ok
return foo(a);
}

void test()
{
int i = bar(3);     // ok
int j = bar.foo(3); // error, bar.foo not visible
}

Nested functions have access to the variables and other symbols defined by the lexically enclosing function. This access includes both the ability to read and write them.

int bar(int a)
{
int c = 3;

int foo(int b)
{
b += c;       // 4 is added to b
c++;          // bar.c is now 5
return b + c; // 12 is returned
}
c = 4;
int i = foo(a); // i is set to 12
return i + c;   // returns 17
}

void test()
{
int i = bar(3); // i is assigned 17
}

This access can span multiple nesting levels:

int bar(int a)
{
int c = 3;

int foo(int b)
{
int abc()
{
return c;   // access bar.c
}
return b + c + abc();
}
return foo(3);
}

Static nested functions cannot access any stack variables of any lexically enclosing function, but can access static variables. This is analogous to how static member functions behave.

int bar(int a)
{
int c;
static int d;

static int foo(int b)
{
b = d;          // ok
b = c;          // error, foo() cannot access frame of bar()
return b + 1;
}
return foo(a);
}

Functions can be nested within member functions:

struct Foo
{
int a;

int bar()
{
int c;

int foo()
{
return c + a;
}
return 0;
}
}

Nested functions always have the D function linkage type.

Unlike module level declarations, declarations within function scope are processed in order. This means that two nested functions cannot mutually call each other:

void test()
{
void foo() { bar(); } // error, bar not defined
void bar() { foo(); } // ok
}

There are several workarounds for this limitation:

- Declare the functions to be static members of a nested struct:

void test()
{
static struct S
{
static void foo() { bar(); } // ok
static void bar() { foo(); } // ok
}

S.foo();  // compiles (but note the infinite runtime loop)
}

- Declare one or more of the functions to be function templates even if they take no specific template arguments:

void test()
{
void foo()() { bar(); } // ok (foo is a function template)
void bar()   { foo(); } // ok
}

- Declare the functions inside of a mixin template:

mixin template T()
{
void foo() { bar(); } // ok
void bar() { foo(); } // ok
}

void main()
{
mixin T!();
}

- Use a delegate:

void test()
{
void delegate() fp;
void foo() { fp(); }
void bar() { foo(); }
fp = &bar;
}

Nested functions cannot be overloaded.

Delegates, Function Pointers, and Closures

A function pointer can point to a static nested function:

int function() fp;  // fp is a pointer to a function returning an int

void test()
{
static int a = 7;
static int foo() { return a + 3; }

fp = &foo;
}

void bar()
{
test();
int i = fp();       // i is set to 10
}

Implementation Defined: Two functions with identical bodies, or two functions that compile to identical assembly code, are not guaranteed to have distinct function pointer values. The implementation may merge functions bodies into one if they compile to identical code.

int abc(int x)   { return x + 1; }
uint def(uint y) { return y + 1; }

int delegate(int)   fp1 = &abc;
uint delegate(uint) fp2 = &def;
// Do not rely on fp1 and fp2 being different values; the compiler may merge
// them.

A delegate can be set to a non-static nested function:

int delegate() dg;

void test()
{
int a = 7;
int foo() { return a + 3; }

dg = &foo;
int i = dg(); // i is set to 10
}

The stack variables referenced by a nested function are still valid even after the function exits (NOTE this is different from D 1.0.) This is called a closure.

Those referenced stack variables that make up the closure are allocated on the GC heap. Closures are not allowed for @nogc functions.

Note: Returning addresses of stack variables, however, is not a closure and is an error.

void bar()
{
int b;
test();
int i = dg(); // ok, test.a is in a closure and still exists
}

Delegates to non-static nested functions contain two pieces of data: the pointer to the stack frame of the lexically enclosing function (called the context pointer) and the address of the function. This is analogous to struct/class non-static member function delegates consisting of a this pointer and the address of the member function. Both forms of delegates are indistinguishable, and are the same type.

struct Foo
{
int a = 7;
int bar() { return a; }
}

int foo(int delegate() dg)
{
return dg() + 1;
}

void test()
{
int x = 27;
int abc() { return x; }
Foo f;
int i;

i = foo(&abc);   // i is set to 28
i = foo(&f.bar); // i is set to 8
}

This combining of the environment and the function is called a dynamic closure.

The .ptr property of a delegate will return the context pointer value as a void*.

The .funcptr property of a delegate will return the function pointer value as a function type.

Function pointers are zero-initialized by default. They can be initialized to the address of any function (including a function literal). Initialization with the address of a function that requires a context pointer is not allowed in @safe functions.

struct S
{
static int sfunc();
int member();   // has hidden `this` reference parameter
}

@safe void sun()
{
int function() fp = &S.sfunc;
fp(); // Ok
fp = &S.member; // error
}

@system void moon()
{
int function() fp = &S.member; // Ok because @system
fp(); // undefined behavior
}

Delegates are zero-initialized by default. They can be initialized by taking the address of a non-static member function, but a context pointer must be supplied. They can be initialized by taking the address of a non-static nested function or function literal, where the context pointer will be set to point to the stack frame, closure, or null.

Delegates cannot be initialized by taking the address of a global function, a static member function, or a static nested function.

struct S
{
static int sfunc();
int member() { return 1; }
}

void main()
{
S s;
int delegate() dg = &s.member; // Ok, s supplies context pointer
assert(dg() == 1);

dg = &S.sfunc;  // error
dg = &S.member; // error

int moon() { return 2; }
dg = &moon;     // Ok
assert(dg() == 2);

static int mars() { return 3; }
dg = &mars;     // error

dg = () { return 4; }; // Ok
assert(dg() == 4);
}

Note: Function pointers can be passed to functions taking a delegate argument by passing them through the std.functional.toDelegate template, which converts any callable to a delegate with a null context pointer.

Anonymous Functions and Anonymous Delegates

See FunctionLiterals.

main() Function

For console programs, main() serves as the entry point. It gets called after all the module initializers are run, and after any unittests are run. After it returns, all the module destructors are run. main() must be declared using one of the following forms:

- void main() { ... }

- void main(string[] args) { ... }

- int main() { ... }

- int main(string[] args) { ... }

The main function must have D linkage.

Attributes may be added as needed, e.g. @safe, @nogc, nothrow, etc.

BetterC main() Function

For BetterC programs, the main function is declared using one of the following forms:

- extern (C) int main() { ... }

- extern (C) int main(int argc, char** argv) { ... }

This takes the place of the C main function and serves the identical purpose.

Module constructors, module destructors, and unittests are not run.

Implementation Defined: Other system-specific entry points may exist, such as WinMain and DllMain on Windows systems.

Function Templates

Functions can have compile time arguments in the form of a template. See function templates.

Compile Time Function Execution (CTFE)

In contexts where a compile time value is required, functions can be used to compute those values. This is called Compile Time Function Execution, or CTFE.

These contexts are:

- initialization of a static variable or a manifest constant

- static initializers of struct/class members

- dimension of a static array

- argument for a template value parameter

- static if

- static foreach

- static assert

- mixin statement

- pragma argument

- __traits argument

enum eval(Args...) = Args[0];

int square(int i)
{
return i * i;
}

void foo()
{
static j = square(3);      // CTFE
writeln(j);
assert(square(4));         // run time
static assert(square(3) == 9); // CTFE
writeln(eval!(square(5))); // CTFE
}

The function must have a SpecifiedFunctionBody.

CTFE is subject to the following restrictions:

1. Expressions may not reference any global or local static variables.

2. AsmStatements are not permitted

3. Non-portable casts (eg, from int[] to float[]), including casts which depend on endianness, are not permitted. Casts between signed and unsigned types are permitted.

4. Reinterpretation of overlapped fields in a union is not permitted.

Pointers are permitted in CTFE, provided they are used safely:

- Pointer arithmetic is permitted only on pointers which point to static or dynamic array elements. A pointer may also point to the first element past the array, although such pointers cannot be dereferenced. Pointer arithmetic on pointers which are null, or which point to a non-array, is not allowed.

- Ordered comparison (<, <=, >, >=) between two pointers is permitted when both pointers point to the same array, or when at least one pointer is null.

- Pointer comparisons between discontiguous memory blocks are illegal, unless two such comparisons are combined using && or || to yield a result which is independent of the ordering of memory blocks. Each comparison must consist of two pointer expressions compared with <, <=, >, or >=, and may optionally be negated with !. For example, the expression (p1 > q1 && p2 <= q2) is permitted when p1, p2 are expressions yielding pointers to memory block P, and q1, q2 are expressions yielding pointers to memory block Q, even when P and Q are unrelated memory blocks. It returns true if [p1..p2] lies inside [q1..q2], and false otherwise. Similarly, the expression (p1 < q1 || p2 > q2) is true if [p1..p2] lies outside [q1..q2], and false otherwise.

- Equality comparisons (==, !=, is, !is) are permitted between all pointers, without restriction.

- Any pointer may be cast to void* and from void* back to its original type. Casting between pointer and non-pointer types is illegal.

The above restrictions apply only to expressions which are actually executed. For example:

static int y = 0;

int countTen(int x)
{
if (x > 10)
++y;    // access static variable
return x;
}

static assert(countTen(6) == 6);    // OK
static assert(countTen(12) == 12);  // invalid, modifies y.

The __ctfe boolean pseudo-variable evaluates to true during CTFE but false otherwise.

Note: __ctfe can be used to provide an alternative execution path to avoid operations which are forbidden in CTFE. Every usage of __ctfe is statically evaluated and has no run-time cost.

Non-recoverable errors (such as assert failures) are illegal.

Implementation Defined: Executing functions via CTFE can take considerably longer than executing it at run time. If the function goes into an infinite loop, it may cause the compiler to hang.

Implementation Defined: Functions executed via CTFE can give different results from run time when implementation-defined or undefined-behavior occurs.

String Mixins and Compile Time Function Execution

All functions that execute in CTFE must also be executable at run time. The compile time evaluation of a function does the equivalent of running the function at run time. The semantics of a function cannot depend on compile time values of the function. For example:

int foo(string s)
{
return mixin(s);
}

const int x = foo("1");

is illegal, because the runtime code for foo cannot be generated.
Best Practices: A function template, where s is a template argument, would be the appropriate method to implement this sort of thing.

No-GC Functions

No-GC functions are functions marked with the @nogc attribute. Those functions do not allocate memory on the GC heap. These operations are not allowed in No-GC functions:

1. constructing an array on the heap

2. resizing an array by writing to its .length property

3. array concatenation

4. array appending

5. constructing an associative array

6. indexing an associative array
Note: because it may throw RangeError if the specified key is not present

7. allocating an object with new on the heap

8. calling functions that are not @nogc, unless the call is in a ConditionalStatement controlled by a DebugCondition

@nogc void foo()
{
auto a = ['a'];    // (1) error, allocates
a.length = 1;      // (2) error, array resizing allocates
a = a ~ a;         // (3) error, arrays concatenation allocates
a ~= 'c';          // (4) error, appending to arrays allocates

auto aa = ["x":1]; // (5) error, allocates
aa["abc"];         // (6) error, indexing may allocate and throws

auto p = new int;  // (7) error, operator new allocates
bar();             // (8) error, bar() may allocate
debug bar();       // (8) Ok
}
void bar() { }

No-GC functions cannot be closures.

@nogc int delegate() foo()
{
int n;              // error, variable n cannot be allocated on heap
return (){ return n; } // since `n` escapes `foo()`, a closure is required
}

@nogc affects the type of the function. A @nogc function is covariant with a non-@nogc function.

void function() fp;
void function() @nogc gp;  // pointer to @nogc function

void foo();
@nogc void bar();

void test()
{
fp = &foo; // ok
fp = &bar; // ok, it's covariant
gp = &foo; // error, not contravariant
gp = &bar; // ok
}

Function Safety

Safe Functions

Safe functions are marked with the @safe attribute. @safe can be inferred, see Function Attribute Inference.

Safe functions have safe interfaces. An implementation must enforce this by restricting the function's body to operations that are known safe.

The following operations are not allowed in safe functions:

- No casting from a pointer type to any type with pointers other than void*.

- No casting from any non-pointer type to a pointer type.

- No pointer arithmetic (including pointer indexing).

- Cannot access unions that have pointers or references overlapping with other types.

- Cannot access unions that have fields with invariants overlapping with other types.

- Calling any System Functions.

- No catching of exceptions that are not derived from class Exception.

- No inline assembler.

- No explicit casting of mutable objects to immutable.

- No explicit casting of immutable objects to mutable.

- No explicit casting of thread local objects to shared.

- No explicit casting of shared objects to thread local.

- Cannot access __gshared variables.

- Cannot use void initializers for pointers.

- Cannot use void initializers for class or interface references.

- Cannot use void initializers for types that have invariants.

When indexing or slicing an array, an out of bounds access will cause a runtime error.

Functions nested inside safe functions default to being safe functions.

Safe functions are covariant with trusted or system functions.

Best Practices: Mark as many functions @safe as practical.

Safe External Functions

External functions don't have a function body visible to the compiler:

@safe extern (C) void play();

and so safety cannot be verified automatically.
Best Practices: Explicitly set an attribute for external functions rather than relying on default settings.

Trusted Functions

Trusted functions are marked with the @trusted attribute.

Like safe functions, trusted functions have safe interfaces. Unlike safe functions, this is not enforced by restrictions on the function body. Instead, it is the responsibility of the programmer to ensure that the interface of a trusted function is safe.

Example:

immutable(int)* f(int* p) @trusted
{
version (none) p[2] = 13;
// Invalid. p[2] is out of bounds. This line would exhibit undefined
// behavior.

version (none) p[1] = 13;
// Invalid. In this program, p[1] happens to be in-bounds, so the
// line would not exhibit undefined behavior, but a trusted function
// is not allowed to rely on this.

version (none) return cast(immutable) p;
// Invalid. @safe code still has mutable access and could trigger
// undefined behavior by overwriting the value later on.

int* p2 = new int;
*p2 = 42;
return cast(immutable) p2;
// Valid. After f returns, no mutable aliases of p2 can exist.
}

void main() @safe
{
int[2] a = [10, 20];
int* mp = &a[0];
immutable(int)* ip = f(mp);
assert(a[1] == 20); // Guaranteed. f cannot access a[1].
assert(ip !is mp); // Guaranteed. f cannot introduce unsafe aliasing.
}

Trusted functions may call safe, trusted, or system functions.

Trusted functions are covariant with safe or system functions.

Best Practices: Trusted functions should be kept small so that they are easier to manually verify.

System Functions

System functions are functions not marked with @safe or @trusted and are not nested inside @safe functions. System functions may be marked with the @system attribute. A function being system does not mean it actually is unsafe, it just means that its safety must be manually verified.

System functions are not covariant with trusted or safe functions.

System functions can call safe and trusted functions.

Best Practices: When in doubt, mark extern (C) and extern (C++) functions as @system when their implementations are not in D, as the D compiler will be unable to check them. Most of them are @safe, but will need to be manually checked.

Best Practices: The number and size of system functions should be minimized. This minimizes the work necessary to manually check for safety.

Safe Interfaces

When it is only called with safe values and safe aliasing, a function has a safe interface when:

1. it cannot exhibit undefined behavior, and

2. it cannot create unsafe values that are accessible from other parts of the program (e.g., via return values, global variables, or ref parameters), and

3. it cannot introduce unsafe aliasing that is accessible from other parts of the program.

Functions that meet these requirements may be @safe or @trusted. Function that do not meet these requirements can only be @system.

Examples:

- C's free does not have a safe interface:

extern (C) @system void free(void* ptr);

because free(p) invalidates p, making its value unsafe. free can only be @system.

- C's strlen and memcpy do not have safe interfaces:

extern (C) @system size_t strlen(char* s);
extern (C) @system void* memcpy(void* dst, void* src, size_t nbytes);

because they iterate pointers based on unverified assumptions (strlen assumes that s is zero-terminated; memcpy assumes that the memory objects pointed to by dst and src are at least nbytes big). Any function that traverses a C string passed as an argument can only be @system. Any function that trusts a separate parameter for array bounds can only be @system.

- C's malloc does have a safe interface:

extern (C) @trusted void* malloc(size_t sz);

It does not exhibit undefined behavior for any input. It returns either a valid pointer, which is safe, or null which is also safe. It returns a pointer to a fresh allocation, so it cannot introduce any unsafe aliasing.
Note: The implementation of malloc is most likely @system code.

- A D version of memcpy can have a safe interface:

@safe void memcpy(E)(E[] src, E[] dst)
{
import std.math : min;
foreach (i; 0 .. min(src.length, dst.length))
{
dst[i] = src[i];
}
}

because the rules for safe values ensure that the lengths of the arrays are correct.

Safe Values

For basic data types, all possible bit patterns are safe.

A pointer is a safe value when it is one of:

1. null

2. it points to a memory object that is live and the pointed to value in that memory object is safe.

Examples:

int* n = null; /* n is safe because dereferencing null is a well-defined
crash. */
int* x = cast(int*) 0xDEADBEEF; /* x is (most likely) unsafe because it
is not a valid pointer and cannot be dereferenced. */

import core.stdc.stdlib: malloc, free;
int* p1 = cast(int*) malloc(int.sizeof); /* p1 is safe because the
pointer is valid and *p1 is safe regardless of its actual value. */
free(p1); /* This makes p1 unsafe. */
int** p2 = &p1; /* While it can be dereferenced, p2 is unsafe because p1
is unsafe. */
p1 = null; /* This makes p1 and p2 safe. */

A dynamic array is safe when:

1. its pointer is safe, and

2. its length is in-bounds with the corresponding memory object, and

3. all its elements are safe.

Examples:

int[] f() @system
{
int[3] a;
int[] d1 = a[0 .. 2]; /* d1 is safe. */
int[] d2 = a.ptr[0 .. 3]; /* d2 is unsafe because it goes beyond a's
bounds. */
int*[] d3 = [cast(int*) 0xDEADBEEF]; /* d3 is unsafe because the
element is unsafe. */
return d1; /* Up to here, d1 was safe, but its pointer becomes
invalid when the function returns, so the returned dynamic array
is unsafe. */
}

A static array is safe when all its elements are safe. Regardless of the element type, a static array with length zero is always safe.

An associative array is safe when all its keys and elements are safe.

A struct/union instance is safe when:

1. the values of its accessible fields are safe, and

2. it does not introduce unsafe aliasing with unions.

Examples:

struct S { int* p; }
S s1 = S(new int); /* s1 is safe. */
S s2 = S(cast(int*) 0xDEADBEEF); /* s2 is unsafe, because s2.p is
unsafe. */

union U { int* p; size_t x; }
U u = U(new int); /* Even though both u.p and u.x are safe, u is unsafe
because of unsafe aliasing. */

A class reference is safe when it is null or:

1. it refers to a valid class instance of the class type or a type derived from the class type, and

2. the values of the instance's accessible fields are safe, and

3. it does not introduce unsafe aliasing with unions.

A function pointer is safe when it is null or it refers to a valid function that has the same or a covariant signature.

A delegate is safe when:

1. its .funcptr property is null or refers to a function that matches or is covariant with the delegate type, and

2. its .ptr property is null or refers to a memory object that is in a form expected by the function.

Safe Aliasing

When one memory location is accessible with two different types, that aliasing is considered safe if:

1. both types are const or immutable; or

2. one of the types is mutable while the other is a const-qualified basic data type; or

3. both types are mutable basic data types; or

4. one of the types is a static array type with length zero; or

5. one of the types is a static array type with non-zero length, and aliasing of the array's element type and the other type is safe; or

6. both types are pointer types, and aliasing of the target types is safe, and the target types have the same size.

All other cases of aliasing are considered unsafe.

Note: Safe aliasing may be exposed to functions with safe interfaces without affecting their guaranteed safety. Unsafe aliasing does not guarantee safety.

Note: Safe aliasing does not imply that all aliased views of the data have safe values. Those must examined separately for safety.

Examples:

void f1(ref ubyte x, ref float y) @safe { x = 0; y = float.init; }
union U1 { ubyte x; float y; } // safe aliasing
U1 u1;
f1(u1.x, u1.y); // Ok

void f2(ref int* x, ref int y) @trusted { x = new int; y = 0xDEADBEEF; }
union U2 { int* x; int y; } // unsafe aliasing
U2 u2;
version (none) f1(u2.x, u2.y); // not safe

Function Attribute Inference

FunctionLiterals, Auto Functions, Auto Ref Functions, nested functions and function templates, since their function bodies are always present, infer the pure, nothrow, @safe, @nogc, return ref parameters, scope parameters, return scope parameters and ref return scope parameters attributes unless specifically overridden.

Attribute inference is not done for other functions, even if the function body is present.

The inference is done by determining if the function body follows the rules of the particular attribute.

Cyclic functions (i.e. functions that wind up directly or indirectly calling themselves) are inferred as being impure, throwing, and @system.

If a function attempts to test itself for those attributes, then the function is inferred as not having those attributes.

Rationale: Function attribute inference greatly reduces the need for the user to add attributes to functions, especially for templates.

Uniform Function Call Syntax (UFCS)

A free function can be called with a syntax equivalent to that of a member function of its first parameter type. The free function is called a UFCS function.

void sun(T, int);

void moon(T t)
{
t.sun(1);
// If `T` does not have member function `sun`,
// `t.sun(1)` is interpreted as if it were written `sun(t, 1)`
}

Rationale: This provides a way to add external functions to a class as if they were public final member functions. This enables minimizing the number of functions in a class to only the essentials that are needed to take care of the object's private state, without the temptation to add a kitchen-sink's worth of member functions. It also enables function chaining and component programming.

A more complex example:

stdin.byLine(KeepTerminator.yes)
.map!(a => a.idup)
.array
.sort
.copy(stdout.lockingTextWriter());

is the equivalent of:

copy(sort(array(map!(a => a.idup)(byLine(stdin, KeepTerminator.yes)))), lockingTextWriter(stdout));

UFCS works with @property functions:

@property prop(X thisObj);
@property prop(X thisObj, int value);

X obj;
obj.prop;      // if X does not have member prop, reinterpret as prop(obj);
obj.prop = 1;  // similarly, reinterpret as prop(obj, 1);

Functions declared in a local scope are not found when searching for a matching UFCS function.

Member functions are not found when searching for a matching UFCS function.

Otherwise, UFCS function lookup proceeds normally.

module a;
void foo(X);
alias boo = foo;
void main()
{
void bar(X);     // bar declared in local scope
import b : baz;  // void baz(X);

X obj;
obj.foo();    // OK, calls a.foo;
//obj.bar();  // NG, UFCS does not see nested functions
obj.baz();    // OK, calls b.baz, because it is declared at the
// top level scope of module b

import b : boo = baz;
obj.boo();    // OK, calls aliased b.baz instead of a.boo (== a.foo),
// because the declared alias name 'boo' in local scope
// overrides module scope name
}
class C
{
void mfoo(X);           // member function
static void sbar(X);    // static member function
import b : ibaz = baz;  // void baz(X);
void test()
{
X obj;
//obj.mfoo();  // NG, UFCS does not see member functions
//obj.sbar();  // NG, UFCS does not see static member functions
obj.ibaz();    // OK, ibaz is an alias of baz which declared at
//     the top level scope of module b
}
}

Rationale: Local function symbols are not considered by UFCS to avoid unexpected name conflicts. See below problematic examples.

int front(int[] arr) { return arr[0]; }

void main()
{
int[] a = [1,2,3];
auto x = a.front();   // call .front by UFCS

auto front = x;       // front is now a variable
auto y = a.front();   // Error, front is not a function
}

class C
{
int[] arr;
int front()
{
return arr.front(); // Error, C.front is not callable
// using argument types (int[])
}
}

Operator Overloading

Operator overloading is accomplished by rewriting operators whose operands are class or struct objects into calls to specially named members. No additional syntax is used.

- Unary Operator Overloading

- Cast Operator Overloading

- Binary Operator Overloading

- Overloading the Comparison Operators

- Overloading == and !=

- Overloading <, <=, >, and >=

- Function Call Operator Overloading

- Assignment Operator Overloading

- Op Assignment Operator Overloading

- Array Indexing and Slicing Operators Overloading

- Index Operator Overloading

- Slice Operator Overloading

- Dollar Operator Overloading

- Forwarding

- D1 style operator overloading

Unary Operator Overloading

Overloadable Unary Operators

op

rewrite

-e

e.opUnary!("-")()

+e

e.opUnary!("+")()

~e

e.opUnary!("~")()

*e

e.opUnary!("*")()

++e

e.opUnary!("++")()

--e

e.opUnary!("--")()

For example, in order to overload the - (negation) operator for struct S, and no other operator:

struct S
{
int m;

int opUnary(string s)() if (s == "-")
{
return -m;
}
}

int foo(S s)
{
return -s;
}

Postincrement e++ and Postdecrement e-- Operators

These are not directly overloadable, but instead are rewritten in terms of the ++e and --e prefix operators:

Postfix Operator Rewrites

op

rewrite

e--

(auto t = e, --e, t)

e++

(auto t = e, ++e, t)

Overloading Index Unary Operators

Overloadable Index Unary Operators

op

rewrite

-a[b_1, b_2, ... bn]

a.opIndexUnary!("-")(b_1, b_2, ... bn)

+a[b_1, b_2, ... bn]

a.opIndexUnary!("+")(b_1, b_2, ... bn)

~a[b_1, b_2, ... bn]

a.opIndexUnary!("~")(b_1, b_2, ... bn)

*a[b_1, b_2, ... bn]

a.opIndexUnary!("*")(b_1, b_2, ... bn)

++a[b_1, b_2, ... bn]

a.opIndexUnary!("++")(b_1, b_2, ... bn)

--a[b_1, b_2, ... bn]

a.opIndexUnary!("--")(b_1, b_2, ... bn)

Overloading Slice Unary Operators

Overloadable Slice Unary Operators

op

rewrite

-a[i..j]

a.opIndexUnary!("-")(a.opSlice(i, j))

+a[i..j]

a.opIndexUnary!("+")(a.opSlice(i, j))

~a[i..j]

a.opIndexUnary!("~")(a.opSlice(i, j))

*a[i..j]

a.opIndexUnary!("*")(a.opSlice(i, j))

++a[i..j]

a.opIndexUnary!("++")(a.opSlice(i, j))

--a[i..j]

a.opIndexUnary!("--")(a.opSlice(i, j))

-a[ ]

a.opIndexUnary!("-")()

+a[ ]

a.opIndexUnary!("+")()

~a[ ]

a.opIndexUnary!("~")()

*a[ ]

a.opIndexUnary!("*")()

++a[ ]

a.opIndexUnary!("++")()

--a[ ]

a.opIndexUnary!("--")()

For backward compatibility, if the above rewrites fail to compile and opSliceUnary is defined, then the rewrites a.opSliceUnary!(op)(i, j) and a.opSliceUnary!(op) are tried instead, respectively.

Cast Operator Overloading

To define how one type can be cast to another, define the opCast template method, which is used as follows:

Cast Operators

op

rewrite

cast(type) e

e.opCast!(type)()

Note that opCast is only ever used with an explicit cast expression, except in the case of boolean operations (see next section)

Boolean Operations

Notably absent from the list of overloaded unary operators is the ! logical negation operator. More obscurely absent is a unary operator to convert to a bool result. Instead, these are covered by a rewrite to:

opCast!(bool)(e)

So,

if (e)   =>  if (e.opCast!(bool))
if (!e)  =>  if (!e.opCast!(bool))

etc., whenever a bool result is expected. This only happens, however, for instances of structs. Class references are converted to bool by checking to see if the class reference is null or not.

Binary Operator Overloading

The following binary operators are overloadable:

Overloadable Binary Operators

+

-

*

/

%

^^

&

|

^

<<

>>

>>>

~

in

The expression:

a op b

is rewritten as both:

a.opBinary!("op")(b)
b.opBinaryRight!("op")(a)

and the one with the ‘better’ match is selected. It is an error for both to equally match.

Operator overloading for a number of operators can be done at the same time. For example, if only the + or - operators are supported:

T opBinary(string op)(T rhs)
{
static if (op == "+") return data + rhs.data;
else static if (op == "-") return data - rhs.data;
else static assert(0, "Operator "~op~" not implemented");
}

To do them all en masse:

T opBinary(string op)(T rhs)
{
return mixin("data "~op~" rhs.data");
}

Note that opIn and opIn_r have been deprecated in favor of opBinary!"in" and opBinaryRight!"in" respectively.

Overloading the Comparison Operators

D allows overloading of the comparison operators ==, !=, <, <=, >=, > via two functions, opEquals and opCmp.

The equality and inequality operators are treated separately because while practically all user-defined types can be compared for equality, only a subset of types have a meaningful ordering. For example, while it makes sense to determine if two RGB color vectors are equal, it is not meaningful to say that one color is greater than another, because colors do not have an ordering. Thus, one would define opEquals for a Color type, but not opCmp.

Furthermore, even with orderable types, the order relation may not be linear. For example, one may define an ordering on sets via the subset relation, such that x < y is true if x is a (strict) subset of y. If x and y are disjoint sets, then neither x < y nor y < x holds, but that does not imply that x == y. Thus, it is insufficient to determine equality purely based on opCmp alone. For this reason, opCmp is only used for the inequality operators <, <=, >=, and >. The equality operators == and != always employ opEquals instead.

Therefore, it is the programmer's responsibility to ensure that opCmp and opEquals are consistent with each other. If opEquals is not specified, the compiler provides a default version that does member-wise comparison. If this suffices, one may define only opCmp to customize the behaviour of the inequality operators. But if not, then a custom version of opEquals should be defined as well, in order to preserve consistent semantics between the two kinds of comparison operators.

Finally, if the user-defined type is to be used as a key in the built-in associative arrays, then the programmer must ensure that the semantics of opEquals and toHash are consistent. If not, the associative array may not work in the expected manner.

Overloading == and !=

Expressions of the form a != b are rewritten as !(a == b).

Given a == b :

1. If a and b are both class objects, then the expression is rewritten as:

.object.opEquals(a, b)

and that function is implemented as:

bool opEquals(Object a, Object b)
{
if (a is b) return true;
if (a is null || b is null) return false;
if (typeid(a) == typeid(b)) return a.opEquals(b);
return a.opEquals(b) && b.opEquals(a);
}

2. Otherwise the expressions a.opEquals(b) and b.opEquals(a) are tried. If both resolve to the same opEquals function, then the expression is rewritten to be a.opEquals(b).

3. If one is a better match than the other, or one compiles and the other does not, the first is selected.

4. Otherwise, an error results.

If overridding Object.opEquals() for classes, the class member function signature should look like:

class C
{
override bool opEquals(Object o) { ... }
}

If structs declare an opEquals member function for the identity comparison, it could have several forms, such as:

struct S
{
// lhs should be mutable object
bool opEquals(const S s) { ... }        // for r-values (e.g. temporaries)
bool opEquals(ref const S s) { ... }    // for l-values (e.g. variables)

// both hand side can be const object
bool opEquals(const S s) const { ... }  // for r-values (e.g. temporaries)
}

Alternatively, declare a single templated opEquals function with an auto ref parameter:

struct S
{
// for l-values and r-values,
// with converting both hand side implicitly to const
bool opEquals()(auto ref const S s) const { ... }
}

Overloading <, <=, >, and >=

Comparison operations are rewritten as follows:

Rewriting of comparison operations

comparison

rewrite 1

rewrite 2

a < b

a.opCmp(b) < 0

b.opCmp(a) > 0

a <= b

a.opCmp(b) <= 0

b.opCmp(a) >= 0

a >b

a.opCmp(b) > 0

b.opCmp(a) < 0

a >= b

a.opCmp(b) >= 0

b.opCmp(a) <= 0

Both rewrites are tried. If only one compiles, that one is taken. If they both resolve to the same function, the first rewrite is done. If they resolve to different functions, the best matching one is used. If they both match the same, but are different functions, an ambiguity error results.

If overriding Object.opCmp() for classes, the class member function signature should look like:

class C
{
override int opCmp(Object o) { ... }
}

If structs declare an opCmp member function, it should have the following form:

struct S
{
int opCmp(ref const S s) const { ... }
}

Note that opCmp is only used for the inequality operators; expressions like a == b always uses opEquals. If opCmp is defined but opEquals isn't, the compiler will supply a default version of opEquals that performs member-wise comparison. If this member-wise comparison is not consistent with the user-defined opCmp, then it is up to the programmer to supply an appropriate version of opEquals. Otherwise, inequalities like a <= b will behave inconsistently with equalities like a == b.

Function Call Operator Overloading f()

The function call operator, (), can be overloaded by declaring a function named opCall:

struct F
{
int opCall();
int opCall(int x, int y, int z);
}

void test()
{
F f;
int i;

i = f();      // same as i = f.opCall();
i = f(3,4,5); // same as i = f.opCall(3,4,5);
}

In this way a struct or class object can behave as if it were a function.

Note that merely declaring opCall automatically disables struct literal syntax. To avoid the limitation, declare a constructor so that it takes priority over opCall in Type(...) syntax.

struct Multiplier
{
int factor;
this(int num) { factor = num; }
int opCall(int value) { return value * factor; }
}
void test()
{
Multiplier m = Multiplier(10);  // invoke constructor
int result = m(5);              // invoke opCall
assert(result == 50);
}

Static opCall

static opCall also works as expected for a function call operator with type names.

struct Double
{
static int opCall(int x) { return x * 2; }
}
void test()
{
int i = Double(2);
assert(i == 4);
}

Mixing struct constructors and static opCall is not allowed.

struct S
{
this(int i) {}
static S opCall()  // disallowed due to constructor
{
return S.init;
}
}

Note: static opCall can be used to simulate struct constructors with no arguments, but this is not recommended practice. Instead, the preferred solution is to use a factory function to create struct instances.

Assignment Operator Overloading

The assignment operator = can be overloaded if the left hand side is a struct aggregate, and opAssign is a member function of that aggregate.
For struct types, operator overloading for the identity assignment is allowed.

struct S
{
// identity assignment, allowed.
void opAssign(S rhs);

// not identity assignment, also allowed.
void opAssign(int);
}
S s;
s = S();      // Rewritten to s.opAssign(S());
s = 1;        // Rewritten to s.opAssign(1);

However for class types, identity assignment is not allowed. All class types have reference semantics, so identity assignment by default rebinds the left-hand-side to the argument at the right, and this is not overridable.

class C
{
// If X is the same type as C or the type which is
// implicitly convertible to C, then opAssign would
// accept identity assignment, which is disallowed.
// C opAssign(...);
// C opAssign(X);
// C opAssign(X, ...);
// C opAssign(X ...);
// C opAssign(X, U = defaultValue, etc.);

// not an identity assignment - allowed
void opAssign(int);
}
C c = new C();
c = new C();  // Rebinding referencee
c = 1;        // Rewritten to c.opAssign(1);

Index Assignment Operator Overloading

If the left hand side of an assignment is an index operation on a struct or class instance, it can be overloaded by providing an opIndexAssign member function. Expressions of the form a[b_1, b_2, ... bn] = c are rewritten as a.opIndexAssign(c, b_1, b_2, ... bn).

struct A
{
int opIndexAssign(int value, size_t i1, size_t i2);
}

void test()
{
A a;
a[i,3] = 7;  // same as a.opIndexAssign(7,i,3);
}

Slice Assignment Operator Overloading

If the left hand side of an assignment is a slice operation on a struct or class instance, it can be overloaded by implementing an opIndexAssign member function that takes the return value of the opSlice function as parameter(s). Expressions of the form a[i..j] = c are rewritten as a.opIndexAssign(c, a.opSlice(i, j)), and a[] = c as a.opIndexAssign(c).

See Array Indexing and Slicing Operators Overloading for more details.

struct A
{
int opIndexAssign(int v);  // overloads a[] = v
int opIndexAssign(int v, size_t[2] x);  // overloads a[i .. j] = v
int[2] opSlice(size_t x, size_t y);     // overloads i .. j
}

void test()
{
A a;
int v;

a[] = v;  // same as a.opIndexAssign(v);
a[3..4] = v;  // same as a.opIndexAssign(v, a.opSlice(3,4));
}

For backward compatibility, if rewriting a[i..j] as a.opIndexAssign(a.opSlice(i, j)) fails to compile, the legacy rewrite opSliceAssign(c, i, j) is used instead.

Op Assignment Operator Overloading

The following op assignment operators are overloadable:

Overloadable Op Assignment Operators

+=

-=

*=

/=

%=

^^=

&=

|=

^=

<<=

>>=

>>>=

~=

The expression:

a op= b

is rewritten as:

a.opOpAssign!("op")(b)

Index Op Assignment Operator Overloading

If the left hand side of an op= is an index expression on a struct or class instance and opIndexOpAssign is a member:

a[b_1, b_2, ... bn] op= c

it is rewritten as:

a.opIndexOpAssign!("op")(c, b_1, b_2, ... bn)

Slice Op Assignment Operator Overloading

If the left hand side of an op= is a slice expression on a struct or class instance and opIndexOpAssign is a member:

a[i..j] op= c

it is rewritten as:

a.opIndexOpAssign!("op")(c, a.opSlice(i, j))

and

a[] op= c

it is rewritten as:

a.opIndexOpAssign!("op")(c)

For backward compatibility, if the above rewrites fail and opSliceOpAssign is defined, then the rewrites a.opSliceOpAssign(c, i, j) and a.opSliceOpAssign(c) are tried, respectively.

Array Indexing and Slicing Operators Overloading

The array indexing and slicing operators are overloaded by implementing the opIndex, opSlice, and opDollar methods. These may be combined to implement multidimensional arrays.

The code example below shows a simple implementation of a 2-dimensional array with overloaded indexing and slicing operators. The explanations of the various constructs employed are given in the sections following.

struct Array2D(E)
{
E[] impl;
int stride;
int width, height;

this(int width, int height, E[] initialData = [])
{
impl = initialData;
this.stride = this.width = width;
this.height = height;
impl.length = width * height;
}

// Index a single element, e.g., arr[0, 1]
ref E opIndex(int i, int j) { return impl[i + stride*j]; }

// Array slicing, e.g., arr[1..2, 1..2], arr[2, 0..$], arr[0..$, 1].
Array2D opIndex(int[2] r1, int[2] r2)
{
Array2D result;

auto startOffset = r1[0] + r2[0]*stride;
auto endOffset = r1[1] + (r2[1] - 1)*stride;
result.impl = this.impl[startOffset .. endOffset];

result.stride = this.stride;
result.width = r1[1] - r1[0];
result.height = r2[1] - r2[0];

return result;
}
auto opIndex(int[2] r1, int j) { return opIndex(r1, [j, j+1]); }
auto opIndex(int i, int[2] r2) { return opIndex([i, i+1], r2); }

// Support for `x..y` notation in slicing operator for the given dimension.
int[2] opSlice(size_t dim)(int start, int end)
if (dim >= 0 && dim < 2)
in { assert(start >= 0 && end <= this.opDollar!dim); }
body
{
return [start, end];
}

// Support `$` in slicing notation, e.g., arr[1..$, 0..$-1].
@property int opDollar(size_t dim : 0)() { return width; }
@property int opDollar(size_t dim : 1)() { return height; }
}

unittest
{
auto arr = Array2D!int(4, 3, [
0, 1, 2,  3,
4, 5, 6,  7,
8, 9, 10, 11
]);

// Basic indexing
assert(arr[0, 0] == 0);
assert(arr[1, 0] == 1);
assert(arr[0, 1] == 4);

// Use of opDollar
assert(arr[$-1, 0] == 3);
assert(arr[0, $-1] == 8);   // Note the value of $ differs by dimension
assert(arr[$-1, $-1] == 11);

// Slicing
auto slice1 = arr[1..$, 0..$];
assert(slice1[0, 0] == 1 && slice1[1, 0] == 2  && slice1[2, 0] == 3 &&
slice1[0, 1] == 5 && slice1[1, 1] == 6  && slice1[2, 1] == 7 &&
slice1[0, 2] == 9 && slice1[1, 2] == 10 && slice1[2, 2] == 11);

auto slice2 = slice1[0..2, 1..$];
assert(slice2[0, 0] == 5 && slice2[1, 0] == 6 &&
slice2[0, 1] == 9 && slice2[1, 1] == 10);

// Thin slices
auto slice3 = arr[2, 0..$];
assert(slice3[0, 0] == 2 &&
slice3[0, 1] == 6 &&
slice3[0, 2] == 10);

auto slice4 = arr[0..3, 2];
assert(slice4[0, 0] == 8 && slice4[1, 0] == 9 && slice4[2, 0] == 10);
}

Index Operator Overloading

Expressions of the form arr[b_1, b_2, ... bn] are translated into arr.opIndex(b_1, b_2, ... bn). For example:

struct A
{
int opIndex(size_t i1, size_t i2, size_t i3);
}

void test()
{
A a;
int i;
i = a[5,6,7];  // same as i = a.opIndex(5,6,7);
}

In this way a struct or class object can behave as if it were an array.

If an index expression can be rewritten using opIndexAssign or opIndexOpAssign, those are preferred over opIndex.

Slice Operator Overloading

Overloading the slicing operator means overloading expressions like a[] or a[i..j], where the expressions inside the square brackets contain slice expressions of the form i..j.

To overload a[], simply define opIndex with no parameters:

struct S
{
int[] impl;
int[] opIndex()
{
return impl[];
}
}
void test()
{
auto s = S([1,2,3]);
auto t = s[]; // calls s.opIndex()
assert(t == [1,2,3]);
}

To overload array indexing of the form a[i..j, ...], two steps are needed. First, the expressions of the form i..j are translated via opSlice into user-defined objects that encapsulate the endpoints i and j. Then these user-defined objects are passed to opIndex to perform the actual slicing. This design was chosen in order to support mixed indexing and slicing in multidimensional arrays; for example, in translating expressions like arr[1, 2..3, 4].

More precisely, an expression of the form arr[b_1, b_2, ... bn] is translated into arr.opIndex(c_1, c_2, ... cn). Each argument bi can be either a single expression, in which case it is passed directly as the corresponding argument ci to opIndex; or it can be a slice expression of the form xi..yi, in which case the corresponding argument ci to opIndex is arr.opSlice!i(xi, yi). Namely:

op

rewrite

arr[1, 2, 3]

arr.opIndex(1, 2, 3)

arr[1..2, 3..4, 5..6]

arr.opIndex(arr.opSlice!0(1,2), arr.opSlice!1(3,4), arr.opSlice!2(5,6))

arr[1, 2..3, 4]

arr.opIndex(1, arr.opSlice!1(2,3), 4)

Similar translations are done for assignment operators involving slicing, for example:

op

rewrite

arr[1, 2..3, 4] = c

arr.opIndexAssign(c, 1, arr.opSlice!1(2, 3), 4)

arr[2, 3..4] += c

arr.opIndexOpAssign!"+"(c, 2, arr.opSlice!1(2, 3))

The intention is that opSlice!i should return a user-defined object that represents an interval of indices along the i'th dimension of the array. This object is then passed to opIndex to perform the actual slicing operation. If only one-dimensional slicing is desired, opSlice may be declared without the compile-time parameter i.

Note that in all cases, arr is only evaluated once. Thus, an expression like getArray()[1, 2..3, $-1]=c has the effect of:

auto __tmp = getArray();
__tmp.opIndexAssign(c, 1, __tmp.opSlice!1(2,3), __tmp.opDollar!2 - 1);

where the initial function call to getArray is only executed once.

For backward compatibility, a[] and a[i..j] can also be overloaded by implementing opSlice() with no arguments and opSlice(i, j) with two arguments, respectively. This only applies for one-dimensional slicing, and dates from when D did not have full support for multidimensional arrays. This usage of opSlice is discouraged.

Dollar Operator Overloading

Within the arguments to array index and slicing operators, $ gets translated to opDollar!i, where i is the position of the expression $ appears in. For example:

op

rewrite

arr[$-1, $-2, 3]

arr.opIndex(arr.opDollar!0 - 1, arr.opDollar!1 - 2, 3)

arr[1, 2, 3..$]

arr.opIndex(1, 2, arr.opSlice!2(3, arr.opDollar!2))

The intention is that opDollar!i should return the length of the array along its i'th dimension, or a user-defined object representing the end of the array along that dimension, that is understood by opSlice and opIndex.

struct Rectangle
{
int width, height;
int[][] impl;
this(int w, int h)
{
width = w;
height = h;
impl = new int[w][h];
}
int opIndex(size_t i1, size_t i2)
{
return impl[i1][i2];
}
int opDollar(size_t pos)()
{
static if (pos==0)
return width;
else
return height;
}
}

void test()
{
auto r = Rectangle(10,20);
int i = r[$-1, 0];    // same as: r.opIndex(r.opDollar!0, 0),
// which is r.opIndex(r.width-1, 0)
int j = r[0, $-1];    // same as: r.opIndex(0, r.opDollar!1)
// which is r.opIndex(0, r.height-1)
}

As the above example shows, a different compile-time argument is passed to opDollar depending on which argument it appears in. A $ appearing in the first argument gets translated to opDollar!0, a $ appearing in the second argument gets translated to opDollar!1, and so on. Thus, the appropriate value for $ can be returned to implement multidimensional arrays.

Note that opDollar!i is only evaluated once for each i where $ occurs in the corresponding position in the indexing operation. Thus, an expression like arr[$-sqrt($), 0, $-1] has the effect of:

auto __tmp1 = arr.opDollar!0;
auto __tmp2 = arr.opDollar!2;
arr.opIndex(__tmp1 - sqrt(__tmp1), 0, __tmp2 - 1);

If opIndex is declared with only one argument, the compile-time argument to opDollar may be omitted. In this case, it is illegal to use $ inside an array indexing expression with more than one argument.

Forwarding

Member names not found in a class or struct can be forwarded to a template function named opDispatch for resolution.

import std.stdio;

struct S
{
void opDispatch(string s, T)(T i)
{
writefln("S.opDispatch('%s', %s)", s, i);
}
}

class C
{
void opDispatch(string s)(int i)
{
writefln("C.opDispatch('%s', %s)", s, i);
}
}

struct D
{
template opDispatch(string s)
{
enum int opDispatch = 8;
}
}

void main()
{
S s;
s.opDispatch!("hello")(7);
s.foo(7);

auto c = new C();
c.foo(8);

D d;
writefln("d.foo = %s", d.foo);
assert(d.foo == 8);
}

D1 style operator overloading

The D1 operator overload mechanisms are deprecated.

Templates

Templates are D's approach to generic programming. Templates are defined with a TemplateDeclaration:

TemplateDeclaration:
template Identifier TemplateParameters Constraintopt { DeclDefsopt }

TemplateParameters:
( TemplateParameterListopt )

TemplateParameterList:
TemplateParameter
TemplateParameter ,
TemplateParameter , TemplateParameterList

TemplateParameter:
TemplateTypeParameter
TemplateValueParameter
TemplateAliasParameter
TemplateSequenceParameter
TemplateThisParameter

The body of the TemplateDeclaration must be syntactically correct even if never instantiated. Semantic analysis is not done until instantiated. A template forms its own scope, and the template body can contain classes, structs, types, enums, variables, functions, and other templates.

Template parameters can be types, values, symbols, or sequences. Types can be any type. Value parameters must be of an integral type, floating point type, or string type and specializations for them must resolve to an integral constant, floating point constant, null, or a string literal. Symbols can be any non-local symbol. Sequences can contain zero or more types, values or symbols.

Template parameter specializations constrain the values or types the TemplateParameter can accept.

Template parameter defaults are the value or type to use for the TemplateParameter in case one is not supplied.

Explicit Template Instantiation

Templates are explicitly instantiated with:

TemplateInstance:
Identifier TemplateArguments

TemplateArguments:
! ( TemplateArgumentListopt )
! TemplateSingleArgument

TemplateArgumentList:
TemplateArgument
TemplateArgument ,
TemplateArgument , TemplateArgumentList

TemplateArgument:
Type
AssignExpression
Symbol

Symbol:
SymbolTail
. SymbolTail

SymbolTail:
Identifier
Identifier . SymbolTail
TemplateInstance
TemplateInstance . SymbolTail

TemplateSingleArgument:
Identifier
FundamentalType
CharacterLiteral
StringLiteral
IntegerLiteral
FloatLiteral
true
false
null
this
SpecialKeyword

Once instantiated, the declarations inside the template, called the template members, are in the scope of the TemplateInstance:

template TFoo(T) { alias Ptr = T*; }
...
TFoo!(int).Ptr x; // declare x to be of type int*

If the TemplateArgument is one token long, the parentheses can be omitted:

TFoo!int.Ptr x;   // same as TFoo!(int).Ptr x;

A template instantiation can be aliased:

template TFoo(T) { alias Ptr = T*; }
alias foo = TFoo!(int);
foo.Ptr x;        // declare x to be of type int*

Multiple instantiations of a TemplateDeclaration with the same TemplateArgumentList will all refer to the same instantiation. For example:

template TFoo(T) { T f; }
alias a = TFoo!(int);
alias b = TFoo!(int);
...
a.f = 3;
assert(b.f == 3);  // a and b refer to the same instance of TFoo

This is true even if the TemplateInstances are done in different modules.

Even if template arguments are implicitly converted to the same template parameter type, they still refer to the same instance. This example uses a struct template:

struct TFoo(int x) { }

// Different template parameters create different struct types
static assert(!is(TFoo!(3) == TFoo!(2)));
// 3 and 2+1 are both 3 of type int - same TFoo instance
static assert(is(TFoo!(3) == TFoo!(2 + 1)));

// 3u is implicitly converted to 3 to match int parameter,
// and refers to exactly the same instance as TFoo!(3)
static assert(is(TFoo!(3) == TFoo!(3u)));

If multiple templates with the same Identifier are declared, they are distinct if they have a different number of arguments or are differently specialized.

Practical Example

A simple generic copy template would be:

template TCopy(T)
{
void copy(out T to, T from)
{
to = from;
}
}

To use this template, it must first be instantiated with a specific type:

int i;
TCopy!(int).copy(i, 3);

See also function templates.

Instantiation Scope

TemplateInstances are always instantiated in the scope of where the TemplateDeclaration is declared, with the addition of the template parameters being declared as aliases for their deduced types.

For example:

module a

template TFoo(T) { void bar() { func(); } }

module b

import a;

void func() { }
alias f = TFoo!(int); // error: func not defined in module a

and:

module a

template TFoo(T) { void bar() { func(1); } }
void func(double d) { }

module b

import a;

void func(int i) { }
alias f = TFoo!(int);
...
f.bar();  // will call a.func(double)

TemplateParameter specializations and default values are evaluated in the scope of the TemplateDeclaration.

Argument Deduction

The types of template parameters are deduced for a particular template instantiation by comparing the template argument with the corresponding template parameter.

For each template parameter, the following rules are applied in order until a type is deduced for each parameter:

1. If there is no type specialization for the parameter, the type of the parameter is set to the template argument.

2. If the type specialization is dependent on a type parameter, the type of that parameter is set to be the corresponding part of the type argument.

3. If after all the type arguments are examined, there are any type parameters left with no type assigned, they are assigned types corresponding to the template argument in the same position in the TemplateArgumentList.

4. If applying the above rules does not result in exactly one type for each template parameter, then it is an error.

For example:

template TFoo(T) { }
alias foo1 = TFoo!(int);     // (1) T is deduced to be int
alias foo2 = TFoo!(char*);   // (1) T is deduced to be char*

template TBar(T : T*) { }
alias bar = TBar!(char*);    // (2) T is deduced to be char

template TAbc(D, U : D[]) { }
alias abc1 = TAbc!(int, int[]);  // (2) D is deduced to be int, U is int[]
alias abc2 = TAbc!(char, int[]); // (4) error, D is both char and int

template TDef(D : E*, E) { }
alias def = TDef!(int*, int);  // (1) E is int
// (3) D is int*

Deduction from a specialization can provide values for more than one parameter:

template Foo(T: T[U], U)
{
...
}

Foo!(int[long])  // instantiates Foo with T set to int, U set to long

When considering matches, a class is considered to be a match for any super classes or interfaces:

class A { }
class B : A { }

template TFoo(T : A) { }
alias foo = TFoo!(B);      // (3) T is B

template TBar(T : U*, U : A) { }
alias bar = TBar!(B*, B);  // (2) T is B*
// (3) U is B

Template Type Parameters

TemplateTypeParameter:
Identifier
Identifier TemplateTypeParameterSpecialization
Identifier TemplateTypeParameterDefault
Identifier TemplateTypeParameterSpecialization TemplateTypeParameterDefault

TemplateTypeParameterSpecialization:
: Type

TemplateTypeParameterDefault:
= Type

Specialization

Templates may be specialized for particular types of arguments by following the template parameter identifier with a : and the specialized type. For example:

template TFoo(T)        { ... } // #1
template TFoo(T : T[])  { ... } // #2
template TFoo(T : char) { ... } // #3
template TFoo(T, U, V)  { ... } // #4

alias foo1 = TFoo!(int);            // instantiates #1
alias foo2 = TFoo!(double[]);       // instantiates #2 with T being double
alias foo3 = TFoo!(char);           // instantiates #3
alias fooe = TFoo!(char, int);      // error, number of arguments mismatch
alias foo4 = TFoo!(char, int, int); // instantiates #4

The template picked to instantiate is the one that is most specialized that fits the types of the TemplateArgumentList. If the result is ambiguous, it is an error.

Template This Parameters

TemplateThisParameter:
this TemplateTypeParameter

TemplateThisParameters are used in member function templates to pick up the type of the this reference. It also will infer the mutability of the this reference. For example, if this is const, then the function is marked const.

import std.stdio;

struct S
{
const void foo(this T)(int i)
{
writeln(typeid(T));
}
}

void main()
{
const(S) s;
(&s).foo(1);
S s2;
s2.foo(2);
immutable(S) s3;
s3.foo(3);
}

Prints:

const(S)
S
immutable(S)

This is especially useful when used with inheritance. For example, consider the implementation of a final base method which returns a derived class type. Typically this would return a base type, but that would prohibit calling or accessing derived properties of the type:

interface Addable(T)
{
final auto add(T t)
{
return this;
}
}

class List(T) : Addable!T
{
List remove(T t)
{
return this;
}
}

void main()
{
auto list = new List!int;
list.add(1).remove(1);  // error: no 'remove' method for Addable!int
}

Here the method add returns the base type, which doesn't implement the remove method. The template this parameter can be used for this purpose:

interface Addable(T)
{
final R add(this R)(T t)
{
return cast(R)this;  // cast is necessary, but safe
}
}

class List(T) : Addable!T
{
List remove(T t)
{
return this;
}
}

void main()
{
auto list = new List!int;
list.add(1).remove(1);  // ok
}

Template Value Parameters

TemplateValueParameter:
BasicType Declarator
BasicType Declarator TemplateValueParameterSpecialization
BasicType Declarator TemplateValueParameterDefault
BasicType Declarator TemplateValueParameterSpecialization TemplateValueParameterDefault

TemplateValueParameterSpecialization:
: ConditionalExpression

TemplateValueParameterDefault:
= AssignExpression
= SpecialKeyword

Template value parameter types can be any type which can be statically initialized at compile time. Template value arguments can be integer values, floating point values, nulls, string values, array literals of template value arguments, associative array literals of template value arguments, or struct literals of template value arguments.

template foo(string s)
{
string bar() { return s ~ " betty"; }
}

void main()
{
writefln("%s", foo!("hello").bar()); // prints: hello betty
}

This example of template foo has a value parameter that is specialized for 10:

template foo(U : int, int v : 10)
{
U x = v;
}

void main()
{
assert(foo!(int, 10).x == 10);
}

Template Alias Parameters

TemplateAliasParameter:
alias Identifier TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt
alias BasicType Declarator TemplateAliasParameterSpecializationopt TemplateAliasParameterDefaultopt

TemplateAliasParameterSpecialization:
: Type
: ConditionalExpression

TemplateAliasParameterDefault:
= Type
= ConditionalExpression

Alias parameters enable templates to be parameterized with symbol names or values computed at compile-time. Almost any kind of D symbol can be used, including user-defined type names, global names, local names, module names, template names, and template instance names.

Symbol examples:

- User-defined type names

class Foo
{
static int x;
}

template Bar(alias a)
{
alias sym = a.x;
}

void main()
{
alias bar = Bar!(Foo);
bar.sym = 3;  // sets Foo.x to 3
}

- Global names

shared int x;

template Foo(alias var)
{
auto ptr = &var;
}

void main()
{
alias bar = Foo!(x);
*bar.ptr = 3;       // set x to 3
static shared int y;
alias abc = Foo!(y);
*abc.ptr = 3;       // set y to 3
}

- Local names

template Foo(alias var)
{
void inc() { var++; }
}

void main()
{
int v = 4;
alias foo = Foo!v;
foo.inc();
assert(v == 5);
}

See also Implicit Template Nesting.

- Module names

import std.conv;

template Foo(alias a)
{
alias sym = a.text;
}

void main()
{
alias bar = Foo!(std.conv);
bar.sym(3);   // calls std.conv.text(3)
}

- Template names

shared int x;

template Foo(alias var)
{
auto ptr = &var;
}

template Bar(alias Tem)
{
alias instance = Tem!(x);
}

void main()
{
alias bar = Bar!(Foo);
*bar.instance.ptr = 3;  // sets x to 3
}

- Template instance names

shared int x;

template Foo(alias var)
{
auto ptr = &var;
}

template Bar(alias sym)
{
alias p = sym.ptr;
}

void main()
{
alias foo = Foo!(x);
alias bar = Bar!(foo);
*bar.p = 3;  // sets x to 3
}

Value examples:

- Literals

template Foo(alias x, alias y)
{
static int i = x;
static string s = y;
}

void main()
{
alias foo = Foo!(3, "bar");
writeln(foo.i, foo.s);  // prints 3bar
}

- Compile-time values

template Foo(alias x)
{
static int i = x;
}

void main()
{
// compile-time argument evaluation
enum two = 1 + 1;
alias foo = Foo!(5 * two);
assert(foo.i == 10);
static assert(foo.stringof == "Foo!10");

// compile-time function evaluation
int get10() { return 10; }
alias bar = Foo!(get10());
// bar is the same template instance as foo
assert(&bar.i is &foo.i);
}

- Lambdas

template Foo(alias fun)
{
enum val = fun(2);
}

alias foo = Foo!((int x) => x * x);
static assert(foo.val == 4);

Typed alias parameters

Alias parameters can also be typed. These parameters will accept symbols of that type:

template Foo(alias int x) { }
int x;
float f;

Foo!x;  // ok
Foo!f;  // fails to instantiate

Specialization

Alias parameters can accept both literals and user-defined type symbols, but they are less specialized than the matches to type parameters and value parameters:

template Foo(T)         { ... }  // #1
template Foo(int n)     { ... }  // #2
template Foo(alias sym) { ... }  // #3

struct S {}
int var;

alias foo1  = Foo!(S);      // instantiates #1
alias foo2  = Foo!(1);      // instantiates #2
alias foo3a = Foo!([1,2]);  // instantiates #3
alias foo3b = Foo!(var);    // instantiates #3

template Bar(alias A) { ... }                 // #4
template Bar(T : U!V, alias U, V...) { ... }  // #5

class C(T) {}
alias bar = Bar!(C!int);    // instantiates #5

Template Sequence Parameters

TemplateSequenceParameter:
Identifier ...

If the last template parameter in the TemplateParameterList is declared as a TemplateSequenceParameter, it is a match with any trailing template arguments. Such a sequence of arguments can be defined using the template std.meta.AliasSeq and will thus henceforth be referred to by that name for clarity. An AliasSeq is not itself a type, value, or symbol. It is a compile-time sequence of any mix of types, values or symbols.

An AliasSeq whose elements consist entirely of types is called a type sequence or TypeSeq. An AliasSeq whose elements consist entirely of values is called a value sequence or ValueSeq.

An AliasSeq can be used as an argument list to instantiate another template, or as the list of parameters for a function.

template print(args...)
{
void print()
{
writeln("args are ", args); // args is a ValueSeq
}
}

template write(Args...)
{
void write(Args args) // Args is a TypeSeq
// args is a ValueSeq
{
writeln("args are ", args);
}
}

void main()
{
print!(1,'a',6.8).print();                    // prints: args are 1a6.8
write!(int, char, double).write(1, 'a', 6.8); // prints: args are 1a6.8
}

The number of elements in an AliasSeq can be retrieved with the .length property. The nth element can be retrieved by indexing the AliasSeq with [n], and sub-sequences are denoted by the slicing syntax.

AliasSeq-s are static compile-time entities, there is no way to dynamically change, add, or remove elements either at compile-time or run-time.

Type sequences can be deduced from the trailing parameters of an implicitly instantiated function template:

template print(T, Args...)
{
void print(T first, Args args)
{
writeln(first);
static if (args.length) // if more arguments
print(args);        // recurse for remaining arguments
}
}

void main()
{
print(1, 'a', 6.8);
}

prints:

1
a
6.8

Type sequences can also be deduced from the type of a delegate or function parameter list passed as a function argument:

import std.stdio;

/* Partially applies a delegate by tying its first argument to a particular value.
* R = return type
* T = first argument type
* Args = TypeSeq of remaining argument types
*/
R delegate(Args) partial(R, T, Args...)(R delegate(T, Args) dg, T first)
{
// return a closure
return (Args args) => dg(first, args);
}

void main()
{
int plus(int x, int y, int z)
{
return x + y + z;
}

auto plus_two = partial(&plus, 2);
writefln("%d", plus_two(6, 8)); // prints 16
}

See also: std.functional.partial
Specialization

If both a template with a sequence parameter and a template without a sequence parameter exactly match a template instantiation, the template without a TemplateSequenceParameter is selected.

template Foo(T)         { pragma(msg, "1"); }   // #1
template Foo(int n)     { pragma(msg, "2"); }   // #2
template Foo(alias sym) { pragma(msg, "3"); }   // #3
template Foo(Args...)   { pragma(msg, "4"); }   // #4

import std.stdio;

// Any sole template argument will never match to #4
alias foo1 = Foo!(int);          // instantiates #1
alias foo2 = Foo!(3);            // instantiates #2
alias foo3 = Foo!(std);          // instantiates #3

alias foo4 = Foo!(int, 3, std);  // instantiates #4

Template Parameter Default Values

Trailing template parameters can be given default values:

template Foo(T, U = int) { ... }
Foo!(uint,long); // instantiate Foo with T as uint, and U as long
Foo!(uint);      // instantiate Foo with T as uint, and U as int

template Foo(T, U = T*) { ... }
Foo!(uint);      // instantiate Foo with T as uint, and U as uint*

See also: Function Template Default Arguments.

Eponymous Templates

If a template contains members whose name is the same as the template identifier then these members are assumed to be referred to in a template instantiation:

template foo(T)
{
T foo; // declare variable foo of type T
}

void main()
{
foo!(int) = 6; // instead of foo!(int).foo
}

Using functions and more types than the template:

template foo(S, T)
{
// each member contains all the template parameters
void foo(S s, T t) {}
void foo(S s, T t, string) {}
}

void main()
{
foo(1, 2, "test"); // foo!(int, int).foo(1, 2, "test")
foo(1, 2); // foo!(int, int).foo(1, 2)
}

When the template parameters must be deduced, the eponymous members can't rely on a static if condition since the deduction relies on how the in members are used:

template foo(T)
{
static if (is(T)) // T is not yet known...
void foo(T t) {} // T is deduced from the member usage
}

void main()
{
foo(0); // Error: cannot deduce function from argument types
foo!int(0); // Ok since no deduction necessary
}

Template Constructors

ConstructorTemplate:
this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt :
this TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt FunctionBody

Templates can be used to form constructors for classes and structs.

Aggregate Templates

ClassTemplateDeclaration:
class Identifier TemplateParameters ;
class Identifier TemplateParameters Constraintopt BaseClassListopt AggregateBody
class Identifier TemplateParameters BaseClassListopt Constraintopt AggregateBody

InterfaceTemplateDeclaration:
interface Identifier TemplateParameters ;
interface Identifier TemplateParameters Constraintopt BaseInterfaceListopt AggregateBody
interface Identifier TemplateParameters BaseInterfaceList Constraint AggregateBody

StructTemplateDeclaration:
struct Identifier TemplateParameters ;
struct Identifier TemplateParameters Constraintopt AggregateBody

UnionTemplateDeclaration:
union Identifier TemplateParameters ;
union Identifier TemplateParameters Constraintopt AggregateBody

If a template declares exactly one member, and that member is a class with the same name as the template (see Eponymous Templates:)

template Bar(T)
{
class Bar
{
T member;
}
}

then the semantic equivalent, called a ClassTemplateDeclaration can be written as:

class Bar(T)
{
T member;
}

See also: Template This Parameters.

Analogously to class templates, struct, union and interfaces can be transformed into templates by supplying a template parameter list.

Function Templates

If a template declares exactly one member, and that member is a function with the same name as the template, it is a function template declaration. Alternatively, a function template declaration is a function declaration with a TemplateParameterList immediately preceding the Parameters.

A function template to compute the square of type T is:

T square(T)(T t)
{
return t * t;
}

It is lowered to:

template square(T)
{
T square(T t)
{
return t * t;
}
}

Function templates can be explicitly instantiated with a !(TemplateArgumentList):

writefln("The square of %s is %s", 3, square!(int)(3));

Implicit Function Template Instantiation (IFTI)

Function templates can be implicitly instantiated if the TemplateArgumentList is deducible from the types of the function arguments:

T square(T)(T t)
{
return t * t;
}

writefln("The square of %s is %s", 3, square(3));  // T is deduced to be int

Type parameter deduction is not influenced by the order of function arguments.

If there are fewer arguments supplied in the TemplateArgumentList than parameters in the TemplateParameterList, the arguments fill parameters from left to right, and the rest of the parameters are then deduced from the function arguments.

Function template type parameters that are to be implicitly deduced may not have specializations:

void foo(T : T*)(T t) { ... }

int x,y;
foo!(int*)(x);   // ok, T is not deduced from function argument
foo(&y);         // error, T has specialization

Type Conversions

If template type parameters match the literal expressions on function arguments, the deduced types may consider narrowing conversions of them.

void foo(T)(T v)        { pragma(msg, "in foo, T = ", T); }
void bar(T)(T v, T[] a) { pragma(msg, "in bar, T = ", T); }

foo(1);
// an integer literal type is analyzed as int by default
// then T is deduced to int

short[] arr;
bar(1, arr);
// arr is short[], and the integer literal 1 is
// implicitly convertible to short.
// then T will be deduced to short.

bar(1, [2.0, 3.0]);
// the array literal is analyzed as double[],
// and the integer literal 1 is implicitly convertible to double.
// then T will be deduced to double.

The deduced type parameter for dynamic array and pointer arguments has an unqualified head:

void foo(T)(T arg) { pragma(msg, T); }

int[] marr;
const(int[]) carr;
immutable(int[]) iarr;
foo(marr);  // T == int[]
foo(carr);  // T == const(int)[]
foo(iarr);  // T == immutable(int)[]

int* mptr;
const(int*) cptr;
immutable(int*) iptr;
foo(mptr);  // T == int*
foo(cptr);  // T == const(int)*
foo(iptr);  // T == immutable(int)*

Return Type Deduction

Function templates can have their return types deduced based on the ReturnStatements in the function, just as with normal functions. See Auto Functions.

auto square(T)(T t)
{
return t * t;
}

auto i = square(2);
static assert(is(typeof(i) == int));

Default Arguments

Template arguments not implicitly deduced can have default values:

void foo(T, U=T*)(T t) { U p; ... }

int x;
foo(x);    // T is int, U is int*

Variadic Function Templates can have parameters with default values. These parameters are always set to their default value in case of IFTI.

size_t fun(T...)(T t, string file = __FILE__)
{
import std.stdio;
writeln(file, " ", t);
return T.length;
}

assert(fun(1, "foo") == 2);  // uses IFTI
assert(fun!int(1, "filename") == 1);  // no IFTI

Enum & Variable Templates

Like aggregates and functions, manifest constant and variable declarations can have template parameters, providing there is an Initializer:

enum string constant(TL...) = TL.stringof;
ubyte[T.sizeof] storage(T) = 0;
auto array(alias a) = a;

These declarations are transformed into templates:

template constant(TL...)
{
enum string constant = TL.stringof;
}
template storage(T)
{
ubyte[T.sizeof] storage = 0;
}
template array(alias a)
{
auto array = a;
}

Alias Templates

AliasDeclaration can also have optional template parameters:

alias Sequence(TL...) = TL;

It is lowered to:

template Sequence(TL...)
{
alias Sequence = TL;
}

Function Templates with Auto Ref Parameters

An auto ref function template parameter becomes a ref parameter if its corresponding argument is an lvalue, otherwise it becomes a value parameter:

int foo(Args...)(auto ref Args args)
{
int result;

foreach (i, v; args)
{
if (v == 10)
assert(__traits(isRef, args[i]));
else
assert(!__traits(isRef, args[i]));
result += v;
}
return result;
}

void main()
{
int y = 10;
int r;
r = foo(8);       // returns 8
r = foo(y);       // returns 10
r = foo(3, 4, y); // returns 17
r = foo(4, 5, y); // returns 19
r = foo(y, 6, y); // returns 26
}

Auto ref parameters can be combined with auto ref return attributes:

auto ref min(T, U)(auto ref T lhs, auto ref U rhs)
{
return lhs > rhs ? rhs : lhs;
}

void main()
{
int x = 7, y = 8;
int i;

i = min(4, 3);     // returns 3
i = min(x, y);     // returns 7
min(x, y) = 10;    // sets x to 10
static assert(!__traits(compiles, min(3, y) = 10));
static assert(!__traits(compiles, min(y, 3) = 10));
}

Nested Templates

If a template is declared in aggregate or function local scope, the instantiated functions will implicitly capture the context of the enclosing scope.

class C
{
int num;

this(int n) { num = n; }

template Foo()
{
// 'foo' can access 'this' reference of class C object.
void foo(int n) { this.num = n; }
}
}

void main()
{
auto c = new C(1);
assert(c.num == 1);

c.Foo!().foo(5);
assert(c.num == 5);

template Bar()
{
// 'bar' can access local variable of 'main' function.
void bar(int n) { c.num = n; }
}
Bar!().bar(10);
assert(c.num == 10);
}

Above, Foo!().foo will work just the same as a member function of class C, and Bar!().bar will work just the same as a nested function within function main().

Implicit Nesting

If a template has a template alias parameter, and is instantiated with a local symbol, the instantiated function will implicitly become nested in order to access runtime data of the given local symbol.

template Foo(alias sym)
{
void foo() { sym = 10; }
}

class C
{
int num;

this(int n) { num = n; }

void main()
{
assert(this.num == 1);

alias fooX = Foo!(C.num).foo;

// fooX will become member function implicitly, so &fooX
//     returns a delegate object.
static assert(is(typeof(&fooX) == delegate));

fooX(); // called by using valid 'this' reference.
assert(this.num == 10);  // OK
}
}

void main()
{
new C(1).main();

int num;
alias fooX = Foo!num.foo;

// fooX will become nested function implicitly, so &fooX
//     returns a delegate object.
static assert(is(typeof(&fooX) == delegate));

fooX();
assert(num == 10);  // OK
}

Not only functions, but also instantiated class and struct types can become nested via implicitly captured context.

class C
{
int num;
this(int n) { num = n; }

class N(T)
{
// instantiated class N!T can become nested in C
T foo() { return num * 2; }
}
}

void main()
{
auto c = new C(10);
auto n = c.new N!int();
assert(n.foo() == 20);
}

See also: Nested Class Instantiation.

void main()
{
int num = 10;
struct S(T)
{
// instantiated struct S!T can become nested in main()
T foo() { return num * 2; }
}
S!int s;
assert(s.foo() == 20);
}

A templated struct can become a nested struct if it is instantiated with a local symbol passed as an aliased argument:

struct A(alias F)
{
int fun(int i) { return F(i); }
}

A!F makeA(alias F)() { return A!F(); }

void main()
{
int x = 40;
int fun(int i) { return x + i; }
A!fun a = makeA!fun();
assert(a.fun(2) == 42);
}

Context Limitation

Currently nested templates can capture at most one context. As a typical example, non-static template member functions cannot take local symbol by using template alias parameter.

class C
{
int num;
void foo(alias sym)() { num = sym * 2; }
}

void main()
{
auto c = new C();
int var = 10;
c.foo!var();    // NG, foo!var requires two contexts, 'this' and 'main()'
}

But, if one context is indirectly accessible from other context, it is allowed.

int sum(alias x, alias y)() { return x + y; }

void main()
{
int a = 10;
void nested()
{
int b = 20;
assert(sum!(a, b)() == 30);
}
nested();
}

Two local variables a and b are in different contexts, but outer context is indirectly accessible from innter context, so nested template instance sum!(a, b) will capture only inner context.

Recursive Templates

Template features can be combined to produce some interesting effects, such as compile time evaluation of non-trivial functions. For example, a factorial template can be written:

template factorial(int n)
{
static if (n == 1)
enum factorial = 1;
else
enum factorial = n * factorial!(n - 1);
}

static assert(factorial!(4) == 24);

For more information and a CTFE (Compile-time Function Execution) factorial alternative, see: Template Recursion.

Template Constraints

Constraint:
if ( Expression )

Constraints are used to impose additional constraints on matching arguments to a template beyond what is possible in the TemplateParameterList. The Expression is computed at compile time and returns a result that is converted to a boolean value. If that value is true, then the template is matched, otherwise the template is not matched.

For example, the following function template only matches with odd values of N:

void foo(int N)()
if (N & 1)
{
...
}
...
foo!(3)();  // OK, matches
foo!(4)();  // Error, no match

Template constraints can be used with aggregate types (structs, classes, unions). Constraints are effectively used with library module std.traits:

import std.traits;

struct Bar(T)
if (isIntegral!T)
{
...
}
...
auto x = Bar!int;       // OK, int is an integral type
auto y = Bar!double;    // Error, double does not satisfy constraint

Limitations

Templates cannot be used to add non-static fields or virtual functions to classes or interfaces. For example:

class Foo
{
template TBar(T)
{
T xx;               // becomes a static field of Foo
int func(T) { ... } // non-virtual

static T yy;                        // Ok
static int func(T t, int y) { ... } // Ok
}
}

Template Mixins

A TemplateMixin takes an arbitrary set of declarations from the body of a TemplateDeclaration and inserts them into the current context.

TemplateMixinDeclaration:
mixin template Identifier TemplateParameters Constraintopt { DeclDefsopt }

TemplateMixin:
mixin MixinTemplateName TemplateArgumentsopt Identifieropt ;

MixinTemplateName:
. MixinQualifiedIdentifier
MixinQualifiedIdentifier
Typeof . MixinQualifiedIdentifier

MixinQualifiedIdentifier:
Identifier
Identifier . MixinQualifiedIdentifier
TemplateInstance . MixinQualifiedIdentifier

A TemplateMixin can occur in declaration lists of modules, classes, structs, unions, and as a statement. The MixinTemplateName refers to a TemplateDeclaration or TemplateMixinDeclaration. If the TemplateDeclaration has no parameters, the mixin form that has no !(TemplateArgumentList) can be used.

Unlike a template instantiation, a template mixin's body is evaluated within the scope where the mixin appears, not where the template declaration is defined. It is analogous to cutting and pasting the body of the template into the location of the mixin into a nested scope. It is useful for injecting parameterized ‘boilerplate’ code, as well as for creating templated nested functions, which is not possible with template instantiations.

mixin template Foo()
{
int x = 5;
}

mixin Foo;

struct Bar
{
mixin Foo;
}

void test()
{
writefln("x = %d", x);         // prints 5
{
Bar b;
int x = 3;

writefln("b.x = %d", b.x); // prints 5
writefln("x = %d", x);     // prints 3
{
mixin Foo;
writefln("x = %d", x); // prints 5
x = 4;
writefln("x = %d", x); // prints 4
}
writefln("x = %d", x);     // prints 3
}
writefln("x = %d", x);         // prints 5
}

Mixins can be parameterized:

mixin template Foo(T)
{
T x = 5;
}

mixin Foo!(int);           // create x of type int

Mixins can add virtual functions to a class:

mixin template Foo()
{
void func() { writeln("Foo.func()"); }
}

class Bar
{
mixin Foo;
}

class Code : Bar
{
override void func() { writeln("Code.func()"); }
}

void test()
{
Bar b = new Bar();
b.func();      // calls Foo.func()

b = new Code();
b.func();      // calls Code.func()
}

Mixins are evaluated in the scope of where they appear, not the scope of the template declaration:

int y = 3;

mixin template Foo()
{
int abc() { return y; }
}

void test()
{
int y = 8;
mixin Foo; // local y is picked up, not global y
assert(abc() == 8);
}

Mixins can parameterize symbols using alias parameters:

mixin template Foo(alias b)
{
int abc() { return b; }
}

void test()
{
int y = 8;
mixin Foo!(y);
assert(abc() == 8);
}

This example uses a mixin to implement a generic Duff's device for an arbitrary statement (in this case, the arbitrary statement is in bold). A nested function is generated as well as a delegate literal, these can be inlined by the compiler:

mixin template duffs_device(alias id1, alias id2, alias s)
{
void duff_loop()
{
if (id1 < id2)
{
typeof(id1) n = (id2 - id1 + 7) / 8;
switch ((id2 - id1) % 8)
{
case 0: do { s(); goto case;
case 7:      s(); goto case;
case 6:      s(); goto case;
case 5:      s(); goto case;
case 4:      s(); goto case;
case 3:      s(); goto case;
case 2:      s(); goto case;
case 1:      s(); continue;
default:     assert(0, "Impossible");
} while (--n > 0);
}
}
}
}

void foo() { writeln("foo"); }

void test()
{
int i = 1;
int j = 11;

mixin duffs_device!(i, j, delegate { foo(); });
duff_loop();  // executes foo() 10 times
}

Mixin Scope

The declarations in a mixin are placed in a nested scope and then ‘imported’ into the surrounding scope. If the name of a declaration in a mixin is the same as a declaration in the surrounding scope, the surrounding declaration overrides the mixin one:

int x = 3;

mixin template Foo()
{
int x = 5;
int y = 5;
}

mixin Foo;
int y = 3;

void test()
{
writefln("x = %d", x);  // prints 3
writefln("y = %d", y);  // prints 3
}

If two different mixins are put in the same scope, and each define a declaration with the same name, there is an ambiguity error when the declaration is referenced:

mixin template Foo()
{
int x = 5;
void func(int x) { }
}

mixin template Bar()
{
int x = 4;
void func(long x) { }
}

mixin Foo;
mixin Bar;

void test()
{
import std.stdio : writefln;
writefln("x = %d", x); // error, x is ambiguous
func(1);               // error, func is ambiguous
}

The call to func() is ambiguous because Foo.func and Bar.func are in different scopes.

If a mixin has an Identifier, it can be used to disambiguate between conflicting symbols:

int x = 6;

mixin template Foo()
{
int x = 5;
int y = 7;
void func() { }
}

mixin template Bar()
{
int x = 4;
void func() { }
}

mixin Foo F;
mixin Bar B;

void test()
{
writefln("y = %d", y);     // prints 7
writefln("x = %d", x);     // prints 6
writefln("F.x = %d", F.x); // prints 5
writefln("B.x = %d", B.x); // prints 4
F.func();                  // calls Foo.func
B.func();                  // calls Bar.func
}

Alias declarations can be used to overload together functions declared in different mixins:

mixin template Foo()
{
void func(int x) {  }
}

mixin template Bar()
{
void func(long x) {  }
}

mixin Foo!() F;
mixin Bar!() B;

alias func = F.func;
alias func = B.func;

void main()
{
func(1);  // calls B.func
func(1L); // calls F.func
}

A mixin has its own scope, even if a declaration is overridden by the enclosing one:

int x = 4;

mixin template Foo()
{
int x = 5;
int bar() { return x; }
}

mixin Foo;

void test()
{
writefln("x = %d", x);         // prints 4
writefln("bar() = %d", bar()); // prints 5
}

Contract Programming

Contracts enable specifying conditions that must hold true when the flow of runtime execution reaches the contract. If a contract is not true, then the program is assumed to have entered an undefined state.

Rationale:

Building contract support into the language provides:

1. a consistent look and feel for the contracts

2. tool support

3. the implementation can generate better code using information gathered from the contracts

4. easier management and enforcement of contracts

5. handling of contract inheritance

Assert Contract

See AssertExpression.

Pre and Post Contracts

See contracts.

Invariants

See Struct Invariants and Class Invariants.

References

- Contracts Reading List

- Adding Contracts to Java

Conditional Compilation

Conditional compilation is the process of selecting which code to compile and which code to not compile.

ConditionalDeclaration:
Condition DeclarationBlock
Condition DeclarationBlock else DeclarationBlock
Condition : DeclDefsopt
Condition DeclarationBlock else : DeclDefsopt

ConditionalStatement:
Condition NoScopeNonEmptyStatement
Condition NoScopeNonEmptyStatement else NoScopeNonEmptyStatement

If the Condition is satisfied, then the following DeclarationBlock or Statement is compiled in. If it is not satisfied, the DeclarationBlock or Statement after the optional else is compiled in.

Any DeclarationBlock or Statement that is not compiled in still must be syntactically correct.

No new scope is introduced, even if the DeclarationBlock or Statement is enclosed by { }.

ConditionalDeclarations and ConditionalStatements can be nested.

The StaticAssert can be used to issue errors at compilation time for branches of the conditional compilation that are errors.

Condition comes in the following forms:

Condition:
VersionCondition
DebugCondition
StaticIfCondition

Version Condition

VersionCondition:
version ( IntegerLiteral )
version ( Identifier )
version ( unittest )
version ( assert )

Versions enable multiple versions of a module to be implemented with a single source file.

The VersionCondition is satisfied if the IntegerLiteral is greater than or equal to the current version level, or if Identifier matches a version identifier.

The version level and version identifier can be set on the command line by the -version switch or in the module itself with a VersionSpecification, or they can be predefined by the compiler.

Version identifiers are in their own unique name space, they do not conflict with debug identifiers or other symbols in the module. Version identifiers defined in one module have no influence over other imported modules.

int k;
version (Demo) // compile in this code block for the demo version
{
int i;
int k;    // error, k already defined

i = 3;
}
x = i;      // uses the i declared above

version (X86)
{
... // implement custom inline assembler version
}
else
{
... // use default, but slow, version
}

The version(unittest) is satisfied if and only if the code is compiled with unit tests enabled (the -unittest option on dmd).

Version Specification

VersionSpecification:
version = Identifier ;
version = IntegerLiteral ;

The version specification makes it straightforward to group a set of features under one major version, for example:

version (ProfessionalEdition)
{
version = FeatureA;
version = FeatureB;
version = FeatureC;
}
version (HomeEdition)
{
version = FeatureA;
}
...
version (FeatureB)
{
... implement Feature B ...
}

Version identifiers or levels may not be forward referenced:

version (Foo)
{
int x;
}
version = Foo;  // error, Foo already used

VersionSpecifications may only appear at module scope.

While the debug and version conditions superficially behave the same, they are intended for very different purposes. Debug statements are for adding debug code that is removed for the release version. Version statements are to aid in portability and multiple release versions.

Here's an example of a full version as opposed to a demo version:

class Foo
{
int a, b;

version(full)
{
int extrafunctionality()
{
...
return 1;  // extra functionality is supported
}
}
else // demo
{
int extrafunctionality()
{
return 0;  // extra functionality is not supported
}
}
}

Various different version builds can be built with a parameter to version:

version(n) // add in version code if version level is >= n
{
... version code ...
}

version(identifier) // add in version code if version
// keyword is identifier
{
... version code ...
}

These are presumably set by the command line as -version=n and -version=identifier.

Predefined Versions

Several environmental version identifiers and identifier name spaces are predefined for consistent usage. Version identifiers do not conflict with other identifiers in the code, they are in a separate name space. Predefined version identifiers are global, i.e. they apply to all modules being compiled and imported.

Predefined Version Identifiers

Version Identifier

Description

DigitalMars

DMD (Digital Mars D) is the compiler

GNU

GDC (GNU D Compiler) is the compiler

LDC

LDC (LLVM D Compiler) is the compiler

SDC

SDC (Stupid D Compiler) is the compiler

Windows

Microsoft Windows systems

Win32

Microsoft 32-bit Windows systems

Win64

Microsoft 64-bit Windows systems

linux

All Linux systems

OSX

macOS

iOS

iOS

TVOS

tvOS

WatchOS

watchOS

FreeBSD

FreeBSD

OpenBSD

OpenBSD

NetBSD

NetBSD

DragonFlyBSD

DragonFlyBSD

BSD

All other BSDs

Solaris

Solaris

Posix

All POSIX systems (includes Linux, FreeBSD, OS X, Solaris, etc.)

AIX

IBM Advanced Interactive eXecutive OS

Haiku

The Haiku operating system

SkyOS

The SkyOS operating system

SysV3

System V Release 3

SysV4

System V Release 4

Hurd

GNU Hurd

Android

The Android platform

Emscripten

The Emscripten platform

PlayStation

The PlayStation platform

PlayStation4

The PlayStation 4 platform

Cygwin

The Cygwin environment

MinGW

The MinGW environment

FreeStanding

An environment without an operating system (such as Bare-metal targets)

CRuntime_Bionic

Bionic C runtime

CRuntime_DigitalMars

DigitalMars C runtime

CRuntime_Glibc

Glibc C runtime

CRuntime_Microsoft

Microsoft C runtime

CRuntime_Musl

musl C runtime

CRuntime_Newlib

newlib C runtime

CRuntime_UClibc

uClibc C runtime

CRuntime_WASI

WASI C runtime

CppRuntime_Clang

Clang Cpp runtime

CppRuntime_DigitalMars

DigitalMars Cpp runtime

CppRuntime_Gcc

Gcc Cpp runtime

CppRuntime_Microsoft

Microsoft Cpp runtime

CppRuntime_Sun

Sun Cpp runtime

X86

Intel and AMD 32-bit processors

X86_64

Intel and AMD 64-bit processors

ARM

The ARM architecture (32-bit) (AArch32 et al)

ARM_Thumb

ARM in any Thumb mode

ARM_SoftFloat

The ARM soft floating point ABI

ARM_SoftFP

The ARM softfp floating point ABI

ARM_HardFloat

The ARM hardfp floating point ABI

AArch64

The Advanced RISC Machine architecture (64-bit)

AsmJS

The asm.js intermediate programming language

AVR

8-bit Atmel AVR Microcontrollers

Epiphany

The Epiphany architecture

PPC

The PowerPC architecture, 32-bit

PPC_SoftFloat

The PowerPC soft float ABI

PPC_HardFloat

The PowerPC hard float ABI

PPC64

The PowerPC architecture, 64-bit

IA64

The Itanium architecture (64-bit)

MIPS32

The MIPS architecture, 32-bit

MIPS64

The MIPS architecture, 64-bit

MIPS_O32

The MIPS O32 ABI

MIPS_N32

The MIPS N32 ABI

MIPS_O64

The MIPS O64 ABI

MIPS_N64

The MIPS N64 ABI

MIPS_EABI

The MIPS EABI

MIPS_SoftFloat

The MIPS soft-float ABI

MIPS_HardFloat

The MIPS hard-float ABI

MSP430

The MSP430 architecture

NVPTX

The Nvidia Parallel Thread Execution (PTX) architecture, 32-bit

NVPTX64

The Nvidia Parallel Thread Execution (PTX) architecture, 64-bit

RISCV32

The RISC-V architecture, 32-bit

RISCV64

The RISC-V architecture, 64-bit

SPARC

The SPARC architecture, 32-bit

SPARC_V8Plus

The SPARC v8+ ABI

SPARC_SoftFloat

The SPARC soft float ABI

SPARC_HardFloat

The SPARC hard float ABI

SPARC64

The SPARC architecture, 64-bit

S390

The System/390 architecture, 32-bit

SystemZ

The System Z architecture, 64-bit

HPPA

The HP PA-RISC architecture, 32-bit

HPPA64

The HP PA-RISC architecture, 64-bit

SH

The SuperH architecture, 32-bit

WebAssembly

The WebAssembly virtual ISA (instruction set architecture), 32-bit

WASI

The WebAssembly System Interface

Alpha

The Alpha architecture

Alpha_SoftFloat

The Alpha soft float ABI

Alpha_HardFloat

The Alpha hard float ABI

LittleEndian

Byte order, least significant first

BigEndian

Byte order, most significant first

ELFv1

The Executable and Linkable Format v1

ELFv2

The Executable and Linkable Format v2

D_BetterC

D as Better C code (command line switch -betterC) is being generated

D_Exceptions

Exception handling is supported. Evaluates to false when compiling with command line switch -betterC

D_ModuleInfo

ModuleInfo is supported. Evaluates to false when compiling with command line switch -betterC

D_TypeInfo

Runtime type information (a.k.a TypeInfo) is supported. Evaluates to false when compiling with command line switch -betterC

D_Coverage

Code coverage analysis instrumentation (command line switch -cov) is being generated

D_Ddoc

Ddoc documentation (command line switch -D) is being generated

D_InlineAsm_X86

Inline assembler for X86 is implemented

D_InlineAsm_X86_64

Inline assembler for X86-64 is implemented

D_LP64

Pointers are 64 bits (command line switch -m64). (Do not confuse this with C's LP64 model)

D_X32

Pointers are 32 bits, but words are still 64 bits (x32 ABI) (This can be defined in parallel to X86_64)

D_HardFloat

The target hardware has a floating-point unit

D_SoftFloat

The target hardware does not have a floating-point unit

D_PIC

Position Independent Code (command line switch -fPIC) is being generated

D_PIE

Position Independent Executable (command line switch -fPIE) is being generated

D_SIMD

Vector extensions (via __simd) are supported

D_AVX

AVX Vector instructions are supported

D_AVX2

AVX2 Vector instructions are supported

D_Version2

This is a D version 2 compiler

D_NoBoundsChecks

Array bounds checks are disabled (command line switch -boundscheck=off)

D_ObjectiveC

The target supports interfacing with Objective-C

Core

Defined when building the standard runtime

Std

Defined when building the standard library

unittest

Unit tests are enabled (command line switch -unittest)

assert

Checks are being emitted for AssertExpressions

none

Never defined; used to just disable a section of code

all

Always defined; used as the opposite of none

The following identifiers are defined, but are deprecated:

Predefined Version Identifiers (deprecated)

Version Identifier

Description

darwin

The Darwin operating system; use OSX instead

Thumb

ARM in Thumb mode; use ARM_Thumb instead

S390X

The System/390X architecture, 64-bit; use SystemZ instead

Others will be added as they make sense and new implementations appear.

To allow for future growth of the language, the version identifier namespace beginning with "D_" is reserved for identifiers indicating D language specification or new feature conformance. Further, all identifiers derived from the ones listed above by appending any character(s) are reserved. This means that e.g. ARM_foo and Windows_bar are reserved while foo_ARM and bar_Windows are not.

Furthermore, predefined version identifiers from this list cannot be set from the command line or from version statements. (This prevents things like both Windows and linux being simultaneously set.)

Compiler vendor specific versions can be predefined if the trademarked vendor identifier prefixes it, as in:

version(DigitalMars_funky_extension)
{
...
}

It is important to use the right version identifier for the right purpose. For example, use the vendor identifier when using a vendor specific feature. Use the operating system identifier when using an operating system specific feature, etc.

Debug Condition

DebugCondition:
debug
debug ( IntegerLiteral )
debug ( Identifier )

Two versions of programs are commonly built, a release build and a debug build. The debug build includes extra error checking code, test harnesses, pretty-printing code, etc. The debug statement conditionally compiles in its statement body. It is D's way of what in C is done with #ifdef DEBUG / #endif pairs.

The debug condition is satisfied when the -debug switch is passed to the compiler or when the debug level is >= 1.

The debug ( IntegerLiteral ) condition is satisfied when the debug level is >= IntegerLiteral.

The debug ( Identifier ) condition is satisfied when the debug identifier matches Identifier.

class Foo
{
int a, b;
debug:
int flag;
}

Debug Statement

A ConditionalStatement that has a DebugCondition is called a DebugStatement. DebugStatements have relaxed semantic checks in that pure, @nogc, nothrow and @safe checks are not done. Neither do DebugStatements influence the inference of pure, @nogc, nothrow and @safe attributes.

Undefined Behavior: Since these checks are bypassed, it is up to the programmer to ensure the code is correct. For example, throwing an exception in a nothrow function is undefined behavior.

Best Practices: This enables the easy insertion of code to provide debugging help, by bypassing the otherwise stringent attribute checks. Never ship release code that has DebugStatements enabled.

Debug Specification

DebugSpecification:
debug = Identifier ;
debug = IntegerLiteral ;

Debug identifiers and levels are set either by the command line switch -debug or by a DebugSpecification.

Debug specifications only affect the module they appear in, they do not affect any imported modules. Debug identifiers are in their own namespace, independent from version identifiers and other symbols.

It is illegal to forward reference a debug specification:

debug(foo) writeln("Foo");
debug = foo;    // error, foo used before set

DebugSpecifications may only appear at module scope.

Various different debug builds can be built with a parameter to debug:

debug(IntegerLiteral) { } // add in debug code if debug level is >= IntegerLiteral
debug(identifier) { } // add in debug code if debug keyword is identifier

These are presumably set by the command line as -debug=n and -debug=identifier.

Static If Condition

StaticIfCondition:
static if ( AssignExpression )

AssignExpression is implicitly converted to a boolean type, and is evaluated at compile time. The condition is satisfied if it evaluates to true. It is not satisfied if it evaluates to false.

It is an error if AssignExpression cannot be implicitly converted to a boolean type or if it cannot be evaluated at compile time.

StaticIfConditions can appear in module, class, template, struct, union, or function scope. In function scope, the symbols referred to in the AssignExpression can be any that can normally be referenced by an expression at that point.

const int i = 3;
int j = 4;

static if (i == 3)    // ok, at module scope
int x;

class C
{
const int k = 5;

static if (i == 3) // ok
int x;
else
long x;

static if (j == 3) // error, j is not a constant
int y;

static if (k == 5) // ok, k is in current scope
int z;
}

template Int(int i)
{
static if (i == 32)
alias Int = int;
else static if (i == 16)
alias Int = short;
else
static assert(0); // not supported
}

Int!(32) a;  // a is an int
Int!(16) b;  // b is a short
Int!(17) c;  // error, static assert trips

A StaticIfCondition differs from an IfStatement in the following ways:

1. It can be used to conditionally compile declarations, not just statements.

2. It does not introduce a new scope even if { } are used for conditionally compiled statements.

3. For unsatisfied conditions, the conditionally compiled code need only be syntactically correct. It does not have to be semantically correct.

4. It must be evaluatable at compile time.

Static Foreach

StaticForeach:
static AggregateForeach
static RangeForeach

StaticForeachDeclaration:
StaticForeach DeclarationBlock
StaticForeach : DeclDefsopt

StaticForeachStatement:
StaticForeach NoScopeNonEmptyStatement

The aggregate/range bounds are evaluated at compile time and turned into a sequence of compile-time entities by evaluating corresponding code with a ForeachStatement/ForeachRangeStatement at compile time. The body of the static foreach is then copied a number of times that corresponds to the number of elements of the sequence. Within the i-th copy, the name of the static foreach variable is bound to the i-th entry of the sequence, either as an enum variable declaration (for constants) or an alias declaration (for symbols). (In particular, static foreach variables are never runtime variables.)

static foreach(i; [0, 1, 2, 3])
{
pragma(msg, i);
}

static foreach supports multiple variables in cases where the corresponding foreach statement supports them. (In this case, static foreach generates a compile-time sequence of tuples, and the tuples are subsequently unpacked during iteration).

static foreach(i, v; ['a', 'b', 'c', 'd'])
{
static assert(i + 'a' == v);
}

Like bodies of ConditionalDeclarations, a static foreach body does not introduce a new scope. Therefore, it can be used to generate declarations:

import std.range : iota;

static foreach(i; iota(0, 3))
{
mixin(`enum x`, i, ` = i;`);
}

pragma(msg, x0, " ", x1," ", x2); // 0 1 2

If a new scope is desired for each expansion, use another set of braces:

static foreach(s; ["hi", "hey", "hello"])
{{
enum len = s.length;    // local to each iteration
static assert(len <= 5);
}}

static assert(!is(typeof(len)));

break and continue

As static foreach is a code generation construct and not a loop, break and continue cannot be used to change control flow within it. Instead of breaking or continuing a suitable enclosing statement, such an usage yields an error (this is to prevent misunderstandings).

int test(int x)
{
int r = -1;
switch(x)
{
static foreach(i; 0 .. 100)
{
case i:
r = i;
break; // error
}
default: break;
}
return r;
}

static foreach(i; 0 .. 200)
{
static assert(test(i) == (i < 100 ? i : -1));
}

An explicit break/continue label can be used to avoid this limitation. (Note that static foreach itself cannot be broken nor continued even if it is explicitly labeled.)

int test(int x)
{
int r = -1;
Lswitch: switch(x)
{
static foreach(i; 0 .. 100)
{
case i:
r = i;
break Lswitch;
}
default: break;
}
return r;
}

static foreach(i; 0 .. 200)
{
static assert(test(i) == (i<100 ? i : -1));
}

Static Assert

StaticAssert:
static assert ( AssertArguments ) ;

The first AssignExpression is evaluated at compile time, and converted to a boolean value. If the value is true, the static assert is ignored. If the value is false, an error diagnostic is issued and the compile fails.

Unlike AssertExpressions, StaticAsserts are always checked and evaluated by the compiler unless they appear in an unsatisfied conditional.

void foo()
{
if (0)
{
assert(0);  // never trips
static assert(0); // always trips
}
version (BAR)
{
}
else
{
static assert(0); // trips when version BAR is not defined
}
}

StaticAssert is useful tool for drawing attention to conditional configurations not supported in the code.

The optional second AssignExpression can be used to supply additional information, such as a text string, that will be printed out along with the error diagnostic.

Traits

Traits are extensions to the language to enable programs, at compile time, to get at information internal to the compiler. This is also known as compile time reflection. It is done as a special, easily extended syntax (similar to Pragmas) so that new capabilities can be added as required.

TraitsExpression:
__traits ( TraitsKeyword , TraitsArguments )

TraitsKeyword:
isAbstractClass
isArithmetic
isAssociativeArray
isFinalClass
isPOD
isNested
isFuture
isDeprecated
isFloating
isIntegral
isScalar
isStaticArray
isUnsigned
isDisabled
isVirtualFunction
isVirtualMethod
isAbstractFunction
isFinalFunction
isStaticFunction
isOverrideFunction
isTemplate
isRef
isOut
isLazy
isReturnOnStack
isCopyable
isZeroInit
isModule
isPackage
hasMember
hasCopyConstructor
hasPostblit
identifier
getAliasThis
getAttributes
getFunctionAttributes
getFunctionVariadicStyle
getLinkage
getLocation
getMember
getOverloads
getParameterStorageClasses
getPointerBitmap
getCppNamespaces
getVisibility
getProtection
getTargetInfo
getVirtualFunctions
getVirtualMethods
getUnitTests
parent
child
classInstanceSize
getVirtualIndex
allMembers
derivedMembers
isSame
compiles
toType

TraitsArguments:
TraitsArgument
TraitsArgument , TraitsArguments

TraitsArgument:
AssignExpression
Type

isArithmetic

If the arguments are all either types that are arithmetic types, or expressions that are typed as arithmetic types, then true is returned. Otherwise, false is returned. If there are no arguments, false is returned.

import std.stdio;

void main()
{
int i;
writeln(__traits(isArithmetic, int));
writeln(__traits(isArithmetic, i, i+1, int));
writeln(__traits(isArithmetic));
writeln(__traits(isArithmetic, int*));
}

Prints:

true
true
false
false

isFloating

Works like isArithmetic, except it's for floating point types (including imaginary and complex types).

import core.simd : float4;

enum E : float { a, b }

static assert(__traits(isFloating, float));
static assert(__traits(isFloating, E));
static assert(__traits(isFloating, float4));

static assert(!__traits(isFloating, float[4]));

isIntegral

Works like isArithmetic, except it's for integral types (including character types).

import core.simd : int4;

enum E { a, b }

static assert(__traits(isIntegral, bool));
static assert(__traits(isIntegral, char));
static assert(__traits(isIntegral, int));
static assert(__traits(isIntegral, E));
static assert(__traits(isIntegral, int4));

static assert(!__traits(isIntegral, float));
static assert(!__traits(isIntegral, int[4]));
static assert(!__traits(isIntegral, void*));

isScalar

Works like isArithmetic, except it's for scalar types.

import core.simd : int4, void16;

enum E { a, b }

static assert(__traits(isScalar, bool));
static assert(__traits(isScalar, char));
static assert(__traits(isScalar, int));
static assert(__traits(isScalar, float));
static assert(__traits(isScalar, E));
static assert(__traits(isScalar, int4));
static assert(__traits(isScalar, void*)); // Includes pointers!

static assert(!__traits(isScalar, int[4]));
static assert(!__traits(isScalar, void16));
static assert(!__traits(isScalar, void));
static assert(!__traits(isScalar, typeof(null)));
static assert(!__traits(isScalar, Object));

isUnsigned

Works like isArithmetic, except it's for unsigned types.

import core.simd : uint4;

enum SignedEnum { a, b }
enum UnsignedEnum : uint { a, b }

static assert(__traits(isUnsigned, bool));
static assert(__traits(isUnsigned, char));
static assert(__traits(isUnsigned, uint));
static assert(__traits(isUnsigned, UnsignedEnum));
static assert(__traits(isUnsigned, uint4));

static assert(!__traits(isUnsigned, int));
static assert(!__traits(isUnsigned, float));
static assert(!__traits(isUnsigned, SignedEnum));
static assert(!__traits(isUnsigned, uint[4]));
static assert(!__traits(isUnsigned, void*));

isStaticArray

Works like isArithmetic, except it's for static array types.

import core.simd : int4;

enum E : int[4] { a = [1, 2, 3, 4] }

static array = [1, 2, 3]; // Not a static array: the type is inferred as int[] not int[3].

static assert(__traits(isStaticArray, void[0]));
static assert(__traits(isStaticArray, E));
static assert(!__traits(isStaticArray, int4));
static assert(!__traits(isStaticArray, array));

isAssociativeArray

Works like isArithmetic, except it's for associative array types.

isAbstractClass

If the arguments are all either types that are abstract classes, or expressions that are typed as abstract classes, then true is returned. Otherwise, false is returned. If there are no arguments, false is returned.

import std.stdio;

abstract class C { int foo(); }

void main()
{
C c;
writeln(__traits(isAbstractClass, C));
writeln(__traits(isAbstractClass, c, C));
writeln(__traits(isAbstractClass));
writeln(__traits(isAbstractClass, int*));
}

Prints:

true
true
false
false

isFinalClass

Works like isAbstractClass, except it's for final classes.

isPOD

Takes one argument, which must be a type. It returns true if the type is a POD type, otherwise false.

isNested

Takes one argument. It returns true if the argument is a nested type which internally stores a context pointer, otherwise it returns false. Nested types can be classes, structs, and functions.

isFuture

Takes one argument. It returns true if the argument is a symbol marked with the @future keyword, otherwise false. Currently, only functions and variable declarations have support for the @future keyword.

isDeprecated

Takes one argument. It returns true if the argument is a symbol marked with the deprecated keyword, otherwise false.

isDisabled

Takes one argument and returns true if it's a function declaration marked with @disable.

struct Foo
{
@disable void foo();
void bar(){}
}

static assert(__traits(isDisabled, Foo.foo));
static assert(!__traits(isDisabled, Foo.bar));

For any other declaration even if @disable is a syntactically valid attribute false is returned because the annotation has no effect.

@disable struct Bar{}

static assert(!__traits(isDisabled, Bar));

isVirtualFunction

The same as isVirtualMethod, except that final functions that don't override anything return true.

isVirtualMethod

Takes one argument. If that argument is a virtual function, true is returned, otherwise false. Final functions that don't override anything return false.

import std.stdio;

struct S
{
void bar() { }
}

class C
{
void bar() { }
}

void main()
{
writeln(__traits(isVirtualMethod, C.bar));  // true
writeln(__traits(isVirtualMethod, S.bar));  // false
}

isAbstractFunction

Takes one argument. If that argument is an abstract function, true is returned, otherwise false.

import std.stdio;

struct S
{
void bar() { }
}

class C
{
void bar() { }
}

class AC
{
abstract void foo();
}

void main()
{
writeln(__traits(isAbstractFunction, C.bar));   // false
writeln(__traits(isAbstractFunction, S.bar));   // false
writeln(__traits(isAbstractFunction, AC.foo));  // true
}

isFinalFunction

Takes one argument. If that argument is a final function, true is returned, otherwise false.

import std.stdio;

struct S
{
void bar() { }
}

class C
{
void bar() { }
final void foo();
}

final class FC
{
void foo();
}

void main()
{
writeln(__traits(isFinalFunction, C.bar));  // false
writeln(__traits(isFinalFunction, S.bar));  // false
writeln(__traits(isFinalFunction, C.foo));  // true
writeln(__traits(isFinalFunction, FC.foo)); // true
}

isOverrideFunction

Takes one argument. If that argument is a function marked with override, true is returned, otherwise false.

import std.stdio;

class Base
{
void foo() { }
}

class Foo : Base
{
override void foo() { }
void bar() { }
}

void main()
{
writeln(__traits(isOverrideFunction, Base.foo)); // false
writeln(__traits(isOverrideFunction, Foo.foo));  // true
writeln(__traits(isOverrideFunction, Foo.bar));  // false
}

isStaticFunction

Takes one argument. If that argument is a static function, meaning it has no context pointer, true is returned, otherwise false.

struct A
{
int foo() { return 3; }
static int boo(int a) { return a; }
}

void main()
{
assert(__traits(isStaticFunction, A.boo));
assert(!__traits(isStaticFunction, A.foo));
assert(__traits(isStaticFunction, main));
}

isRef, isOut, isLazy

Takes one argument. If that argument is a declaration, true is returned if it is ref, out, or lazy, otherwise false.

void fooref(ref int x)
{
static assert(__traits(isRef, x));
static assert(!__traits(isOut, x));
static assert(!__traits(isLazy, x));
}

void fooout(out int x)
{
static assert(!__traits(isRef, x));
static assert(__traits(isOut, x));
static assert(!__traits(isLazy, x));
}

void foolazy(lazy int x)
{
static assert(!__traits(isRef, x));
static assert(!__traits(isOut, x));
static assert(__traits(isLazy, x));
}

isTemplate

Takes one argument. If that argument is a template then true is returned, otherwise false.

void foo(T)(){}
static assert(__traits(isTemplate,foo));
static assert(!__traits(isTemplate,foo!int()));
static assert(!__traits(isTemplate,"string"));

isCopyable

Takes one argument. If that argument is a copyable type then true is returned, otherwise false.

struct S
{
}
static assert( __traits(isCopyable, S));

struct T
{
@disable this(this); // disable copy construction
}
static assert(!__traits(isCopyable, T));

isZeroInit

Takes one argument which must be a type. If the type's default initializer is all zero bits then true is returned, otherwise false.

struct S1 { int x; }
struct S2 { int x = -1; }

static assert(__traits(isZeroInit, S1));
static assert(!__traits(isZeroInit, S2));

void test()
{
int x = 3;
static assert(__traits(isZeroInit, typeof(x)));
}

// `isZeroInit` will always return true for a class C
// because `C.init` is null reference.

class C { int x = -1; }

static assert(__traits(isZeroInit, C));

// For initializing arrays of element type `void`.
static assert(__traits(isZeroInit, void));

isReturnOnStack

Takes one argument which must either be a function symbol, function literal, a delegate, or a function pointer. It returns a bool which is true if the return value of the function is returned on the stack via a pointer to it passed as a hidden extra parameter to the function.

struct S { int[20] a; }
int test1();
S test2();

static assert(__traits(isReturnOnStack, test1) == false);
static assert(__traits(isReturnOnStack, test2) == true);

Implementation Defined: This is determined by the function ABI calling convention in use, which is often complex.

Best Practices: This has applications in:

1. Returning values in registers is often faster, so this can be used as a check on a hot function to ensure it is using the fastest method.

2. When using inline assembly to correctly call a function.

3. Testing that the compiler does this correctly is normally hackish and awkward, this enables efficient, direct, and simple testing.

isModule

Takes one argument. If that argument is a symbol that refers to a spec/module, module then true is returned, otherwise false. Package modules are considered to be modules even if they have not been directly imported as modules.

import std.algorithm.sorting;

// A regular package (no package.d)
static assert(!__traits(isModule, std));
// A package module (has a package.d file)
// Note that we haven't imported std.algorithm directly.
// (In other words, we don't have an "import std.algorithm;" directive.)
static assert(__traits(isModule, std.algorithm));
// A regular module
static assert(__traits(isModule, std.algorithm.sorting));

isPackage

Takes one argument. If that argument is a symbol that refers to a spec/module, package then true is returned, otherwise false.

import std.algorithm.sorting;
static assert(__traits(isPackage, std));
static assert(__traits(isPackage, std.algorithm));
static assert(!__traits(isPackage, std.algorithm.sorting));

hasMember

The first argument is a type that has members, or is an expression of a type that has members. The second argument is a string. If the string is a valid property of the type, true is returned, otherwise false.

import std.stdio;

struct S
{
int m;

import std.stdio; // imports write
}

void main()
{
S s;

writeln(__traits(hasMember, S, "m")); // true
writeln(__traits(hasMember, s, "m")); // true
writeln(__traits(hasMember, S, "y")); // false
writeln(__traits(hasMember, S, "write")); // true
writeln(__traits(hasMember, int, "sizeof")); // true
}

hasCopyConstructor

The argument is a type. If it is a struct with a copy constructor, returns true. Otherwise, return false. Note that a copy constructor is distinct from a postblit.

import std.stdio;

struct S
{
}

class C
{
}

struct P
{
this(ref P rhs) {}
}

struct B
{
this(this) {}
}

void main()
{
writeln(__traits(hasCopyConstructor, S)); // false
writeln(__traits(hasCopyConstructor, C)); // false
writeln(__traits(hasCopyConstructor, P)); // true
writeln(__traits(hasCopyConstructor, B)); // false, this is a postblit
}

hasPostblit

The argument is a type. If it is a struct with a postblit, returns true. Otherwise, return false. Note a postblit is distinct from a copy constructor.

import std.stdio;

struct S
{
}

class C
{
}

struct P
{
this(ref P rhs) {}
}

struct B
{
this(this) {}
}

void main()
{
writeln(__traits(hasPostblit, S)); // false
writeln(__traits(hasPostblit, C)); // false
writeln(__traits(hasPostblit, P)); // false, this is a copy ctor
writeln(__traits(hasPostblit, B)); // true
}

identifier

Takes one argument, a symbol. Returns the identifier for that symbol as a string literal.

import std.stdio;

int var = 123;
pragma(msg, typeof(var));                       // int
pragma(msg, typeof(__traits(identifier, var))); // string
writeln(var);                                   // 123
writeln(__traits(identifier, var));             // "var"

getAliasThis

Takes one argument, a type. If the type has alias this declarations, returns a sequence of the names (as strings) of the members used in those declarations. Otherwise returns an empty sequence.

alias AliasSeq(T...) = T;

struct S1
{
string var;
alias var this;
}
static assert(__traits(getAliasThis, S1) == AliasSeq!("var"));
static assert(__traits(getAliasThis, int).length == 0);

pragma(msg, __traits(getAliasThis, S1));
pragma(msg, __traits(getAliasThis, int));

Prints:

tuple("var")
tuple()

getAttributes

Takes one argument, a symbol. Returns a tuple of all attached user-defined attributes. If no UDAs exist it will return an empty tuple.

For more information, see: User-Defined Attributes

@(3) int a;
@("string", 7) int b;

enum Foo;
@Foo int c;

pragma(msg, __traits(getAttributes, a));
pragma(msg, __traits(getAttributes, b));
pragma(msg, __traits(getAttributes, c));

Prints:

tuple(3)
tuple("string", 7)
tuple((Foo))

getFunctionVariadicStyle

Takes one argument which must either be a function symbol, or a type that is a function, delegate or a function pointer. It returns a string identifying the kind of variadic arguments that are supported.

getFunctionVariadicStyle

result

kind

access

example

"none"

not a variadic function

void foo();

"argptr"

D style variadic function

_argptr and _arguments

void bar(...)

"stdarg"

C style variadic function

core.stdc.stdarg

extern (C) void abc(int, ...)

"typesafe"

typesafe variadic function

array on stack

void def(int[] ...)

import core.stdc.stdarg;

void novar() {}
extern(C) void cstyle(int, ...) {}
extern(C++) void cppstyle(int, ...) {}
void dstyle(...) {}
void typesafe(int[]...) {}

static assert(__traits(getFunctionVariadicStyle, novar) == "none");
static assert(__traits(getFunctionVariadicStyle, cstyle) == "stdarg");
static assert(__traits(getFunctionVariadicStyle, cppstyle) == "stdarg");
static assert(__traits(getFunctionVariadicStyle, dstyle) == "argptr");
static assert(__traits(getFunctionVariadicStyle, typesafe) == "typesafe");

static assert(__traits(getFunctionVariadicStyle, (int[] a...) {}) == "typesafe");
static assert(__traits(getFunctionVariadicStyle, typeof(cstyle)) == "stdarg");

getFunctionAttributes

Takes one argument which must either be a function symbol, function literal, or a function pointer. It returns a string tuple of all the attributes of that function excluding any user-defined attributes (UDAs can be retrieved with the getAttributes trait). If no attributes exist it will return an empty tuple.
Note: The order of the attributes in the returned tuple is implementation-defined and should not be relied upon.
A list of currently supported attributes are:

- pure, nothrow, @nogc, @property, @system, @trusted, @safe, ref and @live

Note: ref is a function attribute even though it applies to the return type.
Additionally the following attributes are only valid for non-static member functions:

- const, immutable, inout, shared

For example:

int sum(int x, int y) pure nothrow { return x + y; }

pragma(msg, __traits(getFunctionAttributes, sum));

struct S
{
void test() const @system { }
}

pragma(msg, __traits(getFunctionAttributes, S.test));

void main(){}

Prints:

tuple("pure", "nothrow", "@system")
tuple("const", "@system")

Note that some attributes can be inferred. For example:

pragma(msg, __traits(getFunctionAttributes, (int x) @trusted { return x * 2; }));

void main(){}

Prints:

tuple("pure", "nothrow", "@nogc", "@trusted")

getLinkage

Takes one argument, which is a declaration symbol, or the type of a function, delegate, pointer to function, struct, class, or interface. Returns a string representing the LinkageAttribute of the declaration. The string is one of:

- "D"

- "C"

- "C++"

- "Windows"

- "Objective-C"

- "System"

extern (C) int fooc();
alias aliasc = fooc;

static assert(__traits(getLinkage, fooc) == "C");
static assert(__traits(getLinkage, aliasc) == "C");

extern (C++) struct FooCPPStruct {}
extern (C++) class FooCPPClass {}
extern (C++) interface FooCPPInterface {}

static assert(__traits(getLinkage, FooCPPStruct) == "C++");
static assert(__traits(getLinkage, FooCPPClass) == "C++");
static assert(__traits(getLinkage, FooCPPInterface) == "C++");

getLocation

Takes one argument which is a symbol. To disambiguate between overloads, pass the result of getOverloads with the desired index, to getLocation. Returns a tuple(string, int, int) whose entries correspond to the filename, line number and column number where the argument was declared.

getMember

Takes two arguments, the second must be a string. The result is an expression formed from the first argument, followed by a ‘.’, followed by the second argument as an identifier.

import std.stdio;

struct S
{
int mx;
static int my;
}

void main()
{
S s;

__traits(getMember, s, "mx") = 1;  // same as s.mx=1;
writeln(__traits(getMember, s, "m" ~ "x")); // 1

// __traits(getMember, S, "mx") = 1;  // error, no this for S.mx
__traits(getMember, S, "my") = 2;  // ok
}

getOverloads

The first argument is an aggregate (e.g. struct/class/module). The second argument is a string that matches the name of the member(s) to return. The third argument is a bool, and is optional. If true, the result will also include template overloads. The result is a tuple of all the overloads of the supplied name.

import std.stdio;

class D
{
this() { }
~this() { }
void foo() { }
int foo(int) { return 2; }
void bar(T)() { return T.init; }
class bar(int n) {}
}

void main()
{
D d = new D();

foreach (t; __traits(getOverloads, D, "foo"))
writeln(typeid(typeof(t)));

alias b = typeof(__traits(getOverloads, D, "foo"));
foreach (t; b)
writeln(typeid(t));

auto i = __traits(getOverloads, d, "foo")[1](1);
writeln(i);

foreach (t; __traits(getOverloads, D, "bar", true))
writeln(t.stringof);
}

Prints:

void()
int()
void()
int()
2
bar(T)()
bar(int n)

getParameterStorageClasses

Takes two arguments. The first must either be a function symbol, or a type that is a function, delegate or a function pointer. The second is an integer identifying which parameter, where the first parameter is 0. It returns a tuple of strings representing the storage classes of that parameter.

ref int foo(return ref const int* p, scope int* a, out int b, lazy int c);

static assert(__traits(getParameterStorageClasses, foo, 0)[0] == "return");
static assert(__traits(getParameterStorageClasses, foo, 0)[1] == "ref");

static assert(__traits(getParameterStorageClasses, foo, 1)[0] == "scope");
static assert(__traits(getParameterStorageClasses, foo, 2)[0] == "out");
static assert(__traits(getParameterStorageClasses, typeof(&foo), 3)[0] == "lazy");

getPointerBitmap

The argument is a type. The result is an array of size_t describing the memory used by an instance of the given type.

The first element of the array is the size of the type (for classes it is the classInstanceSize).

The following elements describe the locations of GC managed pointers within the memory occupied by an instance of the type. For type T, there are T.sizeof / size_t.sizeof possible pointers represented by the bits of the array values.

This array can be used by a precise GC to avoid false pointers.

void main()
{
static class C
{
// implicit virtual function table pointer not marked
// implicit monitor field not marked, usually managed manually
C next;
size_t sz;
void* p;
void function () fn; // not a GC managed pointer
}

static struct S
{
size_t val1;
void* p;
C c;
byte[] arr;          // { length, ptr }
void delegate () dg; // { context, func }
}

static assert (__traits(getPointerBitmap, C) == [6*size_t.sizeof, 0b010100]);
static assert (__traits(getPointerBitmap, S) == [7*size_t.sizeof, 0b0110110]);
}

getCppNamespaces

The argument is a symbol. The result is a tuple of strings, possibly empty, that correspond to the namespaces the symbol resides in.

extern(C++, "ns")
struct Foo {}
struct Bar {}
extern(C++, __traits(getCppNamespaces, Foo)) struct Baz {}
static assert(__traits(getCppNamespaces, Foo) ==  __traits(getCppNamespaces, Baz));
void main()
{
static assert(__traits(getCppNamespaces, Foo)[0] == "ns");
static assert(!__traits(getCppNamespaces, Bar).length);
static assert(__traits(getCppNamespaces, Foo) ==  __traits(getCppNamespaces, Baz));
}

getVisibility

The argument is a symbol. The result is a string giving its visibility level: "public", "private", "protected", "export", or "package".

import std.stdio;

class D
{
export void foo() { }
public int bar;
}

void main()
{
D d = new D();

auto i = __traits(getVisibility, d.foo);
writeln(i);

auto j = __traits(getVisibility, d.bar);
writeln(j);
}

Prints:

export
public

getProtection

A backward-compatible alias for getVisibility.

getTargetInfo

Receives a string key as argument. The result is an expression describing the requested target information.

version (CppRuntime_Microsoft)
static assert(__traits(getTargetInfo, "cppRuntimeLibrary") == "libcmt");

Keys are implementation defined, allowing relevant data for exotic targets. A reliable subset exists which are always available:

- "cppRuntimeLibrary" - The C++ runtime library affinity for this toolchain

- "cppStd" - The version of the C++ standard supported by extern(C++) code, equivalent to the __cplusplus macro in a C++ compiler

- "floatAbi" - Floating point ABI; may be "hard", "soft", or "softfp"

- "objectFormat" - Target object format

getVirtualFunctions

The same as getVirtualMethods, except that final functions that do not override anything are included.

getVirtualMethods

The first argument is a class type or an expression of class type. The second argument is a string that matches the name of one of the functions of that class. The result is a tuple of the virtual overloads of that function. It does not include final functions that do not override anything.

import std.stdio;

class D
{
this() { }
~this() { }
void foo() { }
int foo(int) { return 2; }
}

void main()
{
D d = new D();

foreach (t; __traits(getVirtualMethods, D, "foo"))
writeln(typeid(typeof(t)));

alias b = typeof(__traits(getVirtualMethods, D, "foo"));
foreach (t; b)
writeln(typeid(t));

auto i = __traits(getVirtualMethods, d, "foo")[1](1);
writeln(i);
}

Prints:

void()
int()
void()
int()
2

getUnitTests

Takes one argument, a symbol of an aggregate (e.g. struct/class/module). The result is a tuple of all the unit test functions of that aggregate. The functions returned are like normal nested static functions, CTFE will work and UDAs will be accessible.

Note:

The -unittest flag needs to be passed to the compiler. If the flag is not passed __traits(getUnitTests) will always return an empty tuple.

module foo;

import core.runtime;
import std.stdio;

struct name { string name; }

class Foo
{
unittest
{
writeln("foo.Foo.unittest");
}
}

@name("foo") unittest
{
writeln("foo.unittest");
}

template Tuple (T...)
{
alias Tuple = T;
}

shared static this()
{
// Override the default unit test runner to do nothing. After that, "main" will
// be called.
Runtime.moduleUnitTester = { return true; };
}

void main()
{
writeln("start main");

alias tests = Tuple!(__traits(getUnitTests, foo));
static assert(tests.length == 1);

alias attributes = Tuple!(__traits(getAttributes, tests[0]));
static assert(attributes.length == 1);

foreach (test; tests)
test();

foreach (test; __traits(getUnitTests, Foo))
test();
}

By default, the above will print:

start main
foo.unittest
foo.Foo.unittest

parent

Takes a single argument which must evaluate to a symbol. The result is the symbol that is the parent of it.

child

Takes two arguments. The first must be a symbol or expression. The second is a symbol, such as an alias to a member of the first argument. The result is the second argument interpreted with its this context set to the value of the first argument.

import std.stdio;

struct A
{
int i;
int foo(int j) {
return i * j;
}
T bar(T)(T t) {
return i + t;
}
}

alias Ai = A.i;
alias Abar = A.bar!int;

void main()
{
A a;

__traits(child, a, Ai) = 3;
writeln(a.i);
writeln(__traits(child, a, A.foo)(2));
writeln(__traits(child, a, Abar)(5));
}

Prints:

3
6
8

classInstanceSize

Takes a single argument, which must evaluate to either a class type or an expression of class type. The result is of type size_t, and the value is the number of bytes in the runtime instance of the class type. It is based on the static type of a class, not the polymorphic type.

getVirtualIndex

Takes a single argument which must evaluate to a function. The result is a ptrdiff_t containing the index of that function within the vtable of the parent type. If the function passed in is final and does not override a virtual function, -1 is returned instead.

allMembers

Takes a single argument, which must evaluate to either a type or an expression of type. A tuple of string literals is returned, each of which is the name of a member of that type combined with all of the members of the base classes (if the type is a class). No name is repeated. Builtin properties are not included.

import std.stdio;

class D
{
this() { }
~this() { }
void foo() { }
int foo(int) { return 0; }
}

void main()
{
auto b = [ __traits(allMembers, D) ];
writeln(b);
// ["__ctor", "__dtor", "foo", "toString", "toHash", "opCmp", "opEquals",
// "Monitor", "factory"]
}

The order in which the strings appear in the result is not defined.

derivedMembers

Takes a single argument, which must evaluate to either a type or an expression of type. A tuple of string literals is returned, each of which is the name of a member of that type. No name is repeated. Base class member names are not included. Builtin properties are not included.

import std.stdio;

class D
{
this() { }
~this() { }
void foo() { }
int foo(int) { return 0; }
}

void main()
{
auto a = [__traits(derivedMembers, D)];
writeln(a);    // ["__ctor", "__dtor", "foo"]
}

The order in which the strings appear in the result is not defined.

isSame

Takes two arguments and returns bool true if they are the same symbol, false if not.

import std.stdio;

struct S { }

int foo();
int bar();

void main()
{
writeln(__traits(isSame, foo, foo)); // true
writeln(__traits(isSame, foo, bar)); // false
writeln(__traits(isSame, foo, S));   // false
writeln(__traits(isSame, S, S));     // true
writeln(__traits(isSame, std, S));   // false
writeln(__traits(isSame, std, std)); // true
}

If the two arguments are expressions made up of literals or enums that evaluate to the same value, true is returned.

If the two arguments are both lambda functions (or aliases to lambda functions), then they are compared for equality. For the comparison to be computed correctly, the following conditions must be met for both lambda functions:

1. The lambda function arguments must not have a template instantiation as an explicit argument type. Any other argument types (basic, user-defined, template) are supported.

2. The lambda function body must contain a single expression (no return statement) which contains only numeric values, manifest constants, enum values, function arguments and function calls. If the expression contains local variables or return statements, the function is considered incomparable.

If these constraints aren't fulfilled, the function is considered incomparable and isSame returns false.

int f() { return 2; }
void test(alias pred)()
{
// f() from main is a different function from top-level f()
static assert(!__traits(isSame, (int a) => a + f(), pred));
}

void main()
{
static assert(__traits(isSame, (a, b) => a + b, (c, d) => c + d));
static assert(__traits(isSame, a => ++a, b => ++b));
static assert(!__traits(isSame, (int a, int b) => a + b, (a, b) => a + b));
static assert(__traits(isSame, (a, b) => a + b + 10, (c, d) => c + d + 10));

// lambdas accessing local variables are considered incomparable
int b;
static assert(!__traits(isSame, a => a + b, a => a + b));

// lambdas calling other functions are comparable
int f() { return 3;}
static assert(__traits(isSame, a => a + f(), a => a + f()));
test!((int a) => a + f())();

class A
{
int a;
this(int a)
{
this.a = a;
}
}

class B
{
int a;
this(int a)
{
this.a = a;
}
}

static assert(__traits(isSame, (A a) => ++a.a, (A b) => ++b.a));
// lambdas with different data types are considered incomparable,
// even if the memory layout is the same
static assert(!__traits(isSame, (A a) => ++a.a, (B a) => ++a.a));
}

If the two arguments are tuples then isSame returns true if the two tuples, after expansion, have the same length and if each pair of nth argument respects the constraints previously specified.

import std.stdio;
import std.meta;

struct S { }

void main()
{
// true, like __traits(isSame(0,0)) && __traits(isSame(1,1))
writeln(__traits(isSame, AliasSeq!(0,1), AliasSeq!(0,1)));
// false, like __traits(isSame(S,std.meta)) && __traits(isSame(1,1))
writeln(__traits(isSame, AliasSeq!(S,1), AliasSeq!(std.meta,1)));
// false, the length of the sequences is different
writeln(__traits(isSame, AliasSeq!(1), AliasSeq!(1,2)));
}

compiles

Returns a bool true if all of the arguments compile (are semantically correct). The arguments can be symbols, types, or expressions that are syntactically correct. The arguments cannot be statements or declarations.

If there are no arguments, the result is false.

import std.stdio;

struct S
{
static int s1;
int s2;
}

int foo();
int bar();

void main()
{
writeln(__traits(compiles));                      // false
writeln(__traits(compiles, foo));                 // true
writeln(__traits(compiles, foo + 1));             // true
writeln(__traits(compiles, &foo + 1));            // false
writeln(__traits(compiles, typeof(1)));           // true
writeln(__traits(compiles, S.s1));                // true
writeln(__traits(compiles, S.s3));                // false
writeln(__traits(compiles, 1,2,3,int,long,std));  // true
writeln(__traits(compiles, 3[1]));                // false
writeln(__traits(compiles, 1,2,3,int,long,3[1])); // false
}

This is useful for:

- Giving better error messages inside generic code than the sometimes hard to follow compiler ones.

- Doing a finer grained specialization than template partial specialization allows for.

toType

Takes a single argument, which must evaluate to an expression of type string. The contents of the string must correspond to the mangled contents of a type that has been seen by the implementation.

Only D mangling is supported. Other manglings, such as C++ mangling, are not.

The value returned is a type.

template Type(T) { alias Type = T; }

Type!(__traits(toType, "i")) j = 3; // j is declared as type `int`

static assert(is(Type!(__traits(toType, (int*).mangleof)) == int*));

__traits(toType, "i") x = 4; // x is also declared as type `int`

Rationale: Provides the inverse operation of the .mangleof property.

Error Handling

I came, I coded, I crashed. Julius C'ster

All programs have to deal with errors. Errors are unexpected conditions that are not part of the normal operation of a program. Examples of common errors are:

- Out of memory.

- Out of disk space.

- Invalid file name.

- Attempting to write to a read-only file.

- Attempting to read a non-existent file.

- Requesting a system service that is not supported.

The Error Handling Problem
The traditional C way of detecting and reporting errors is not traditional, it is ad-hoc and varies from function to function, including:

- Returning a NULL pointer.

- Returning a 0 value.

- Returning a non-zero error code.

- Requiring errno to be checked.

- Requiring that a function be called to check if the previous function failed.

To deal with these possible errors, tedious error handling code must be added to each function call. If an error happened, code must be written to recover from the error, and the error must be reported to the user in some user friendly fashion. If an error cannot be handled locally, it must be explicitly propagated back to its caller. The long list of errno values needs to be converted into appropriate text to be displayed. Adding all the code to do this can consume a large part of the time spent coding a project - and still, if a new errno value is added to the runtime system, the old code can not properly display a meaningful error message.

Good error handling code tends to clutter up what otherwise would be a neat and clean looking implementation.

Even worse, good error handling code is itself error prone, tends to be the least tested (and therefore buggy) part of the project, and is frequently simply omitted. The end result is likely a "blue screen of death" as the program failed to deal with some unanticipated error.

Quick and dirty programs are not worth writing tedious error handling code for, and so such utilities tend to be like using a table saw with no blade guards.
What's needed is an error handling philosophy and methodology such that:

- It is standardized - consistent usage makes it more useful.

- The result is reasonable even if the programmer fails to check for errors.

- Old code can be reused with new code without having to modify the old code to be compatible with new error types.

- No errors get inadvertently ignored.

- ‘Quick and dirty’ utilities can be written that still correctly handle errors.

- It is easy to make the error handling source code look good.

The D Error Handling Solution
Let's first make some observations and assumptions about errors:

- Errors are not part of the normal flow of a program. Errors are exceptional, unusual, and unexpected.

- Because errors are unusual, execution of error handling code is not performance critical.

- The normal flow of program logic is performance critical.

- All errors must be dealt with in some way, either by code explicitly written to handle them, or by some system default handling.

- The code that detects an error knows more about the error than the code that must recover from the error.

The solution is to use exception handling to report errors. All errors are objects derived from the abstract class Error. Error has a pure virtual function called toString() which produces a string with a human readable description of the error.

If code detects an error like "out of memory," then an Error is thrown with a message saying "Out of memory". The function call stack is unwound, looking for a handler for the Error. Finally blocks are executed as the stack is unwound. If an error handler is found, execution resumes there. If not, the default Error handler is run, which displays the message and terminates the program.

How does this meet our criteria?

It is standardized - consistent usage makes it more useful.

This is the D way, and is used consistently in the D runtime library and examples.

The result is reasonable result even if the programmer fails to check for errors.

If no catch handlers are there for the errors, then the program gracefully exits through the default error handler with an appropriate message.

Old code can be reused with new code without having to modify the old code to be compatible with new error types.

Old code can decide to catch all errors, or only specific ones, propagating the rest upwards. In any case, there is no more need to correlate error numbers with messages, the correct message is always supplied.

No errors get inadvertently ignored.

Error exceptions get handled one way or another. There is nothing like a NULL pointer return indicating an error, followed by trying to use that NULL pointer.

'Quick and dirty' utilities can be written that still correctly handle errors.

Quick and dirty code need not write any error handling code at all, and don't need to check for errors. The errors will be caught, an appropriate message displayed, and the program gracefully shut down all by default.

It is easy to make the error handling source code look good.

The try/catch/finally statements look a lot nicer than endless if (error) goto errorhandler; statements.

How does this meet our assumptions about errors?

Errors are not part of the normal flow of a program. Errors are exceptional, unusual, and unexpected.

D exception handling fits right in with that.

Because errors are unusual, execution of error handling code is not performance critical.

Exception handling stack unwinding is a relatively slow process.

The normal flow of program logic is performance critical.

Since the normal flow code does not have to check every function call for error returns, it can be realistically faster to use exception handling for the errors.

All errors must be dealt with in some way, either by code explicitly written to handle them, or by some system default handling.

If there's no handler for a particular error, it is handled by the runtime library default handler. If an error is ignored, it is because the programmer specifically added code to ignore an error, which presumably means it was intentional.

The code that detects an error knows more about the error than the code that must recover from the error.

There is no more need to translate error codes into human readable strings, the correct string is generated by the error detection code, not the error recovery code. This also leads to consistent error messages for the same error between applications.

Using exceptions to handle errors leads to another issue - how to write exception safe programs. Here's how.

Unit Tests

UnitTest:
unittest BlockStatement

Unit tests are a builtin framework of test cases applied to a module to determine if it is working properly. A D program can be run with unit tests enabled or disabled.

Unit tests are a special function defined like:

unittest
{
...test code...
}

Individual tests are specified in the unit test using AssertExpressions. Unlike AssertExpressions used elsewhere, the assert is not assumed to hold, and upon assert failure the program is still in a defined state.

There can be any number of unit test functions in a module, including within struct, union and class declarations. They are executed in lexical order.

Unit tests, when enabled, are run after all static initialization is complete and before the main() function is called.

For example, given a class Sum that is used to add two values, a unit test can be given:

class Sum
{
int add(int x, int y) { return x + y; }

unittest
{
Sum sum = new Sum;
assert(sum.add(3,4) == 7);
assert(sum.add(-2,0) == -2);
}
}

When unit tests are enabled, the version identifier unittest is predefined.

Attributed Unittests

A unittest may be attributed with any of the global function attributes. Such unittests are useful in verifying the given attribute(s) on a template function:

void myFunc(T)(T[] data)
{
if (data.length > 2)
data[0] = data[1];
}

@safe nothrow unittest
{
auto arr = [1,2,3];
myFunc(arr);
assert(arr == [2,2,3]);
}

This unittest verifies that myFunc contains only @safe, nothrow code. Although this can also be accomplished by attaching these attributes to myFunc itself, that would prevent myFunc from being instantiated with types T that have @system or throwing code in their opAssign method, or other methods that myFunc may call. The above idiom allows myFunc to be instantiated with such types, yet at the same time verify that the @system and throwing behavior is not introduced by the code within myFunc itself.

Implementation Defined:

1. If unit tests are not enabled, the implementation is not required to check the UnitTest for syntactic or semantic correctness. This is to reduce the compile time impact of larger unit test sections. The tokens must still be valid, and the implementation can merely count { and } tokens to find the end of the UnitTest's BlockStatement.

2. The presentation of unit test results to the user.

3. The method used to enable or disable the unit tests. Use of a compiler switch such as -unittest to enable them is suggested.

4. The order in which modules are called to run their unit tests.

5. Whether the program stops on the first unit test failure, or continues running the unit tests.

Best Practices:

1. Using unit tests in conjunction with coverage testing (such as -cov) is effective.

2. A unit test for a function should appear immediately following it.

Documented Unittests

Documented unittests allow the developer to deliver code examples to the user, while at the same time automatically verifying that the examples are valid. This avoids the frequent problem of having outdated documentation for some piece of code.

If a declaration is followed by a documented unittest, the code in the unittest will be inserted in the example section of the declaration:

/// Math class
class Math
{
/// add function
static int add(int x, int y) { return x + y; }

///
unittest
{
assert(add(2, 2) == 4);
}
}

///
unittest
{
auto math = new Math();
auto result = math.add(2, 2);
}

The above will generate the following documentation:

class Math;

Math class

Example:

auto math = new Math;
auto result = math.add(2, 2);

int add(int x, int y);

add function

Example:

assert(add(2, 2) == 4);

A unittest which is not documented, or is marked as private will not be used to generate code samples.

There can be multiple documented unittests and they can appear in any order. They will be attached to the last non-unittest declaration:

/// add function
int add(int x, int y) { return x + y; }

/// code sample generated
unittest
{
assert(add(1, 1) == 2);
}

/// code sample not generated because the unittest is private
private unittest
{
assert(add(2, 2) == 4);
}

unittest
{
/// code sample not generated because the unittest isn't documented
assert(add(3, 3) == 6);
}

/// code sample generated, even if it only includes comments (or is empty)
unittest
{
/** assert(add(4, 4) == 8); */
}

The above will generate the following documentation:

int add(int x, int y);

add function

Examples:
code sample generated

assert(add(1, 1) == 2);

Examples:
code sample generated, even if it is empty or only includes comments

/** assert(add(4, 4) == 8); */

Garbage Collection

D is a systems programming language with support for garbage collection. Usually it is not necessary to free memory explicitly. Just allocate as needed, and the garbage collector will periodically return all unused memory to the pool of available memory.

D also provides the mechanisms to write code where the garbage collector is not involved. More information is provided below.

Programmers accustomed to explicitly managing memory allocation and deallocation will likely be skeptical of the benefits and efficacy of garbage collection. Experience both with new projects written with garbage collection in mind, and converting existing projects to garbage collection shows that:

- Garbage collected programs are often faster. This is counterintuitive, but the reasons are:

- Reference counting is a common solution to solve explicit memory allocation problems. The code to implement the increment and decrement operations whenever assignments are made is one source of slowdown. Hiding it behind smart pointer classes doesn't help the speed. (Reference counting methods are not a general solution anyway, as circular references never get deleted.)

- Destructors are used to deallocate resources acquired by an object. For most classes, this resource is allocated memory. With garbage collection, most destructors then become empty and can be discarded entirely.

- All those destructors freeing memory can become significant when objects are allocated on the stack. For each one, some mechanism must be established so that if an exception happens, the destructors all get called in each frame to release any memory they hold. If the destructors become irrelevant, then there's no need to set up special stack frames to handle exceptions, and the code runs faster.

- Garbage collection kicks in only when memory gets tight. When memory is not tight, the program runs at full speed and does not spend any time tracing and freeing memory.

- Garbage collected programs do not suffer from gradual deterioration due to an accumulation of memory leaks.

- Garbage collectors reclaim unused memory, therefore they do not suffer from "memory leaks" which can cause long running applications to gradually consume more and more memory until they bring down the system. GC programs have longer term stability.

- Garbage collected programs have fewer hard-to-find pointer bugs. This is because there are no dangling references to freed memory. There is no code to explicitly manage memory, hence no bugs in such code.

- Garbage collected programs are faster to develop and debug, because there's no need for developing, debugging, testing, or maintaining the explicit deallocation code.

Garbage collection is not a panacea. There are some downsides:

- It is not always obvious when the GC allocates memory, which in turn can trigger a collection, so the program can pause unexpectedly.

- The time it takes for a collection to complete is not bounded. While in practice it is very quick, this cannot normally be guaranteed.

- Normally, all threads other than the collector thread must be halted while the collection is in progress.

- Garbage collectors can keep around some memory that an explicit deallocator would not.

- Garbage collection should be implemented as a basic operating system kernel service. But since it is not, garbage collecting programs must carry around with them the garbage collection implementation. While this can be a shared library, it is still there.

These constraints are addressed by techniques outlined in Memory Management, including the mechanisms provided by D to control allocations outside the GC heap.

There is currently work in progress to make the runtime library free of GC heap allocations, to allow its use in scenarios where the use of GC infrastructure is not possible.

How Garbage Collection Works

The GC works by:

1. Stopping all other threads than the thread currently trying to allocate GC memory.

2. ‘Hijacking’ the current thread for GC work.

3. Scanning all ‘root’ memory ranges for pointers into GC allocated memory.

4. Recursively scanning all allocated memory pointed to by roots looking for more pointers into GC allocated memory.

5. Freeing all GC allocated memory that has no active pointers to it and do not need destructors to run.

6. Queueing all unreachable memory that needs destructors to run.

7. Resuming all other threads.

8. Running destructors for all queued memory.

9. Freeing any remaining unreachable memory.

10. Returning the current thread to whatever work it was doing.

Interfacing Garbage Collected Objects With Foreign Code

The garbage collector looks for roots in:

1. the static data segment

2. the stacks and register contents of each thread

3. the TLS (thread-local storage) areas of each thread

4. any roots added by core.memory.GC.addRoot() or core.memory.GC.addRange()

If the only pointer to an object is held outside of these areas, then the collector will miss it and free the memory.

To avoid this from happening, either

- maintain a pointer to the object in an area the collector does scan for pointers;

- add a root where a pointer to the object is stored using core.memory.GC.addRoot() or core.memory.GC.addRange().

- reallocate and copy the object using the foreign code's storage allocator or using the C runtime library's malloc/free.

Pointers and the Garbage Collector

Pointers in D can be broadly divided into two categories: Those that point to garbage collected memory, and those that do not. Examples of the latter are pointers created by calls to C's malloc(), pointers received from C library routines, pointers to static data, pointers to objects on the stack, etc. For those pointers, anything that is legal in C can be done with them.

For garbage collected pointers and references, however, there are some restrictions. These restrictions are minor, but they are intended to enable the maximum flexibility in garbage collector design.

Undefined Behavior:

- Do not xor pointers with other values, like the xor pointer linked list trick used in C.

- Do not use the xor trick to swap two pointer values.

- Do not store pointers into non-pointer variables using casts and other tricks.

void* p;
...
int x = cast(int)p;   // error: undefined behavior

The garbage collector does not scan non-pointer fields for GC pointers.

- Do not take advantage of alignment of pointers to store bit flags in the low order bits:

p = cast(void*)(cast(int)p | 1);  // error: undefined behavior

- Do not store into pointers values that may point into the garbage collected heap:

p = cast(void*)12345678;   // error: undefined behavior

A copying garbage collector may change this value.

- Do not store magic values into pointers, other than null.

- Do not write pointer values out to disk and read them back in again.

- Do not use pointer values to compute a hash function. A copying garbage collector can arbitrarily move objects around in memory, thus invalidating the computed hash value.

- Do not depend on the ordering of pointers:

if (p1 < p2)  // error: undefined behavior
...

since, again, the garbage collector can move objects around in memory.

- Do not add or subtract an offset to a pointer such that the result points outside of the bounds of the garbage collected object originally allocated.

char* p = new char[10];
char* q = p + 6; // ok
q = p + 11;      // error: undefined behavior
q = p - 1;       // error: undefined behavior

- Do not misalign pointers if those pointers may point into the GC heap, such as:

struct Foo
{
align (1):
byte b;
char* p;  // misaligned pointer
}

Misaligned pointers may be used if the underlying hardware supports them and the pointer is never used to point into the GC heap.

- Do not use byte-by-byte memory copies to copy pointer values. This may result in intermediate conditions where there is not a valid pointer, and if the gc pauses the thread in such a condition, it can corrupt memory. Most implementations of memcpy() will work since the internal implementation of it does the copy in aligned chunks greater than or equal to the pointer size, but since this kind of implementation is not guaranteed by the C standard, use memcpy() only with extreme caution.

- Do not have pointers in a struct instance that point back to the same instance. The trouble with this is if the instance gets moved in memory, the pointer will point back to where it came from, with likely disastrous results.

Things that are reliable and can be done:

- Use a union to share storage with a pointer:

union U { void* ptr; int value }

- A pointer to the start of a garbage collected object need not be maintained if a pointer to the interior of the object exists.

char[] p = new char[10];
char[] q = p[3..6];
// q is enough to hold on to the object, don't need to keep
// p as well.

One can avoid using pointers anyway for most tasks. D provides features rendering most explicit pointer uses obsolete, such as reference objects, dynamic arrays, and garbage collection. Pointers are provided in order to interface successfully with C APIs and for some low level work.

Working with the Garbage Collector

Garbage collection doesn't solve every memory deallocation problem. For example, if a pointer to a large data structure is kept, the garbage collector cannot reclaim it, even if it is never referred to again. To eliminate this problem, it is good practice to set a reference or pointer to an object to null when no longer needed.

This advice applies only to static references or references embedded inside other objects. There is not much point for such stored on the stack to be nulled because new stack frames are initialized anyway.

Object Pinning and a Moving Garbage Collector

Although D does not currently use a moving garbage collector, by following the rules listed above one can be implemented. No special action is required to pin objects. A moving collector will only move objects for which there are no ambiguous references, and for which it can update those references. All other objects will be automatically pinned.

D Operations That Involve the Garbage Collector

Some sections of code may need to avoid using the garbage collector. The following constructs may allocate memory using the garbage collector:

- NewExpression

- Array appending

- Array concatenation

- Array literals (except when used to initialize static data)

- Associative array literals

- Any insertion, removal, or lookups in an associative array

- Extracting keys or values from an associative array

- Taking the address of (i.e. making a delegate to) a nested function that accesses variables in an outer scope

- A function literal that accesses variables in an outer scope

- An AssertExpression that fails its condition

Configuring the Garbage Collector

Since version 2.067, The garbage collector can now be configured through the command line, the environment or by options embedded into the executable.

By default, GC options can only be passed on the command line of the program to run, e.g.

app "--DRT-gcopt=profile:1 minPoolSize:16" arguments to app

Available GC options are:

- disable:0|1 - start disabled

- profile:0|1 - enable profiling with summary when terminating program

- gc:conservative|precise|manual - select gc implementation (default = conservative)

- initReserve:N - initial memory to reserve in MB

- minPoolSize:N - initial and minimum pool size in MB

- maxPoolSize:N - maximum pool size in MB

- incPoolSize:N - pool size increment MB

- parallel:N - number of additional threads for marking

- heapSizeFactor:N - targeted heap size to used memory ratio

- cleanup:none|collect|finalize - how to treat live objects when terminating

- collect: run a collection (the default for backward compatibility)

- none: do nothing

- finalize: all live objects are finalized unconditionally

In addition, --DRT-gcopt=help will show the list of options and their current settings.

Command line options starting with "--DRT-" are filtered out before calling main, so the program will not see them. They are still available via rt_args.

Configuration via the command line can be disabled by declaring a variable for the linker to pick up before using its default from the runtime:

extern(C) __gshared bool rt_cmdline_enabled = false;

Likewise, declare a boolean rt_envvars_enabled to enable configuration via the environment variable DRT_GCOPT:

extern(C) __gshared bool rt_envvars_enabled = true;

Setting default configuration properties in the executable can be done by specifying an array of options named rt_options:

extern(C) __gshared string[] rt_options = [ "gcopt=initReserve:100 profile:1" ];

Evaluation order of options is rt_options, then environment variables, then command line arguments, i.e. if command line arguments are not disabled, they can override options specified through the environment or embedded in the executable.

Precise Heap Scanning

Selecting precise as the garbage collector via the options above means type information will be used to identify actual or possible pointers or references within heap allocated data objects. Non-pointer data will not be interpreted as a reference to other memory as a "false pointer". The collector has to make pessimistic assumptions if a memory slot can contain both a pointer or an integer value, it will still be scanned (e.g. in a union).

To use the GC memory functions from core.memory for data with a mixture of pointers and non-pointer data, pass the TypeInfo of the allocated struct, class, or type as the optional parameter. The default null is interpreted as memory that might contain pointers everywhere.

struct S { size_t hash; Data* data; }
S* s = cast(S*)GC.malloc(S.sizeof, 0, typeid(S));

Attention: Enabling precise scanning needs slightly more caution with type declarations. For example, when reserving a buffer as part of a struct and later emplacing an object instance with references to other allocations into this memory, do not use basic integer types to reserve the space. Doing so will cause the garbage collector not to detect the references. Instead, use an array type that will scan this area conservatively. Using void* is usually the best option as it also ensures proper alignment for pointers being scanned by the GC.

Precise Scanning of the DATA and TLS segment

Windows only: As of version 2.075, the DATA (global shared data) and TLS segment (thread local data) of an executable or DLL can be configured to be scanned precisely by the garbage collector instead of conservatively. This takes advantage of information emitted by the compiler to identify possible mutable pointers inside these segments. Immutable pointers with initializers are excluded from scanning, too, as they can only point to preallocated memory.

Precise scanning can be enabled with the D runtime option "scanDataSeg". Possible option values are "conservative" (default) and "precise". As with the GC options, it can be specified on the command line, in the environment or embedded into the executable, e.g.

extern(C) __gshared string[] rt_options = [ "scanDataSeg=precise" ];

Attention: Enabling precise scanning needs slightly more caution typing global memory. For example, to pre-allocate memory in the DATA/TLS segment and later emplace an object instance with references to other allocations into this memory, do not use basic integer types to reserve the space. Doing so will cause the garbage collector not to detect the references. Instead, use an array type that will scan this area conservatively. Using void* is usually the best option as it also ensures proper alignment for pointers being scanned by the GC.

class Singleton { void[] mem; }
void*[(__traits(classInstanceSize, Singleton) - 1) / (void*).sizeof + 1]
singleton_store;
static this()
{
emplace!Singleton(singleton_store).mem = allocateMem();
}
Singleton singleton() { return cast(Singleton)singleton_store.ptr; }

For precise typing of that area, let the compiler generate the class instance into the DATA segment:

class Singleton { void[] mem; }
shared(Singleton) singleton = new Singleton;
shared static this() { singleton.mem = allocateSharedMem(); }

This doesn't work for TLS memory, though.
Parallel marking

By default the garbage collector uses all available CPU cores to mark the heap.

This might affect your application if it has threads that are not suspended during the mark phase of the collection. Configure the number of additional threads used for marking by GC option parallel, e.g. by passing --DRT-gcopt=parallel:2 on the command line or embedding the option into the binary via rt_options. The number of threads actually created is limited to core.cpuid.threadsPerCPU-1. A value of 0 disables parallel marking completely.

Adding your own Garbage Collector

GC implementations are added to a registry that allows to supply more implementations by just linking them into the binary. To do so add a function that is executed before the D runtime initialization using pragma(crt_constructor):

import core.gc.gcinterface, core.gc.registry;
extern (C) pragma(crt_constructor) void registerMyGC()
{
registerGCFactory("mygc", &createMyGC);
}

GC createMyGC()
{
__gshared instance = new MyGC;
instance.initialize();
return instance;
}

class MyGC : GC { /*...*/ }

[The GC modules defining the interface (gc.interface) and registration (gc.registry) are currently not public and are subject to change from version to version. Add an import search path to the druntime/src path to compile the example.]

The new GC is added to the list of available garbage collectors that can be selected via the usual configuration options, e.g. by embedding rt_options into the binary:

extern (C) __gshared string[] rt_options = ["gcopt=gc:mygc"];

The standard GC implementation from a statically linked binary can be removed by redefining the function extern(C) void* register_default_gcs(). If no custom garbage collector has been registered all attempts to allocate GC managed memory will terminate the application with an appropriate message.

References

- Wikipedia

- GC FAQ

- Uniprocessor Garbage Collector Techniques

- Garbage Collection: Algorithms for Automatic Dynamic Memory Management

Floating-Point

Floating-Point Intermediate Values

For floating-point operations and expression intermediate values, a greater precision can be used than the type of the expression. Only the minimum precision is set by the types of the operands, not the maximum. Implementation Note: On Intel x86 machines, for example, it is expected (but not required) that the intermediate calculations be done to the full 80 bits of precision implemented by the hardware.

Execution of floating-point expressions may yield a result of greater precision than dictated by the source.

Floating-Point Constant Folding

Regardless of the type of the operands, floating-point constant folding is done in real or greater precision. It is always done following IEEE-754 rules and round-to-nearest is used.

Floating-point constants are internally represented in the implementation in at least real precision, regardless of the constant's type. The extra precision is available for constant folding. Committing to the precision of the result is done as late as possible in the compilation process. For example:

const float f = 0.2f;
writeln(f - 0.2);

will print 0. A non-const static variable's value cannot be propagated at compile time, so:

static float f = 0.2f;
writeln(f - 0.2);

will print 2.98023e-09. Hex floating-point constants can also be used when specific floating-point bit patterns are needed that are unaffected by rounding. To find the hex value of 0.2f:

import std.stdio;

void main()
{
writefln("%a", 0.2f);
}

which is 0x1.99999ap-3. Using the hex constant:

const float f = 0x1.99999ap-3f;
writeln(f - 0.2);

prints 2.98023e-09.

Different compiler settings, optimization settings, and inlining settings can affect opportunities for constant folding, therefore the results of floating-point calculations may differ depending on those settings.

Rounding Control

IEEE 754 floating-point arithmetic includes the ability to set 4 different rounding modes. These are accessible via the functions in core.stdc.fenv.

If the floating-point rounding mode is changed within a function, it must be restored before the function exits. If this rule is violated (for example, by the use of inline asm), the rounding mode used for subsequent calculations is undefined.

Exception Flags

IEEE 754 floating-point arithmetic can set several flags based on what happened with a computation:

FE_INVALID

FE_DENORMAL

FE_DIVBYZERO

FE_OVERFLOW

FE_UNDERFLOW

FE_INEXACT

These flags can be set/reset via the functions in core.stdc.fenv.

Floating-Point Transformations

An implementation may perform transformations on floating-point computations in order to reduce their strength.

Not all transformations are valid: The following transformations of floating-point expressions are not allowed because under IEEE rules they could produce different results.

Disallowed Floating-Point Transformations

transformation

comments

x + 0 → x

not valid if x is -0

x - 0 → x

not valid if x is ±0 and rounding is towards -∞

-x ↔ 0 - x

not valid if x is +0

x - x → 0

not valid if x is NaN or ±∞

x - y ↔ -(y - x)

not valid because (1-1=+0) whereas -(1-1)=-0

x * 0 → 0

not valid if x is NaN or ±∞

x / c ↔ x * (1/c)

valid if (1/c) yields an exact result

x != x → false

not valid if x is a NaN

x == x → true

not valid if x is a NaN

x !op y ↔ !(x op y)

not valid if x or y is a NaN

Of course, transformations that would alter side effects are also invalid.

D x86 Inline Assembler

D, being a systems programming language, provides an inline assembler. The inline assembler is standardized for D implementations across the same CPU family, for example, the Intel Pentium inline assembler for a Win32 D compiler will be syntax compatible with the inline assembler for Linux running on an Intel Pentium.

Implementations of D on different architectures, however, are free to innovate upon the memory model, function call/return conventions, argument passing conventions, etc.

This document describes the x86 and x86_64 implementations of the inline assembler. The inline assembler platform support that a compiler provides is indicated by the D_InlineAsm_X86 and D_InlineAsm_X86_64 version identifiers, respectively.

Asm statement

AsmStatement:
asm FunctionAttributesopt { AsmInstructionListopt }

AsmInstructionList:
AsmInstruction ;
AsmInstruction ; AsmInstructionList

Assembler instructions must be located inside an asm block. Like functions, asm statements must be anotated with adequate function attributes to be compatible with the caller. Asm statements attributes must be explicitly defined, they are not infered.

void func1() pure nothrow @safe @nogc
{
asm pure nothrow @trusted @nogc
{}
}

void func2() @safe @nogc
{
asm @nogc // Error: asm statement is assumed to be @system - mark it with '@trusted' if it is not
{}
}

Asm instruction

AsmInstruction:
Identifier : AsmInstruction
align IntegerExpression
even
naked
db Operands
ds Operands
di Operands
dl Operands
df Operands
dd Operands
de Operands
db StringLiteral
ds StringLiteral
di StringLiteral
dl StringLiteral
dw StringLiteral
dq StringLiteral
Opcode
Opcode Operands

Opcode:
Identifier

Operands:
Operand
Operand , Operands

Labels

Assembler instructions can be labeled just like other statements. They can be the target of goto statements. For example:

void *pc;
asm
{
call L1          ;
L1:                ;
pop  EBX         ;
mov  pc[EBP],EBX ; // pc now points to code at L1
}

align IntegerExpression

IntegerExpression:
IntegerLiteral
Identifier

Causes the assembler to emit NOP instructions to align the next assembler instruction on an IntegerExpression boundary. IntegerExpression must evaluate at compile time to an integer that is a power of 2.

Aligning the start of a loop body can sometimes have a dramatic effect on the execution speed.

even

Causes the assembler to emit NOP instructions to align the next assembler instruction on an even boundary.

naked

Causes the compiler to not generate the function prolog and epilog sequences. This means such is the responsibility of inline assembly programmer, and is normally used when the entire function is to be written in assembler.

db, ds, di, dl, df, dd, de

These pseudo ops are for inserting raw data directly into the code. db is for bytes, ds is for 16 bit words, di is for 32 bit words, dl is for 64 bit words, df is for 32 bit floats, dd is for 64 bit doubles, and de is for 80 bit extended reals. Each can have multiple operands. If an operand is a string literal, it is as if there were length operands, where length is the number of characters in the string. One character is used per operand. For example:

asm
{
db 5,6,0x83;   // insert bytes 0x05, 0x06, and 0x83 into code
ds 0x1234;     // insert bytes 0x34, 0x12
di 0x1234;     // insert bytes 0x34, 0x12, 0x00, 0x00
dl 0x1234;     // insert bytes 0x34, 0x12, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
df 1.234;      // insert float 1.234
dd 1.234;      // insert double 1.234
de 1.234;      // insert real 1.234
db "abc";      // insert bytes 0x61, 0x62, and 0x63
ds "abc";      // insert bytes 0x61, 0x00, 0x62, 0x00, 0x63, 0x00
}

Opcodes

A list of supported opcodes is at the end.

The following registers are supported. Register names are always in upper case.

Register:
AL
AH
AX
EAX

BL
BH
BX
EBX

CL
CH
CX
ECX

DL
DH
DX
EDX

BP
EBP

SP
ESP

DI
EDI

SI
ESI

ES
CS
SS
DS
GS
FS

CR0
CR2
CR3
CR4

DR0
DR1
DR2
DR3
DR6
DR7

TR3
TR4
TR5
TR6
TR7

ST

ST(0)
ST(1)
ST(2)
ST(3)
ST(4)
ST(5)
ST(6)
ST(7)

MM0
MM1
MM2
MM3
MM4
MM5
MM6
MM7

XMM0
XMM1
XMM2
XMM3
XMM4
XMM5
XMM6
XMM7

x86_64 adds these additional registers.

Register64:
RAX
RBX
RCX
RDX

BPL
RBP

SPL
RSP

DIL
RDI

SIL
RSI

R8B
R8W
R8D
R8

R9B
R9W
R9D
R9

R10B
R10W
R10D
R10

R11B
R11W
R11D
R11

R12B
R12W
R12D
R12

R13B
R13W
R13D
R13

R14B
R14W
R14D
R14

R15B
R15W
R15D
R15

XMM8
XMM9
XMM10
XMM11
XMM12
XMM13
XMM14
XMM15

YMM0
YMM1
YMM2
YMM3
YMM4
YMM5
YMM6
YMM7

YMM8
YMM9
YMM10
YMM11
YMM12
YMM13
YMM14
YMM15

Special Cases

lock, rep, repe, repne, repnz, repz

These prefix instructions do not appear in the same statement as the instructions they prefix; they appear in their own statement. For example:

asm { rep ; movsb ; }
pause

This opcode is not supported by the assembler, instead use

asm
{
rep  ;
nop  ;
}

which produces the same result.

floating point ops

Use the two operand form of the instruction format;

fdiv ST(1);     // wrong
fmul ST;        // wrong
fdiv ST,ST(1);  // right
fmul ST,ST(0);  // right

Operands

Operand:
AsmExp

AsmExp:
AsmLogOrExp
AsmLogOrExp ? AsmExp : AsmExp

AsmLogOrExp:
AsmLogAndExp
AsmLogOrExp || AsmLogAndExp

AsmLogAndExp:
AsmOrExp
AsmLogAndExp && AsmOrExp

AsmOrExp:
AsmXorExp
AsmOrExp | AsmXorExp

AsmXorExp:
AsmAndExp
AsmXorExp ^ AsmAndExp

AsmAndExp:
AsmEqualExp
AsmAndExp & AsmEqualExp

AsmEqualExp:
AsmRelExp
AsmEqualExp == AsmRelExp
AsmEqualExp != AsmRelExp

AsmRelExp:
AsmShiftExp
AsmRelExp < AsmShiftExp
AsmRelExp <= AsmShiftExp
AsmRelExp > AsmShiftExp
AsmRelExp >= AsmShiftExp

AsmShiftExp:
AsmAddExp
AsmShiftExp << AsmAddExp
AsmShiftExp >> AsmAddExp
AsmShiftExp >>> AsmAddExp

AsmAddExp:
AsmMulExp
AsmAddExp + AsmMulExp
AsmAddExp - AsmMulExp

AsmMulExp:
AsmBrExp
AsmMulExp * AsmBrExp
AsmMulExp / AsmBrExp
AsmMulExp % AsmBrExp

AsmBrExp:
AsmUnaExp
AsmBrExp [ AsmExp ]

AsmUnaExp:
AsmTypePrefix AsmExp
offsetof AsmExp
seg AsmExp
+ AsmUnaExp
- AsmUnaExp
! AsmUnaExp
~ AsmUnaExp
AsmPrimaryExp

AsmPrimaryExp:
IntegerLiteral
FloatLiteral
__LOCAL_SIZE
$
Register
Register : AsmExp
Register64
Register64 : AsmExp
DotIdentifier
this

DotIdentifier:
Identifier
Identifier . DotIdentifier
FundamentalType . Identifier

The operand syntax more or less follows the Intel CPU documentation conventions. In particular, the convention is that for two operand instructions the source is the right operand and the destination is the left operand. The syntax differs from that of Intel's in order to be compatible with the D language tokenizer and to simplify parsing.

The seg means load the segment number that the symbol is in. This is not relevant for flat model code. Instead, do a move from the relevant segment register.

A dotted expression is evaluated during the compilation and then must either give a constant or indicate a higher level variable that fits in the target register or variable.

Operand Types

AsmTypePrefix:
near ptr
far ptr
word ptr
dword ptr
qword ptr
FundamentalType ptr

In cases where the operand size is ambiguous, as in:

add [EAX],3     ;

it can be disambiguated by using an AsmTypePrefix:

add  byte ptr [EAX],3 ;
add  int ptr [EAX],7  ;

far ptr is not relevant for flat model code.

Struct/Union/Class Member Offsets

To access members of an aggregate, given a pointer to the aggregate is in a register, use the .offsetof property of the qualified name of the member:

struct Foo { int a,b,c; }
int bar(Foo *f)
{
asm
{
mov EBX,f                   ;
mov EAX,Foo.b.offsetof[EBX] ;
}
}
void main()
{
Foo f = Foo(0, 2, 0);
assert(bar(&f) == 2);
}

Alternatively, inside the scope of an aggregate, only the member name is needed:

struct Foo   // or class
{
int a,b,c;
int bar()
{
asm
{
mov EBX, this   ;
mov EAX, b[EBX] ;
}
}
}
void main()
{
Foo f = Foo(0, 2, 0);
assert(f.bar() == 2);
}

Stack Variables

Stack variables (variables local to a function and allocated on the stack) are accessed via the name of the variable indexed by EBP:

int foo(int x)
{
asm
{
mov EAX,x[EBP] ; // loads value of parameter x into EAX
mov EAX,x      ; // does the same thing
}
}

If the [EBP] is omitted, it is assumed for local variables. If naked is used, this no longer holds.

Special Symbols

$

Represents the program counter of the start of the next instruction. So,

jmp  $  ;

branches to the instruction following the jmp instruction. The $ can only appear as the target of a jmp or call instruction.

__LOCAL_SIZE

This gets replaced by the number of local bytes in the local stack frame. It is most handy when the naked is invoked and a custom stack frame is programmed.

Opcodes Supported

Opcodes

aaa

aad

aam

aas

adc

add

addpd

addps

addsd

addss

and

andnpd

andnps

andpd

andps

arpl

bound

bsf

bsr

bswap

bt

btc

btr

bts

call

cbw

cdq

clc

cld

clflush

cli

clts

cmc

cmova

cmovae

cmovb

cmovbe

cmovc

cmove

cmovg

cmovge

cmovl

cmovle

cmovna

cmovnae

cmovnb

cmovnbe

cmovnc

cmovne

cmovng

cmovnge

cmovnl

cmovnle

cmovno

cmovnp

cmovns

cmovnz

cmovo

cmovp

cmovpe

cmovpo

cmovs

cmovz

cmp

cmppd

cmpps

cmps

cmpsb

cmpsd

cmpss

cmpsw

cmpxchg

cmpxchg8b

cmpxchg16b

comisd

comiss

cpuid

cvtdq2pd

cvtdq2ps

cvtpd2dq

cvtpd2pi

cvtpd2ps

cvtpi2pd

cvtpi2ps

cvtps2dq

cvtps2pd

cvtps2pi

cvtsd2si

cvtsd2ss

cvtsi2sd

cvtsi2ss

cvtss2sd

cvtss2si

cvttpd2dq

cvttpd2pi

cvttps2dq

cvttps2pi

cvttsd2si

cvttss2si

cwd

cwde

da

daa

das

db

dd

de

dec

df

di

div

divpd

divps

divsd

divss

dl

dq

ds

dt

dw

emms

enter

f2xm1

fabs

fadd

faddp

fbld

fbstp

fchs

fclex

fcmovb

fcmovbe

fcmove

fcmovnb

fcmovnbe

fcmovne

fcmovnu

fcmovu

fcom

fcomi

fcomip

fcomp

fcompp

fcos

fdecstp

fdisi

fdiv

fdivp

fdivr

fdivrp

feni

ffree

fiadd

ficom

ficomp

fidiv

fidivr

fild

fimul

fincstp

finit

fist

fistp

fisub

fisubr

fld

fld1

fldcw

fldenv

fldl2e

fldl2t

fldlg2

fldln2

fldpi

fldz

fmul

fmulp

fnclex

fndisi

fneni

fninit

fnop

fnsave

fnstcw

fnstenv

fnstsw

fpatan

fprem

fprem1

fptan

frndint

frstor

fsave

fscale

fsetpm

fsin

fsincos

fsqrt

fst

fstcw

fstenv

fstp

fstsw

fsub

fsubp

fsubr

fsubrp

ftst

fucom

fucomi

fucomip

fucomp

fucompp

fwait

fxam

fxch

fxrstor

fxsave

fxtract

fyl2x

fyl2xp1

hlt

idiv

imul

in

inc

ins

insb

insd

insw

int

into

invd

invlpg

iret

iretd

iretq

ja

jae

jb

jbe

jc

jcxz

je

jecxz

jg

jge

jl

jle

jmp

jna

jnae

jnb

jnbe

jnc

jne

jng

jnge

jnl

jnle

jno

jnp

jns

jnz

jo

jp

jpe

jpo

js

jz

lahf

lar

ldmxcsr

lds

lea

leave

les

lfence

lfs

lgdt

lgs

lidt

lldt

lmsw

lock

lods

lodsb

lodsd

lodsw

loop

loope

loopne

loopnz

loopz

lsl

lss

ltr

maskmovdqu

maskmovq

maxpd

maxps

maxsd

maxss

mfence

minpd

minps

minsd

minss

mov

movapd

movaps

movd

movdq2q

movdqa

movdqu

movhlps

movhpd

movhps

movlhps

movlpd

movlps

movmskpd

movmskps

movntdq

movnti

movntpd

movntps

movntq

movq

movq2dq

movs

movsb

movsd

movss

movsw

movsx

movupd

movups

movzx

mul

mulpd

mulps

mulsd

mulss

neg

nop

not

or

orpd

orps

out

outs

outsb

outsd

outsw

packssdw

packsswb

packuswb

paddb

paddd

paddq

paddsb

paddsw

paddusb

paddusw

paddw

pand

pandn

pavgb

pavgw

pcmpeqb

pcmpeqd

pcmpeqw

pcmpgtb

pcmpgtd

pcmpgtw

pextrw

pinsrw

pmaddwd

pmaxsw

pmaxub

pminsw

pminub

pmovmskb

pmulhuw

pmulhw

pmullw

pmuludq

pop

popa

popad

popf

popfd

por

prefetchnta

prefetcht0

prefetcht1

prefetcht2

psadbw

pshufd

pshufhw

pshuflw

pshufw

pslld

pslldq

psllq

psllw

psrad

psraw

psrld

psrldq

psrlq

psrlw

psubb

psubd

psubq

psubsb

psubsw

psubusb

psubusw

psubw

punpckhbw

punpckhdq

punpckhqdq

punpckhwd

punpcklbw

punpckldq

punpcklqdq

punpcklwd

push

pusha

pushad

pushf

pushfd

pxor

rcl

rcpps

rcpss

rcr

rdmsr

rdpmc

rdtsc

rep

repe

repne

repnz

repz

ret

retf

rol

ror

rsm

rsqrtps

rsqrtss

sahf

sal

sar

sbb

scas

scasb

scasd

scasw

seta

setae

setb

setbe

setc

sete

setg

setge

setl

setle

setna

setnae

setnb

setnbe

setnc

setne

setng

setnge

setnl

setnle

setno

setnp

setns

setnz

seto

setp

setpe

setpo

sets

setz

sfence

sgdt

shl

shld

shr

shrd

shufpd

shufps

sidt

sldt

smsw

sqrtpd

sqrtps

sqrtsd

sqrtss

stc

std

sti

stmxcsr

stos

stosb

stosd

stosw

str

sub

subpd

subps

subsd

subss

syscall

sysenter

sysexit

sysret

test

ucomisd

ucomiss

ud2

unpckhpd

unpckhps

unpcklpd

unpcklps

verr

verw

wait

wbinvd

wrmsr

xadd

xchg

xlat

xlatb

xor

xorpd

xorps

Pentium 4 (Prescott) Opcodes Supported

Pentium 4 Opcodes

addsubpd

addsubps

fisttp

haddpd

haddps

hsubpd

hsubps

lddqu

monitor

movddup

movshdup

movsldup

mwait

AMD Opcodes Supported

AMD Opcodes

pavgusb

pf2id

pfacc

pfadd

pfcmpeq

pfcmpge

pfcmpgt

pfmax

pfmin

pfmul

pfnacc

pfpnacc

pfrcp

pfrcpit1

pfrcpit2

pfrsqit1

pfrsqrt

pfsub

pfsubr

pi2fd

pmulhrw

pswapd

SIMD

SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX are supported.

GCC syntax

The GNU D Compiler uses an alternative, GCC-based syntax for inline assembler:

GccAsmStatement:
asm FunctionAttributesopt { GccAsmInstructionList }

GccAsmInstructionList:
GccAsmInstruction ;
GccAsmInstruction ; GccAsmInstructionList

GccAsmInstruction:
GccBasicAsmInstruction
GccExtAsmInstruction
GccGotoAsmInstruction

GccBasicAsmInstruction:
AssignExpression

GccExtAsmInstruction:
AssignExpression : GccAsmOperandsopt
AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt
AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt : GccAsmClobbersopt

GccGotoAsmInstruction:
AssignExpression : : GccAsmOperandsopt : GccAsmClobbersopt : GccAsmGotoLabelsopt

GccAsmOperands:
GccSymbolicNameopt StringLiteral ( AssignExpression )
GccSymbolicNameopt StringLiteral ( AssignExpression ) , GccAsmOperands

GccSymbolicName:
[ Identifier ]

GccAsmClobbers:
StringLiteral
StringLiteral , GccAsmClobbers

GccAsmGotoLabels:
Identifier
Identifier , GccAsmGotoLabels

Embedded Documentation

The D programming language enables embedding both contracts and test code along side the actual code, which helps to keep them all consistent with each other. One thing lacking is the documentation, as ordinary comments are usually unsuitable for automated extraction and formatting into manual pages. Embedding the user documentation into the source code has important advantages, such as not having to write the documentation twice, and the likelihood of the documentation staying consistent with the code.

Some existing approaches to this are:

- Doxygen which already has some support for D

- Java's Javadoc, probably the most well-known

- C#'s embedded XML

- Other documentation tools

D's goals for embedded documentation are:

1. It looks good as embedded documentation, not just after it is extracted and processed.

2. It's easy and natural to write, i.e. minimal reliance on <tags> and other clumsy forms one would never see in a finished document.

3. It does not repeat information that the compiler already knows from parsing the code.

4. It doesn't rely on embedded HTML, as such will impede extraction and formatting for other purposes.

5. It's based on existing D comment forms, so it is completely independent of parsers only interested in D code.

6. It should look and feel different from code, so it won't be visually confused with code.

7. It should be possible for the user to use Doxygen or other documentation extractor if desired.

Specification

The specification for the form of embedded documentation comments only specifies how information is to be presented to the compiler. It is implementation-defined how that information is used and the form of the final presentation. Whether the final presentation form is an HTML web page, a man page, a PDF file, etc. is not specified as part of the D Programming Language.

Phases of Processing

Embedded documentation comments are processed in a series of phases:

1. Lexical - documentation comments are identified and attached to tokens.

2. Parsing - documentation comments are associated with specific declarations and combined.

3. Sections - each documentation comment is divided up into a sequence of sections.

4. Special sections are processed.

5. Highlighting of non-special sections is done.

6. All sections for the module are combined.

7. Macro and Escape text substitution is performed to produce the final result.

Lexical

Embedded documentation comments are one of the following forms:

1. /** ... */ The two *'s after the opening /

2. /++ ... +/ The two +'s after the opening /

3. /// The three slashes

The following are all embedded documentation comments:

/// This is a one line documentation comment.

/** So is this. */

/++ And this. +/

/**
This is a brief documentation comment.
*/

/**
* The leading * on this line is not part of the documentation comment.
*/

/*********************************
The extra *'s immediately following the /** are not
part of the documentation comment.
*/

/++
This is a brief documentation comment.
+/

/++
+ The leading + on this line is not part of the documentation comment.
+/

/+++++++++++++++++++++++++++++++++
The extra +'s immediately following the / ++ are not
part of the documentation comment.
+/

/**************** Closing *'s are not part *****************/

The extra *'s and +'s on the comment opening, closing and left margin are ignored and are not part of the embedded documentation. Comments not following one of those forms are not documentation comments.

Parsing

Each documentation comment is associated with a declaration. If the documentation comment is on a line by itself or with only whitespace to the left, it refers to the next declaration. Multiple documentation comments applying to the same declaration are concatenated. Documentation comments not associated with a declaration are ignored. Documentation comments preceding the ModuleDeclaration apply to the entire module. If the documentation comment appears on the same line to the right of a declaration, it applies to that.

If a documentation comment for a declaration consists only of the identifier ditto then the documentation comment for the previous declaration at the same declaration scope is applied to this declaration as well.

If there is no documentation comment for a declaration, that declaration may not appear in the output. To ensure it does appear in the output, put an empty declaration comment for it.

int a;  /// documentation for a; b has no documentation
int b;

/** documentation for c and d */
/** more documentation for c and d */
int c;
/** ditto */
int d;

/** documentation for e and f */ int e;
int f;  /// ditto

/** documentation for g */
int g; /// more documentation for g

/// documentation for C and D
class C
{
int x; /// documentation for C.x

/** documentation for C.y and C.z */
int y;
int z; /// ditto
}

/// ditto
class D { }

Sections

The document comment is a series of Sections. A Section is a name that is the first non-blank character on a line immediately followed by a ':'. This name forms the section name. The section name is not case-sensitive.

Section names starting with 'http://' or 'https://' are not recognized as section names.

Summary

The first section is the Summary, and does not have a section name. It is first paragraph, up to a blank line or a section name. While the summary can be any length, try to keep it to one line. The Summary section is optional.

Description

The next unnamed section is the Description. It consists of all the paragraphs following the Summary until a section name is encountered or the end of the comment.

While the Description section is optional, there cannot be a Description without a Summary section.

/***********************************
* Brief summary of what
* myfunc does, forming the summary section.
*
* First paragraph of synopsis description.
*
* Second paragraph of
* synopsis description.
*/

void myfunc() { }

Named sections follow the Summary and Description unnamed sections.

Standard Sections

For consistency and predictability, there are several standard sections. None of these are required to be present.

Authors:

Lists the author(s) of the declaration.

/** * Authors: Melvin D. Nerd, melvin@mailinator.com */
Bugs:

Lists any known bugs.

/** * Bugs: Doesn't work for negative values. */
Date:

Specifies the date of the current revision. The date should be in a form parseable by std.date.

/** * Date: March 14, 2003 */
Deprecated:

Provides an explanation for and corrective action to take if the associated declaration is marked as deprecated.

/** * Deprecated: superseded by function bar(). */ deprecated void foo() { ... }
Examples:

Any usage examples

/** * Examples: * -------------------- * writeln("3"); // writes '3' to stdout * -------------------- */
History:

Revision history.

/** * History: * V1 is initial version * * V2 added feature X */
License:

Any license information for copyrighted code.

/** * License: use freely for any purpose */ void bar() { ... }
Returns:

Explains the return value of the function. If the function returns void, don't redundantly document it.

/** * Read the file. * Returns: The contents of the file. */ void[] readFile(const(char)[] filename) { ... }
See_Also:

List of other symbols and URLs to related items.

/** * See_Also: * foo, bar, http://www.digitalmars.com/d/phobos/index.html */
Standards:

If this declaration is compliant with any particular standard, the description of it goes here.

/** * Standards: Conforms to DSPEC-1234 */
Throws:

Lists exceptions thrown and under what circumstances they are thrown.

/** * Write the file. * Throws: WriteException on failure. */ void writeFile(string filename) { ... }
Version:

Specifies the current version of the declaration.

/** * Version: 1.6a */

Special Sections

Some sections have specialized meanings and syntax.

Copyright:

This contains the copyright notice. The macro COPYRIGHT is set to the contents of the section when it documents the module declaration. The copyright section only gets this special treatment when it is for the module declaration.

/** Copyright: Public Domain */ module foo;
Params:

Function parameters can be documented by listing them in a params section. Each line that starts with an identifier followed by an '=' starts a new parameter description. A description can span multiple lines.

/*********************************** * foo does this. * Params: * x = is for this * and not for that * y = is for that */ void foo(int x, int y) { }
Macros:

The macros section follows the same syntax as the Params: section. It's a series of NAME=value pairs. The NAME is the macro name, and value is the replacement text.

/** * Macros: * FOO = now is the time for * all good men * BAR = bar * MAGENTA = <font color="magenta">$0</font> */
Escapes=

The escapes section is a series of substitutions which replace special characters with a string. It's useful when the output format requires escaping of certain characters, for example in HTML & should be escaped with &amp;.

The syntax is /c/string/, where c is either a single character, or multiple characters separated by whitespace or commas, and string is the replacement text.

/** * ESCAPES = /&/AddressOf!/ * /!/Exclamation/ * /?/QuestionMark/ * /,/Comma/ * /{ }/Parens/ * /<,>/Arrows/ */

Highlighting

Embedded Comments

The documentation comments can themselves be commented using the $(DDOC_COMMENT comment text) syntax. These comments do not nest.

Embedded Code

D code can be embedded using lines beginning with at least three hyphens -, backticks ` or tildes ~ (ignoring whitespace) to delineate the code section:

/++
+ Our function.
+
+ Example:
+ ---
+ import std.stdio;
+
+ void foo()
+ {
+     writeln("foo!");  /* print the string */
+ }
+ ---
+/

Note that in the above example the documentation comment uses the /++ ... +/ form so that /* ... */ can be used inside the code section.

D code gets automatic syntax highlighting. To include code in another language without syntax highlighting, add a language string at the end of the top delimiter line:

/++
+ Some C++
+ ``` cpp
+ #include <iostream>
+
+ void foo()
+ {
+     std::cout << "foo!";
+ }
+ ```
+/

Inline Code

Inline code can be written between backtick characters (`), similarly to the syntax used on GitHub, Reddit, Stack Overflow, and other websites. Both the opening and closing ` character must appear on the same line to trigger this behavior.

Text inside these sections will be escaped according to the rules described above, then wrapped in a $(DDOC_BACKQUOTED) macro. By default, this macro expands to be displayed as an inline text span, formatted as code.

A literal backtick character can be output either as a non-paired ` on a single line or by using the $(BACKTICK) macro.

/// Returns `true` if `a == b`.
void foo() {}

/// Backquoted `<html>` will be displayed to the user instead
/// of passed through as embedded HTML (see below).
void bar() {}

Embedded HTML

HTML can be embedded into the documentation comments, and it will be passed through to the HTML output unchanged. However, since it is not necessarily true that HTML will be the desired output format of the embedded documentation comment extractor, it is best to avoid using it where practical.

/**
* Example of embedded HTML:
*
* <ol>
*   <li><a href="http://www.digitalmars.com">Digital Mars</a></li>
*   <li><a href="http://www.classicempire.com">Empire</a></li>
* </ol>
*/

Headings

A long documentation section can be subdivided by adding headings. A heading is a line of text that starts with one to six # characters followed by whitespace and then the heading text. The number of # characters determines the heading level. Headings may optionally end with any number of trailing # characters.

/**
* # H1
* ## H2
* ### H3
* #### H4 ###
* ##### H5 ##
* ###### H6 #
*/

Links

Documentation may link to other documentation or to a URL. There are four styles of links:

/**
* Some links:
*
* 1. A [reference link][ref] and bare reference links: [ref] or [Object]
* 2. An [inline link](https://dlang.org)
* 3. A bare URL: https://dlang.org
* 4. An ![image](https://dlang.org/images/d3.png)
*
* [ref]: https://dlang.org "The D Language Website"
*/

Reference Links

Reference-style links enclose a reference label in square brackets. They may optionally be preceded by some link text, also enclosed in square brackets.

The reference label must match a reference defined elsewhere. This may be a D symbol in scope of the source code being documented, like [Object] in the example above, or it may be an explicit reference that is defined in the same documentation comment, like [ref] in the example above. In the example both instances of [ref] in item 1. will be replaced with the URL and title text from the matching definition at the bottom of the example. The first link will read reference link and the second will read ref.

Reference definitions start with a label in square brackets, followed by a colon, a URL and an optional title wrapped in single or double quotes, or in parentheses. If a reference label would match both a D symbol and a reference definition then the reference definition is used.

The generated links to D symbols are relative if they have the same root package as the module being documented. If not, their URLs are preceded by a $(DDOC_ROOT_pkg) macro, where pkg is the root package of the symbol being linked to. Links to D symbols are generated with a $(DOC_EXTENSION) macro after the module name. Then the generated URL for [Object] in the above example is as if it had been written:

$(DOC_ROOT_object)object$(DOC_EXTENSION)#.Object

DOC_ROOT_ macros can be defined for any external packages to link to using a Macros section.

Inline Links

Inline-style links enclose link text in square brackets and the link URL in parentheses. Like reference links, the URL may optionally be followed by title text wrapped in single or double quotes, or in parentheses:

/// [a link with title text](https://dlang.org 'Some title text')

Bare URLs

Bare URLs are sequences of characters that start with http:// or https://, continue with one or more characters from the set of letters, digits and -_?=%&/+#~., and contain at least one period. URL recognition happens before all macro text substitution. The URL is wrapped in a $(DDOC_LINK_AUTODETECT) macro and is otherwise left untouched.

Images

Images have the same form as reference or inline links, but add an exclamation point ! before the initial square bracket. What would be the link text in a normal link is used as the image's alt text.

Lists

Documentation may contain lists. Start an ordered list with a number followed by a period:

/**
* 1. First this
* 2. Then this
*    1. A sub-item
*/

Start an unordered list with a hyphen (-), an asterisk (*) or a plus (+). Subsequent items in the same list must also start with the same symbol:

/**
* - A list
* - With a second item
*
* + A different list
*   - With a sub-item
*
* * A third list (note the double asterisks)
*/

Note the double asterisks in the example above. This is because the list is inside a documentation comment that is delimited with asterisks, so the initial asterisk is considered part of the documentation comment, not a list item. This is even true when other lines don't start with an asterisk:

/**
- A list
* Not a list because the asterisk is part of the documentation comment
*/

/++
+ + The caveat also applies to plus-delimited documentation comments
+/

List items can include content like new paragraphs, headings, embedded code, or child list items. Simply indent the content to match the indent of the text after the list symbol:

/**
* - A parent list item
*
*   With a second paragraph
*
*   - A sub-item
*     ---
*     // A code example inside the sub-item
*     ---
*/

Tables

Data may be placed into a table. Tables consist of a single header row, a delimiter row, and zero or more data rows. Cells in each row are separated by pipe (|) characters. Initial and trailing |'s are optional. The number of cells in the delimiter row must match the number of cells in the header row:

/**
*  | Item | Price |
*  | ---- | ----: |
*  | Wigs | $10 |
*    Wheels | $13
*  | Widgets | $200 |
*/

Cells in the delimiter row contain hyphens (-) and optional colons (:). A : to the left of the hyphens creates a left-aligned column, a : to the right of the hyphens creates a right-aligned column (like the example above), and :'s on both sides of the hyphens create a center-aligned column.

Quotes

Documentation may include a section of quoted material by prefixing each line of the section with a >. Quotes may include headings, lists, embedded code, etc.

/**
* > To D, or not to D. -- Willeam NerdSpeare
*/

Lines of text that directly follow a quoted line are considered part of the quote:

/**
* > This line
* and this line are both part of the quote
*
* This line is not part of the quote.
*/

Horizontal Rules

Create a horizontal rule by adding a line containing three or more asterisks, underscores or hyphens:

/**
* ***
* ___
*/

As with lists, note that the initial * in the example above will be stripped because it is part of a documentation comment that is delimited with asterisks. At least three subsequent asterisks are needed.

To create a horizontal rule with hyphens, add spaces between the hyphens. Without the spaces they would be treated as the start or end of an embedded code block. Note that any horizontal rule may contain spaces:

/**
* - - -
* _ _ _
* * * *
*/

Text Emphasis

A span of text wrapped in asterisks (*) is emphasized, and text wrapped in two asterisks (**) is strongly emphasized:

*single asterisks* is rendered as single asterisks.

**double asterisks** is rendered as double asterisks.

Insert a literal asterisk by backslash-escaping it: \* is rendered as *.

Unlike Markdown, underscores (_) are not supported for emphasizing text because it would break snake_case names and underscore prefix processing in identifier emphasis.

Identifier Emphasis

Identifiers in documentation comments that are function parameters or are names that are in scope at the associated declaration are emphasized in the output. This emphasis can take the form of italics, boldface, a hyperlink, etc. How it is emphasized depends on what it is  —  a function parameter, type, D keyword, etc. To prevent unintended emphasis of an identifier, it can be preceded by an underscore (_). The underscore will be stripped from the output.

Character Entities

Some characters have special meaning to the documentation processor, to avoid confusion it can be best to replace them with their corresponding character entities:

Characters and Entities

Character

Entity

<

&lt;

>

&gt;

&

&amp;

It is not necessary to do this inside a code section, or if the special character is not immediately followed by a # or a letter.

Punctuation Escapes

Escape any ASCII punctuation symbol with a backslash \. Doing so outputs the original character without the backslash, except for the following characters which output predefined macros instead:

Characters and Escape Macros

Character

Macro

(

$(LPAREN)

)

$(RPAREN)

,

$(COMMA)

$

$(DOLLAR)

To output a backslash, simply use two backslashes in a row: \\. Note that backslashes inside embedded or inline code do not escape punctuation and are included in the output as-is. Backslashes before non-punctation are also included in the output as-is. For example, C:\dmd2\bin\dmd.exe does not require escaping its embedded backslashes.

No Documentation

No documentation is generated for the following constructs, even if they have a documentation comment:

- Invariants

- Postblits

- Destructors

- Static constructors and static destructors

- Class info, type info, and module info

Macros

The documentation comment processor includes a simple macro text preprocessor. When a $(NAME) appears in section text it is replaced with NAMEs corresponding replacement text.
For example:

/**
Macros:
PARAM = <u>$1</u>
MATH_DOCS = <a href="https://dlang.org/phobos/std_math.html">Math Docs</a>
*/
module math;

/**
* This function returns the sum of $(PARAM a) and $(PARAM b).
* See also the $(MATH_DOCS).
*/
int sum(int a, int b) { return a + b; }

The above would generate the following output:

<h1>test</h1>
<dl><dt><big><a name="sum"></a>int <u>sum</u>(int <i>a</i>, int <i>b</i>);
</big></dt>
<dd>This function returns the <u>sum</u> of <u><i>a</i></u> and <u><i>b</i></u>.
See also the <a href="https://dlang.org/phobos/std_math.html">Math Docs</a>.
</dd>
</dl>

The replacement text is recursively scanned for more macros. If a macro is recursively encountered, with no argument or with the same argument text as the enclosing macro, it is replaced with no text. Macro invocations that cut across replacement text boundaries are not expanded. If the macro name is undefined, the replacement text has no characters in it. If a $(NAME) is desired to exist in the output without being macro expanded, the $ should be backslash-escaped: \$.

Macros can have arguments. Any text from the end of the identifier to the closing ‘)’ is the $0 argument. A $0 in the replacement text is replaced with the argument text. If there are commas in the argument text, $1 will represent the argument text up to the first comma, $2 from the first comma to the second comma, etc., up to $9. $+ represents the text from the first comma to the closing ‘)’. The argument text can contain nested parentheses, "" or '' strings, <!-- ... --> comments, or tags. If stray, unnested parentheses are used, they can be backslash-escaped: \( or \).

Macro definitions come from the following sources, in the specified order:

1. Predefined macros.

2. Definitions from file specified by sc.ini's or dmd.conf DDOCFILE setting.

3. Definitions from *.ddoc files specified on the command line.

4. Runtime definitions generated by Ddoc.

5. Definitions from any Macros: sections.

Macro redefinitions replace previous definitions of the same name. This means that the sequence of macro definitions from the various sources forms a hierarchy.

Macro names beginning with "D_" and "DDOC_" are reserved.

Predefined Macros

A number of macros are predefined Ddoc, and represent the minimal definitions needed by Ddoc to format and highlight the presentation. The definitions are for simple HTML.

The implementations of all predefined macros are implementation-defined. The reference implementation's macro definitions can be found here.

Ddoc does not generate HTML code. It formats into the basic formatting macros, which (in their predefined form) are then expanded into HTML. If output other than HTML is desired, then these macros need to be redefined.

Predefined Formatting Macros

Name

Description

B

boldface the argument

I

italicize the argument

U

underline the argument

P

argument is a paragraph

DL

argument is a definition list

DT

argument is a definition in a definition list

DD

argument is a description of a definition

TABLE

argument is a table

TR

argument is a row in a table

TH

argument is a header entry in a row

TD

argument is a data entry in a row

OL

argument is an ordered list

UL

argument is an unordered list

LI

argument is an item in a list

BIG

argument is one font size bigger

SMALL

argument is one font size smaller

BR

start new line

LINK

generate clickable link on argument

LINK2

generate clickable link, first arg is address

RED

argument is set to be red

BLUE

argument is set to be blue

GREEN

argument is set to be green

YELLOW

argument is set to be yellow

BLACK

argument is set to be black

WHITE

argument is set to be white

D_CODE

argument is D code

D_INLINECODE

argument is inline D code

LF

Insert a line feed (newline)

LPAREN

Insert a left parenthesis

RPAREN

Insert a right parenthesis

BACKTICK

Insert a backtick

DOLLAR

Insert a dollar sign

DDOC

overall template for output

DDOC is special in that it specifies the boilerplate into which the entire generated text is inserted (represented by the Ddoc generated macro BODY). For example, in order to use a style sheet, DDOC would be redefined as:

DDOC = <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html><head>
<META http-equiv="content-type" content="text/html; charset=utf-8">
<title>$(TITLE)</title>
<link rel="stylesheet" type="text/css" href="style.css">
</head><body>
<h1>$(TITLE)</h1>
$(BODY)
</body></html>

Highlighting of D code is performed by the following macros:

D Code Formatting Macros

Name

Description

D_COMMENT

Highlighting of comments

D_STRING

Highlighting of string literals

D_KEYWORD

Highlighting of D keywords

D_PSYMBOL

Highlighting of current declaration name

D_PARAM

Highlighting of current function declaration parameters

The highlighting macros start with DDOC_. They control the formatting of individual parts of the presentation.

Ddoc Section Formatting Macros

Name

Description

DDOC_CONSTRAINT

Highlighting of a template constraint.

DDOC_COMMENT

Inserts a comment in the output.

DDOC_DECL

Highlighting of the declaration.

DDOC_DECL_DD

Highlighting of the description of a declaration.

DDOC_DITTO

Highlighting of ditto declarations.

DDOC_SECTIONS

Highlighting of all the sections.

DDOC_SUMMARY

Highlighting of the summary section.

DDOC_DESCRIPTION

Highlighting of the description section.

DDOC_AUTHORS

Highlighting of the authors section.

DDOC_BUGS

Highlighting of the bugs section.

DDOC_COPYRIGHT

Highlighting of the copyright section.

DDOC_DATE

Highlighting of the date section.

DDOC_DEPRECATED

Highlighting of the deprecated section.

DEPRECATED

Wrapper for deprecated declarations.

DDOC_EXAMPLES

Highlighting of the examples section.

DDOC_HISTORY

Highlighting of the history section.

DDOC_LICENSE

Highlighting of the license section.

DDOC_OVERLOAD_SEPARATOR

Inserts a separator between overloads of a given name.

DDOC_RETURNS

Highlighting of the returns section.

DDOC_SEE_ALSO

Highlighting of the see-also section.

DDOC_STANDARDS

Highlighting of the standards section.

DDOC_THROWS

Highlighting of the throws section.

DDOC_VERSION

Highlighting of the version section.

DDOC_SECTION_H

Highlighting of the section name of a non-standard section.

DDOC_SECTION

Highlighting of the contents of a non-standard section.

DDOC_MEMBERS

Default highlighting of all the members of a class, struct, etc.

DDOC_MODULE_MEMBERS

Highlighting of all the members of a module.

DDOC_CLASS_MEMBERS

Highlighting of all the members of a class.

DDOC_STRUCT_MEMBERS

Highlighting of all the members of a struct.

DDOC_ENUM_MEMBERS

Highlighting of all the members of an enum.

DDOC_TEMPLATE_PARAM

Highlighting of a template's individual parameters.

DDOC_TEMPLATE_PARAM_LIST

Highlighting of a template's parameter list.

DDOC_TEMPLATE_MEMBERS

Highlighting of all the members of a template.

DDOC_ENUM_BASETYPE

Highlighting of the type an enum is based upon

DDOC_PARAMS

Highlighting of a function parameter section.

DDOC_PARAM_ROW

Highlighting of a name=value function parameter.

DDOC_PARAM_ID

Highlighting of the parameter name.

DDOC_PARAM_DESC

Highlighting of the parameter value.

DDOC_BLANKLINE

Inserts a blank line.

DDOC_ANCHOR

Expands to a named anchor used for hyperlinking to a particular declaration section. Argument $1 expands to the qualified declaration name.

DDOC_PSYMBOL

Highlighting of declaration name to which a particular section is referring.

DDOC_PSUPER_SYMBOL

Highlighting of the base type of a class.

DDOC_KEYWORD

Highlighting of D keywords.

DDOC_PARAM

Highlighting of function parameters.

DDOC_BACKQUOTED

Inserts inline code.

DDOC_AUTO_PSYMBOL_SUPPRESS

Highlighting of auto-detected symbol that starts with underscore

DDOC_AUTO_PSYMBOL

Highlighting of auto-detected symbol

DDOC_AUTO_KEYWORD

Highlighting of auto-detected keywords

DDOC_AUTO_PARAM

Highlighting of auto-detected parameters

For example, one could redefine DDOC_SUMMARY:

DDOC_SUMMARY = $(GREEN $0)

And all the summary sections will now be green.

Macro Definitions from sc.ini's DDOCFILE

A text file of macro definitions can be created, and specified in sc.ini:

DDOCFILE=myproject.ddoc

Macro Definitions from .ddoc Files on the Command Line

File names on the DMD command line with the extension .ddoc are text files that are read and processed in order.

Macro Definitions Generated by Ddoc

Generated Macro Definitions

Macro Name

Content

BODY

Set to the generated document text.

TITLE

Set to the module name.

DATETIME

Set to the current date and time.

YEAR

Set to the current year.

COPYRIGHT

Set to the contents of any Copyright: section that is part of the module comment.

DOCFILENAME

Set to the name of the generated output file.

SRCFILENAME

Set to the name of the source file the documentation is being generated from.

Using Ddoc to generate examples from unit tests

Ddoc can automatically generate usage examples for declarations using unit tests. If a declaration is followed by a documented unit test, the code from the test will be inserted into the example section of the declaration. This avoids the frequent problem of having outdated documentation for pieces of code.

To create a documented unit test just add three forward slashes before the unittest block, like this:

///
unittest
{
...
}

For more information please see the full section on documented unit tests.

Using Ddoc for other Documentation

Ddoc is primarily designed for use in producing documentation from embedded comments. It can also, however, be used for processing other general documentation. The reason for doing this would be to take advantage of the macro capability of Ddoc and the D code syntax highlighting capability.

If the .d source file starts with the string "Ddoc" then it is treated as general purpose documentation, not as a D code source file. From immediately after the "Ddoc" string to the end of the file or any "Macros:" section forms the document. No automatic highlighting is done to that text, other than highlighting of D code embedded between lines delineated with --- lines. Only macro processing is done.

Much of the D documentation itself is generated this way, including this page. Such documentation is marked at the bottom as being generated by Ddoc.

Security considerations

Note that DDoc comments may embed raw HTML, including <script> tags. Be careful when publishing or distributing rendered DDoc HTML generated from untrusted sources, as this may allow cross-site scripting.

Links to D documentation generators

A list of current D documentation generators which use Ddoc can be found on our wiki page.

Interfacing to C

D is designed to fit comfortably with a C compiler for the target system. D makes up for not having its own VM by relying on the target environment's C runtime library. It would be senseless to attempt to port to D or write D wrappers for the vast array of C APIs available. How much easier it is to just call them directly.

This is done by matching the C compiler's data types, layouts, and function call/return sequences.

Calling C Functions

C functions can be called directly from D. There is no need for wrapper functions, argument swizzling, and the C functions do not need to be put into a separate DLL.

The C function must be declared and given a calling convention, most likely the "C" calling convention, for example:

extern (C) int strcmp(const char* string1, const char* string2);

and then it can be called within D code in the obvious way:

import std.string;
int myDfunction(char[] s)
{
return strcmp(std.string.toStringz(s), "foo");
}

There are several things going on here:

- D understands how C function names are "mangled" and the correct C function call/return sequence.

- C functions cannot be overloaded with another C function with the same name.

- There are no __cdecl, __far, __stdcall, __declspec, or other such C extended type modifiers in D. These are handled by linkage attributes, such as extern (C).

- There is no volatile type modifier in D. To declare a C function that uses volatile, just drop the keyword from the declaration.

- Strings are not 0 terminated in D. See "Data Type Compatibility" for more information about this. However, string literals in D are 0 terminated.

C code can correspondingly call D functions, if the D functions use an attribute that is compatible with the C compiler, most likely the extern (C):

// myfunc() can be called from any C function
extern (C)
{
void myfunc(int a, int b)
{
...
}
}

Storage Allocation

C code explicitly manages memory with calls to malloc() and free(). D allocates memory using the D garbage collector, so no explicit frees are necessary.

D can still explicitly allocate memory using core.stdc.stdlib.malloc() and core.stdc.stdlib.free(), these are useful for connecting to C functions that expect malloc'd buffers, etc.

If pointers to D garbage collector allocated memory are passed to C functions, it's critical to ensure that that memory will not be collected by the garbage collector before the C function is done with it. This is accomplished by:

- Making a copy of the data using core.stdc.stdlib.malloc() and passing the copy instead.

- Leaving a pointer to it on the stack (as a parameter or automatic variable), as the garbage collector will scan the stack.

- Leaving a pointer to it in the static data segment, as the garbage collector will scan the static data segment.

- Registering the pointer with the garbage collector with the std.gc.addRoot() or std.gc.addRange() calls.

An interior pointer to the allocated memory block is sufficient to let the GC know the object is in use; i.e. it is not necessary to maintain a pointer to the beginning of the allocated memory.

The garbage collector does not scan the stacks of threads not created by the D Thread interface. Nor does it scan the data segments of other DLLs, etc.

Data Type Compatibility

D And C Type Equivalence

D

C

32 bit

64 bit

void

void

byte

signed char

ubyte

unsigned char

char

char (chars are unsigned in D)

wchar

wchar_t (when sizeof(wchar_t) is 2)

dchar

wchar_t (when sizeof(wchar_t) is 4)

short

short

ushort

unsigned short

int

int

uint

unsigned

ulong

unsigned long long

unsigned long

core.stdc.config.c_long

long

long

core.stdc.config.c_ulong

unsigned long

unsigned long

long

long long

long (or long long)

ulong

unsigned long long

unsigned long (or unsigned long long)

float

float

double

double

real

long double

cdouble

double _Complex

creal

long double _Complex

struct

struct

union

union

enum

enum

class

no equivalent

type *

type *

type[dim]

type[dim]

type[dim], type()[dim]

type[dim], type()[dim]

type[]

no equivalent

type1[type2]

no equivalent

type function(params)

type(*)(params)

type delegate(params)

no equivalent

size_t

size_t

ptrdiff_t

ptrdiff_t

These equivalents hold for most C compilers. The C standard does not pin down the sizes of the types, so some care is needed.

Passing D Array Arguments to C Functions

In C, arrays are passed to functions as pointers even if the function prototype says its an array. In D, static arrays are passed by value, not by reference. Thus, the function prototype must be adjusted to match what C expects.

D And C Function Prototype Equivalence

D type

C type

T*

T[]

ref T[dim]

T[dim]

For example:

void foo(int a[3]) { ... } // C code

extern (C)
{
void foo(ref int[3] a); // D prototype
}

Calling printf()

printf can be directly called from D code:

import core.stdc.stdio;

int main()
{
printf("hello world\n");
return 0;
}

Printing values works as it does in C:

int apples;
printf("there are %d apples\n", apples);

Correctly matching the format specifier to the D type is necessary. The D compiler recognizes the printf formats and diagnoses mismatches with the supplied arguments. The specification for the formats used by D is the C99 specification 7.19.6.1.

A generous interpretation of what is a match between the argument and format specifier is taken, for example, an unsigned type can be printed with a signed format specifier. Diagnosed incompatibilites are:

- incompatible sizes which may cause argument misalignment

- dereferencing arguments that are not pointers

- insufficient number of arguments

- struct, array and slice arguments are not allowed

- non-pointer arguments to s specifier

- non-Standard formats

- undefined behavior per C99

Strings

A string cannot be printed directly. But %.*s can be used:

string s = "betty";
printf("hello %.*s\n", cast(int) s.length, s.ptr);

The cast to int is required.

size_t and ptrdiff_t

These use the zd and dt format specifiers respectively:

int* p, q;
printf("size of an int is %zt, pointer difference is %td\n", int.sizeof, p - q);

Non-Standard Format Specifiers

Non-Standard format specifiers will be rejected by the compiler. Since the checking is only done for formats as string literals, non-Standard ones can be used:

const char* format = "value: %K\n";
printf(format, value);

Modern Formatted Writing

An improved D function for formatted output is std.stdio.writef().

Structs and Unions

D structs and unions are analogous to C's.

C code often adjusts the alignment and packing of struct members with a command line switch or with various implementation specific #pragmas. D supports explicit alignment attributes that correspond to the C compiler's rules. Check what alignment the C code is using, and explicitly set it for the D struct declaration.

D does not support bit fields. If needed, they can be emulated with shift and mask operations, or use the std.bitmanip.bitfields library type. htod will convert bit fields to inline functions that do the right shift and masks.

D does not support declaring variables of anonymous struct types. In such a case, define a named struct in D and make it private:

union Info  // C code
{
struct
{
char *name;
} file;
};

union Info  // D code
{
private struct File
{
char* name;
}
File file;
}

Callbacks

D can easily call C callbacks (function pointers), and C can call callbacks provided by D code if the callback is an extern(C) function, or some other linkage that both sides have agreed to (e.g. extern(Windows)).

Here's an example of C code providing a callback to D code:

void someFunc(void *arg) { printf("Called someFunc!\n"); }  // C code
typedef void (*Callback)(void *);
extern "C" Callback getCallback(void)
{
return someFunc;
}

extern(C) alias Callback = int function(int, int);  // D code
extern(C) Callback getCallback();
void main()
{
Callback cb = getCallback();
cb();  // invokes the callback
}

And an example of D code providing a callback to C code:

extern "C" void printer(int (*callback)(int, int))  // C code
{
printf("calling callback with 2 and 4 returns: %d\n", callback(2, 4));
}

extern(C) alias Callback = int function(int, int);  // D code
extern(C) void printer(Callback callback);
extern(C) int sum(int x, int y) { return x + y; }
void main()
{
printer(&sum);
}

For more info about callbacks read the closures section.

Using Existing C Libraries

Since D can call C code directly, it can also call any C library functions, giving D access to the smorgasbord of existing C libraries. To do so, however, one needs to write a D interface (.di) file, which is a translation of the C .h header file for the C library into D.

For popular C libraries, the first place to look for the corresponding D interface file is the Deimos Project. If it isn't there already, please write and contribute one to the Deimos Project.

Accessing C Globals

C globals can be accessed directly from D. C globals have the C naming convention, and so must be in an extern (C) block. Use the extern storage class to indicate that the global is allocated in the C code, not the D code. C globals default to being in global, not thread local, storage. To reference global storage from D, use the __gshared storage class.

extern (C) extern __gshared int x;

Interfacing to C++

This document specifies how to interface with C++ directly.

It is also possible to indirectly interface with C++ code, either through a C interface or a COM interface.

The General Idea

Being 100% compatible with C++ means more or less adding a fully functional C++ compiler front end to D. Anecdotal evidence suggests that writing such is a minimum of a 10 man-year project, essentially making a D compiler with such capability unimplementable. Other languages looking to hook up to C++ face the same problem, and the solutions have been:

1. Support the COM interface (but that only works for Windows).

2. Laboriously construct a C wrapper around the C++ code.

3. Use an automated tool such as SWIG to construct a C wrapper.

4. Reimplement the C++ code in the other language.

5. Give up.

D takes a pragmatic approach that assumes a couple modest accommodations can solve a significant chunk of the problem:

- matching C++ name mangling conventions

- matching C++ function calling conventions

- matching C++ virtual function table layout for single inheritance

Global Functions

C++ global functions, including those in namespaces, can be declared and called in D, or defined in D and called in C++.

Calling C++ Global Functions from D

Given a C++ function in a C++ source file:

#include <iostream>

using namespace std;

int foo(int i, int j, int k)
{
cout << "i = " << i << endl;
cout << "j = " << j << endl;
cout << "k = " << k << endl;

return 7;
}

In the corresponding D code, foo is declared as having C++ linkage and function calling conventions:

extern (C++) int foo(int i, int j, int k);

and then it can be called within the D code:

extern (C++) int foo(int i, int j, int k);

void main()
{
foo(1, 2, 3);
}

Compiling the two files, the first with a C++ compiler, the second with a D compiler, linking them together, and then running it yields:

> g++ -c foo.cpp
> dmd bar.d foo.o -L-lstdc++ && ./bar
i = 1
j = 2
k = 3

There are several things going on here:

- D understands how C++ function names are "mangled" and the correct C++ function call/return sequence.

- Because modules are not part of C++, each function with C++ linkage in the global namespace must be globally unique within the program.

- There are no __cdecl, __far, __stdcall, __declspec, or other such nonstandard C++ extensions in D.

- There are no volatile type modifiers in D.

- Strings are not 0 terminated in D. See "Data Type Compatibility" for more information about this. However, string literals in D are 0 terminated.

Calling Global D Functions From C++

To make a D function accessible from C++, give it C++ linkage:

import std.stdio;

extern (C++) int foo(int i, int j, int k)
{
writefln("i = %s", i);
writefln("j = %s", j);
writefln("k = %s", k);
return 1;
}

extern (C++) void bar();

void main()
{
bar();
}

The C++ end looks like:

int foo(int i, int j, int k);

void bar()
{
foo(6, 7, 8);
}

Compiling, linking, and running produces the output:

> dmd -c foo.d
> g++ bar.cpp foo.o -lphobos2 -pthread -o bar && ./bar
i = 6
j = 7
k = 8

C++ Namespaces

C++ symbols that reside in namespaces can be accessed from D. A namespace can be added to the extern (C++) LinkageAttribute:

extern (C++, N) int foo(int i, int j, int k);

void main()
{
N.foo(1, 2, 3);   // foo is in C++ namespace 'N'
}

C++ can open the same namespace in the same file and multiple files. In D, this can be done as follows:

module ns;
extern (C++, `ns`)
{
int foo() { return 1; }
}

Any expression that resolves to either a tuple of strings or an empty tuple is accepted. When the expression resolves to an empty tuple, it is equivalent to extern (C++)

extern(C++, (expression))
{
int bar() { return 2; }
}

or in multiple files, by organizing them in a package consisting of several modules:

ns/
|-- a.d
|-- b.d
|-- package.d

File ns/a.d:

module a; extern (C++, `ns`) { int foo() { return 1; } }

File ns/b.d:

module b; extern (C++, `ns`) { int bar() { return 2; } }

File ns/package.d:

module ns;
public import a, b;

Then import the package containing the extern C++ declarations as follows:

import ns;
static assert(foo() == 1 && bar() == 2);

Note that the extern (C++, ns) linkage attribute affects only the ABI (name mangling and calling convention) of these declarations. Importing them follows the usual D module import semantics.

Alternatively, the non-string form can be used to introduce a scope. Note that the enclosing module already provides a scope for the symbols declared in the namespace. This form does not allow closing and reopening the same namespace with in the same module. That is:

module a; extern (C++, ns1) { int foo() { return 1; } }

module b; extern (C++, ns1) { int bar() { return 2; } }

import a, b;
static assert(foo() == 1 && bar() == 2);

works, but:

extern (C++, ns1) { int foo() { return 1; } }
extern (C++, ns1) { int bar() { return 2; } }

does not. Additionally, aliases can be used to avoid collision of symbols:

module a; extern (C++, ns) { int foo() { return 1; } }

module b; extern (C++, ns) { int bar() { return 2; } }

module ns;
import a, b;
alias foo = a.ns.foo;
alias bar = b.ns.bar;

import ns;
static assert(foo() == 1 && bar() == 2);

Classes

C++ classes can be declared in D by using the extern (C++) attribute on class, struct and interface declarations. extern (C++) interfaces have the same restrictions as D interfaces, which means that Multiple Inheritance is supported to the extent that only one base class can have member fields.

extern (C++) structs do not support virtual functions but can be used to map C++ value types.

Unlike classes and interfaces with D linkage, extern (C++) classes and interfaces are not rooted in Object and cannot be used with typeid.

D structs and classes have different semantics whereas C++ structs and classes are basically the same. The use of a D struct or class depends on the C++ implementation and not on the used C++ keyword. When mapping a D class onto a C++ struct, use extern(C++, struct) to avoid linking problems with C++ compilers (notably MSVC) that distinguish between C++'s class and struct when mangling. Conversely, use extern(C++, class) to map a D struct onto a C++ class.

extern(C++, class) and extern(C++, struct) can be combined with C++ namespaces:

extern (C++, struct) extern (C++, foo)
class Bar
{
}

Using C++ Classes From D

The following example shows binding of a pure virtual function, its implementation in a derived class, a non-virtual member function, and a member field:

#include <iostream>

using namespace std;

class Base
{
public:
virtual void print3i(int a, int b, int c) = 0;
};

class Derived : public Base
{
public:
int field;
Derived(int field) : field(field) {}

void print3i(int a, int b, int c)
{
cout << "a = " << a << endl;
cout << "b = " << b << endl;
cout << "c = " << c << endl;
}

int mul(int factor);
};

int Derived::mul(int factor)
{
return field * factor;
}

Derived *createInstance(int i)
{
return new Derived(i);
}

void deleteInstance(Derived *&d)
{
delete d;
d = 0;
}

We can use it in D code like:

extern(C++)
{
abstract class Base
{
void print3i(int a, int b, int c);
}

class Derived : Base
{
int field;
@disable this();
override void print3i(int a, int b, int c);
final int mul(int factor);
}

Derived createInstance(int i);
void deleteInstance(ref Derived d);
}

void main()
{
import std.stdio;

auto d1 = createInstance(5);
writeln(d1.field);
writeln(d1.mul(4));

Base b1 = d1;
b1.print3i(1, 2, 3);

deleteInstance(d1);
assert(d1 is null);

auto d2 = createInstance(42);
writeln(d2.field);

deleteInstance(d2);
assert(d2 is null);
}

Compiling, linking, and running produces the output:

> g++ base.cpp
> dmd main.d base.o -L-lstdc++ && ./main
5
20
a = 1
b = 2
c = 3
42

Note how in the above example, the constructor is not bindable and is instead disabled on the D side; an alternative would be to reimplement the constructor in D. See the section below on lifetime management for more information.

Using D Classes From C++

Given D code like:

extern (C++) int callE(E);

extern (C++) interface E
{
int bar(int i, int j, int k);
}

class F : E
{
extern (C++) int bar(int i, int j, int k)
{
import std.stdio : writefln;
writefln("i = %s", i);
writefln("j = %s", j);
writefln("k = %s", k);
return 8;
}
}

void main()
{
F f = new F();
callE(f);
}

The C++ code to access it looks like:

class E
{
public:
virtual int bar(int i, int j, int k);
};

int callE(E *e)
{
return e->bar(11, 12, 13);
}

> dmd -c base.d
> g++ klass.cpp base.o -lphobos2 -pthread -o klass && ./klass
i = 11
j = 12
k = 13

Structs

C++ allows a struct to inherit from a base struct. This is done in D using alias this:

struct Base { ... members ... };

struct Derived
{
Base base;       // make it the first field
alias base this;

... members ...
}

In both C++ and D, if a struct has zero fields, the struct still has a size of 1 byte. But, in C++ if the struct with zero fields is used as a base struct, its size is zero (called the Empty Base Optimization). There are two methods for emulating this behavior in D. The first forwards references to a function returning a faked reference to the base:

struct Base { ... members ... };

struct DerivedStruct
{
static if (Base.tupleof.length > 0)
Base base;
else
ref inout(Base) base() inout
{
return *cast(inout(Base)*)&this;
}
alias base this;

... members ...
}

The second makes use of template mixins:

mixin template BaseMembers()
{
void memberFunction() { ... }
}

struct Base
{
mixin BaseMembers!();
}

struct Derived
{
mixin BaseMembers!();

... members ...
}

Note that the template mixin is evaluated in the context of its instantiation, not declaration. If this is a problem, the template mixin can use local imports, or have the member functions forward to the actual functions.

C++ Templates

C++ function and type templates can be bound by using the extern (C++) attribute on a function or type template declaration.

Note that all instantiations used in D code must be provided by linking to C++ object code or shared libraries containing the instantiations.

For example:

#include <iostream>

template<class T>
struct Foo
{
private:
T field;

public:
Foo(T t) : field(t) {}
T get();
void set(T t);
};

template<class T>
T Foo<T>::get()
{
return field;
}

template<class T>
void Foo<T>::set(T t)
{
field = t;
}

Foo<int> makeIntFoo(int i)
{
return Foo<int>(i);
}

Foo<char> makeCharFoo(char c)
{
return Foo<char>(c);
}

template<class T>
void increment(Foo<T> &foo)
{
foo.set(foo.get() + 1);
}

template<class T>
void printThreeNext(Foo<T> foo)
{
for(size_t i = 0; i < 3; ++i)
{
std::cout << foo.get() << std::endl;
increment(foo);
}
}

// The following two functions ensure that the required instantiations of
// printThreeNext are provided by this code module
void printThreeNexti(Foo<int> foo)
{
printThreeNext(foo);
}

void printThreeNextc(Foo<char> foo)
{
printThreeNext(foo);
}

extern(C++):
struct Foo(T)
{
private:
T field;

public:
@disable this();
T get();
void set(T t);
}

Foo!int makeIntFoo(int i);
Foo!char makeCharFoo(char c);
void increment(T)(ref Foo!T foo);
void printThreeNext(T)(Foo!T foo);

extern(D) void main()
{
auto i = makeIntFoo(42);
assert(i.get() == 42);
i.set(1);
increment(i);
assert(i.get() == 2);

auto c = makeCharFoo('a');
increment(c);
assert(c.get() == 'b');

c.set('A');
printThreeNext(c);
}

Compiling, linking, and running produces the output:

> g++ -c template.cpp
> dmd main.d template.o -L-lstdc++ && ./main
A
B
C

Function Overloading

C++ and D follow different rules for function overloading. D source code, even when calling extern (C++) functions, will still follow D overloading rules.

Memory Allocation

C++ code explicitly manages memory with calls to ::operator new() and ::operator delete(). D's new operator allocates memory using the D garbage collector, so no explicit delete is necessary. D's new operator is not compatible with C++'s ::operator new and ::operator delete. Attempting to allocate memory with D's new and deallocate with C++ ::operator delete will result in miserable failure.

D can explicitly manage memory using a variety of library tools, such as with std.experimental.allocator. Additionally, core.stdc.stdlib.malloc and core.stdc.stdlib.free can be used directly for connecting to C++ functions that expect malloc'd buffers.

If pointers to memory allocated on the D garbage collector heap are passed to C++ functions, it's critical to ensure that the referenced memory will not be collected by the D garbage collector before the C++ function is done with it. This is accomplished by:

- Making a copy of the data using std.experimental.allocator or core.stdc.stdlib.malloc and passing the copy instead.

- Leaving a pointer to it on the stack (as a parameter or automatic variable), as the garbage collector will scan the stack.

- Leaving a pointer to it in the static data segment, as the garbage collector will scan the static data segment.

- Registering the pointer with the garbage collector using the core.memory.GC.addRoot or core.memory.GC.addRange functions.

An interior pointer to the allocated memory block is sufficient to let the GC know the object is in use; i.e. it is not necessary to maintain a pointer to the beginning of the allocated memory.

The garbage collector does not scan the stacks of threads not registered with the D runtime, nor does it scan the data segments of shared libraries that aren't registered with the D runtime.

Data Type Compatibility

D And C++ Type Equivalence

D type

C++ type

void

void

byte

signed char

ubyte

unsigned char

char

char (chars are unsigned in D)

core.stdc.stddef.wchar_t

wchar_t

short

short

ushort

unsigned short

int

int

uint

unsigned

long

long if it is 64 bits wide, otherwise long long

ulong

unsigned long if it is 64 bits wide, otherwise unsigned long long

core.stdc.config.cpp_long

long

core.stdc.config.cpp_ulong

unsigned long

float

float

double

double

real

long double

extern (C++) struct

struct or class

extern (C++) class

struct or class

extern (C++) interface

struct or class with no member fields

union

union

enum

enum

type*

type *

ref type (in parameter lists only)

type &

type[dim]

type[dim]

type[dim]*

type(*)[dim]

type[]

no equivalent

type[type]

no equivalent

type function(parameters)

type(*)(parameters)

type delegate(parameters)

no equivalent

These equivalents hold when the D and C++ compilers used are companions on the host platform.

Packing and Alignment

D structs and unions are analogous to C's.

C code often adjusts the alignment and packing of struct members with a command line switch or with various implementation specific #pragmas. D supports explicit alignment attributes that correspond to the C compiler's rules. Check what alignment the C code is using, and explicitly set it for the D struct declaration.

D supports bitfields in the standard library: see std.bitmanip.bitfields.

Lifetime Management

C++ constructors, copy constructors, move constructors and destructors cannot be called directly in D code, and D constructors, postblit operators and destructors cannot be directly exported to C++ code. Interoperation of types with these special operators is possible by either 1) disabling the operator in the client language and only using it in the host language, or 2) faithfully reimplementing the operator in the client language. With the latter approach, care needs to be taken to ensure observable semantics remain the same with both implementations, which can be difficult, or in some edge cases impossible, due to differences in how the operators work in the two languages. For example, in D all objects are movable and there is no move constructor.

Special Member Functions

D cannot directly call C++ special member functions, and vice versa. These include constructors, destructors, conversion operators, operator overloading, and allocators.

Runtime Type Identification

D runtime type identification uses completely different techniques than C++. The two are incompatible.

Exception Handling

Exception interoperability is a work in progress.

At present, C++ exceptions cannot be caught in or thrown from D, and D exceptions cannot be caught in or thrown from C++. Additionally, objects in C++ stack frames are not guaranteed to be destroyed when unwinding the stack due to a D exception, and vice versa.

The plan is to support all of the above except throwing D exceptions directly in C++ code (but they will be throwable indirectly by calling into a D function with C++ linkage).

Comparing D Immutable and Const with C++ Const

Const, Immutable Comparison

Feature

D

C++98

const keyword

Yes

Yes

immutable keyword

Yes

No

const notation

// Functional:
//ptr to const ptr to const int
const(int*)* p;

// Postfix:
//ptr to const ptr to const int
const int *const *p;

transitive const

// Yes:
//const ptr to const ptr to const int
const int** p;
**p = 3; // error

// No:
// const ptr to ptr to int
int** const p;
**p = 3;    // ok

cast away const

// Yes:
// ptr to const int
const(int)* p;
int* q = cast(int*)p; // ok

// Yes:
// ptr to const int
const int* p;
int* q = const_cast<int*>p; //ok

cast+mutate

// No:
// ptr to const int
const(int)* p;
int* q = cast(int*)p;
*q = 3;   // undefined behavior

// Yes:
// ptr to const int
const int* p;
int* q = const_cast<int*>p;
*q = 3;   // ok

overloading

// Yes:
void foo(int x);
void foo(const int x);  //ok

// No:
void foo(int x);
void foo(const int x);  //error

const/mutable aliasing

// Yes:
void foo(const int* x, int* y)
{
bar(*x); // bar(3)
*y = 4;
bar(*x); // bar(4)
}
...
int i = 3;
foo(&i, &i);

// Yes:
void foo(const int* x, int* y)
{
bar(*x); // bar(3)
*y = 4;
bar(*x); // bar(4)
}
...
int i = 3;
foo(&i, &i);

immutable/mutable aliasing

// No:
void foo(immutable int* x, int* y)
{
bar(*x); // bar(3)
*y = 4;  // undefined behavior
bar(*x); // bar(??)
}
...
int i = 3;
foo(cast(immutable)&i, &i);

No immutables

type of string literal

immutable(char)[]

const char*

string literal to non-const

not allowed

allowed, but deprecated

Interfacing to Objective-C

D supports interfacing with Objective-C. It supports protocols, classes, subclasses, instance variables, instance methods and class methods. Platform support might vary between different compilers.

Fully working example is available at the bottom.

Classes

Declaring an External Class

import core.attribute : selector;

extern (Objective-C)
extern class NSString
{
const(char)* UTF8String() @selector("UTF8String");
}

All Objective-C classes that should be accessible from within D need to be declared with the Objective-C linkage. If the class is declared as extern (in addition to extern (Objective-C)) it is expected to be defined externally.

The @selector attribute indicates which Objective-C selector should be used when calling this method. This attribute needs to be attached to all methods with the Objective-C linkage.

Defining a Class

import core.attribute : selector;

// externally defined
extern (Objective-C)
extern class NSObject
{
static NSObject alloc() @selector("alloc");
NSObject init() @selector("init");
}

extern (Objective-C)
class Foo : NSObject
{
override static Foo alloc() @selector("alloc");
override Foo init() @selector("init");

final int bar(int a) @selector("bar:")
{
return a;
}
}

void main()
{
assert(Foo.alloc.init.bar(3) == 3);
}

Defining an Objective-C class is exactly the same as declaring an external class but it should not be declared as extern.

To match the Objective-C semantics, static and final methods are virtual. static methods are overridable as well.

Protocols

Declaring a Protocol

import core.attribute : selector;
import core.stdc.stdio : printf;

extern (Objective-C)
interface Foo
{
static void foo() @selector("foo");
void bar() @selector("bar");
}

extern (Objective-C)
class Bar : Foo
{
static void foo() @selector("foo")
{
printf("foo\n");
}

void bar() @selector("bar")
{
printf("bar\n");
}
}

Objective-C protocols are represented as interfaces in D and are declared using the interface keyword.

All Objective-C protocols that should be accessible from within D need to be declared with the Objective-C linkage.

Objective-C protocols support virtual class (static) methods. These methods must be implemented by the class that implements the protocol (unless they are optional). To match these semantics, static methods are virtual. That also means that static methods with Objective-C linkage, inside an interface cannot have a body.

Optional Methods

import core.attribute : optional, selector;
import core.stdc.stdio : printf;

struct objc_selector;
alias SEL = objc_selector*;

extern (C) SEL sel_registerName(in char* str);

extern (Objective-C)
extern class NSObject
{
static NSObject alloc() @selector("alloc");
NSObject init() @selector("init");
}

extern (Objective-C)
interface Foo
{
bool respondsToSelector(SEL sel) @selector("respondsToSelector:");
void foo() @selector("foo");

// this is an optional method
@optional void bar() @selector("bar");
}

extern (Objective-C)
class Bar : NSObject, Foo
{
override static Bar alloc() @selector("alloc");
override Bar init() @selector("init");

bool respondsToSelector(SEL sel) @selector("respondsToSelector:");

void foo() @selector("foo")
{
printf("foo\n");
}
}

void main()
{
Foo f = Bar.alloc.init;

// check, at runtime, if the instance `f` implements the method `bar`
if (f.respondsToSelector(sel_registerName("bar")))
f.bar();
else
f.foo();
}

Objective-C protocols support optional methods. Optional methods are not required to be implemented by the class that implements the protocol. To safely call an optional method, a runtime check should be performed to make sure the receiver implements the method.

In D, optional methods are represented using the @optional attribute.

Instance Variables

import core.attribute : selector;

// externally defined
extern (Objective-C)
extern class NSObject
{
static NSObject alloc() @selector("alloc");
NSObject init() @selector("init");
}

extern (Objective-C)
class Foo : NSObject
{
int bar_;

override static Foo alloc() @selector("alloc");
override Foo init() @selector("init");

int bar() @selector("bar")
{
return bar_;
}
}

void main()
{
auto foo = Foo.alloc.init;
foo.bar_ = 3;
assert(foo.bar == 3);
}

Declaring an instance variable looks exactly the same as for a regular D class.

To solve the fragile base class problem, instance variables in Objective-C has a dynamic offset. That means that the base class can change (add or remove instance variables) without the subclasses needing to recompile or relink. Thanks to this feature it's not necessary to declare instance variables when creating bindings to Objective-C classes.

Calling an Instance Method

Calling an Objective-C instance method uses the same syntax as calling regular D methods:

const(char)* result = object.UTF8String();

When the compiler sees a call to a method with Objective-C linkage it will generate a call similar to how an Objective-C compiler would call the method.

The @selector Attribute

The @selector attribute is a compiler recognized UDA. It is used to tell the compiler which selector to use when calling an Objective-C method.

Selectors in Objective-C can contain the colon character, which is not valid in D identifiers. D supports method overloading while Objective-C achieves something similar by using different selectors. For these two reasons it is better to be able to specify the selectors manually in D, instead of trying to infer it. This allows to have a more natural names for the methods in D. Example:

import core.attribute : selector;

extern (Objective-C)
extern class NSString
{
NSString initWith(in char*) @selector("initWithUTF8String:");
NSString initWith(NSString) @selector("initWithString:");
}

Here the method initWith is overloaded with two versions, one accepting in char*, the other one NSString. These two methods are mapped to two different Objective-C selectors, initWithUTF8String: and initWithString:.

The attribute is defined in druntime in core.attribute. The attribute is only defined when the version identifier D_ObjectiveC is enabled.

Compiler Checks

The compiler performs the following checks to enforce the correct usage of the @selector attribute:

- The attribute can only be attached to methods with Objective-C linkage

- The attribute can only be attached once to a method

- The attribute cannot be attached to a template method

- The number of colons in the selector needs to match the number of parameters the method is declared with

If any of the checks fail, a compile error will occur.

The @optional Attribute

The @optional attribute is a compiler recognized UDA. It is used to tell the compiler that a method, with Objective-C linkage, declared inside an interface is optional. That means that the class that implements the interface does not have to implement the method.

To safely call an optional method, a runtime check should be performed to make sure the receiver implements the method.

The attribute is defined in druntime in core.attribute. The attribute is only defined when the version identifier D_ObjectiveC is enabled.

Compiler Checks

The compiler performs the following checks to enforce the correct usage of the @optional attribute:

- The attribute can only be attached to methods with Objective-C linkage

- The attribute can only be attached to a method inside an interface

- The attribute can only be attached once to a method

- The attribute cannot be attached to a template method

If any of the checks fail, a compile error will occur.

The D_ObjectiveC Version Identifier

The D_ObjectiveC version identifier is a predefined version identifier. It is enabled if Objective-C support is available for the target.

Objective-C Linkage

Objective-C linkage is achieved by attaching the extern (Objective-C) attribute to a class. Example:

import core.attribute : selector;

extern (Objective-C)
extern class NSObject
{
NSObject init() @selector("init");
}

All methods inside a class declared as extern (Objective-C) will get implicit Objective-C linkage.

The linkage is recognized on all platforms but will issue a compile error if it is used on a platform where Objective-C support is not available. This allows to easily hide Objective-C declarations from platforms where it is not available using the version statement, without resorting to string mixins or other workarounds.

Memory Management

The preferred way to do memory management in Objective-C is to use Automatic Reference Counting, ARC. This is not supported in D, therefore manual memory management is required to be used instead. This is achieved by calling release on an Objective-C instance, like in the old days of Objective-C.

Frameworks

Most Objective-C code is bundled in something called a "Framework". This is basically a regular directory, with the .framework extension and a specific directory layout. A framework contains a dynamic library, all public header files and any resources (images, sounds and so on) required by the framework.

These directories are recognized by some tools, like the Objective-C compiler and linker, to be frameworks. To link with a framework from DMD, use the following flags:

-L-framework -L<Framework>

where <Framework> is the name of the framework to link with, without the .framework extension. The two -L flags are required because the linker expects a space between the -framework flag and the name of the framework. DMD cannot handle this and will instead interpret the name of the framework as a separate flag.
Framework Paths

Using the above flag, the linker will search in the standard framework paths. The standard search paths for frameworks are:

- /System/Library/Frameworks

- /Library/Frameworks

The following flag from DMD can be used to add a new path in which to search for frameworks:

-L-F<framework_path>

For more information see the reference documentation and the ld man page.

Full Usage Example

This example will create an Objective-C string, NSString, and log the message using NSLog to stderr.

import core.attribute : selector;

extern (Objective-C)
extern class NSString
{
static NSString alloc() @selector("alloc");
NSString initWithUTF8String(in char* str) @selector("initWithUTF8String:");
void release() @selector("release");
}

This is a simplified declaration of the NSString class. The alloc method allocates an instance of the class. The initWithUTF8String: method will be used to convert a C string in UTF-8 to an Objective-C string, NSString. The release method is used to release an deallocate the string. Since D doesn't support ARC it's needed to manually release Objective-C instances.

extern (C) void NSLog(NSString, ...);

This NSLog function prints a message to the System Log facility, i.e. to stderr and Console.

auto str = NSString.alloc();

Allocate an instance of the class, NSString.

str = str.initWithUTF8String("Hello World!")

Initialize the Objective-C string using a C string.

NSLog(str);

Log the string to stderr, this will print something like this in the terminal:

2015-07-18 13:14:27.978 main[11045:2934950] Hello World!

str.release();

Release and deallocate the string.

All steps combined look like this:

module main;

import core.attribute : selector;

extern (Objective-C)
extern class NSString
{
static NSString alloc() @selector("alloc");
NSString initWithUTF8String(in char* str) @selector("initWithUTF8String:");
void release() @selector("release");
}

extern (C) void NSLog(NSString, ...);

void main()
{
auto str = NSString.alloc().initWithUTF8String("Hello World!");
NSLog(str);
str.release();
}

When compiling the application remember to link with the required libraries, in this case the Foundation framework. Example:

dmd -L-framework -LFoundation main.d

Portability Guide

It's good software engineering practice to minimize gratuitous portability problems in the code. Techniques to minimize potential portability problems are:

- The integral and floating type sizes should be considered as minimums. Algorithms should be designed to continue to work properly if the type size increases.

- Floating point computations can be carried out at a higher precision than the size of the floating point variable can hold. Floating point algorithms should continue to work properly if precision is arbitrarily increased.

- Avoid depending on the order of side effects in a computation that may get reordered by the compiler. For example:

a + b + c

can be evaluated as (a + b) + c, a + (b + c), (a + c) + b, (c + b) + a, etc. Parentheses control operator precedence, parentheses do not control order of evaluation.

If the operands of an associative operator + or * are floating point values, the expression is not reordered.

- Avoid dependence on byte order; i.e. whether the CPU is big-endian or little-endian.

- Avoid dependence on the size of a pointer or reference being the same size as a particular integral type.

- If size dependencies are inevitable, put a static assert in the code to verify it:

static assert(int.sizeof == (int*).sizeof);

32 to 64 Bit Portability

64 bit processors and operating systems are here. With that in mind:

- Integral types will remain the same sizes between 32 and 64 bit code.

- Pointers and object references will increase in size from 4 bytes to 8 bytes going from 32 to 64 bit code.

- Use size_t as an alias for an unsigned integral type that can span the address space. Array indices should be of type size_t.

- Use ptrdiff_t as an alias for a signed integral type that can span the address space. A type representing the difference between two pointers should be of type ptrdiff_t.

- The .length, .size, .sizeof, .offsetof and .alignof properties will be of type size_t.

Endianness

Endianness refers to the order in which multibyte types are stored. The two main orders are big endian and little endian. The compiler predefines the version identifier BigEndian or LittleEndian depending on the order of the target system. The x86 systems are all little endian.

The times when endianness matters are:

- When reading data from an external source (like a file) written in a different endian format.

- When reading or writing individual bytes of a multibyte type like longs or doubles.

OS Specific Code

System specific code is handled by isolating the differences into separate modules. At compile time, the correct system specific module is imported.

Minor differences can be handled by constant defined in a system specific import, and then using that constant in an IfStatement or StaticIfStatement.

Named Character Entities

NamedCharacterEntity:
& Identifier ;

The full list of named character entities from the HTML 5 Spec is supported except for the named entities which contain multiple code points. Below is a partial list of the named character entities.

Note: Not all glyphs will display properly in the Glyph column in all browsers.

Named Character Entities

Name

Value

Glyph

quot

34

"

amp

38

&

lt

60

<

gt

62

>

OElig

338

Œ

oelig

339

œ

Scaron

352

Š

scaron

353

š

Yuml

376

ÿ

circ

710

ˆ

tilde

732

˜

ensp

8194

emsp

8195

thinsp

8201

zwnj

8204

‌

zwj

8205

‍

lrm

8206

‎

rlm

8207

‏

ndash

8211

–

mdash

8212

—

lsquo

8216

‘

rsquo

8217

’

sbquo

8218

‚

ldquo

8220

“

rdquo

8221

”

bdquo

8222

„

dagger

8224

†

Dagger

8225

‡

permil

8240

‰

lsaquo

8249

‹

rsaquo

8250

›

euro

8364

€

Latin-1 (ISO-8859-1) Entities

Name

Value

Glyph

nbsp

160

iexcl

161

¡

cent

162

¢

pound

163

£

curren

164

¤

yen

165

¥

brvbar

166

¦

sect

167

§

uml

168

¨

copy

169

©

ordf

170

ª

laquo

171

«

not

172

¬

shy

173

reg

174

®

macr

175

¯

deg

176

°

plusmn

177

±

sup2

178

²

sup3

179

³

acute

180

´

micro

181

µ

para

182

¶

middot

183

·

cedil

184

¸

sup1

185

¹

ordm

186

º

raquo

187

»

frac14

188

¼

frac12

189

½

frac34

190

¾

iquest

191

¿

Agrave

192

À

Aacute

193

Á

Acirc

194

Â

Atilde

195

Ã

Auml

196

Ä

Aring

197

Å

AElig

198

Æ

Ccedil

199

Ç

Egrave

200

È

Eacute

201

É

Ecirc

202

Ê

Euml

203

Ë

Igrave

204

Ì

Iacute

205

Í

Icirc

206

Î

Iuml

207

Ï

ETH

208

Ð

Ntilde

209

Ñ

Ograve

210

Ò

Oacute

211

Ó

Ocirc

212

Ô

Otilde

213

Õ

Ouml

214

Ö

times

215

×

Oslash

216

Ø

Ugrave

217

Ù

Uacute

218

Ú

Ucirc

219

Û

Uuml

220

Ü

Yacute

221

Ý

THORN

222

Þ

szlig

223

ß

agrave

224

à

aacute

225

á

acirc

226

â

atilde

227

ã

auml

228

ä

aring

229

å

aelig

230

æ

ccedil

231

ç

egrave

232

è

eacute

233

é

ecirc

234

ê

euml

235

ë

igrave

236

ì

iacute

237

í

icirc

238

î

iuml

239

ï

eth

240

ð

ntilde

241

ñ

ograve

242

ò

oacute

243

ó

ocirc

244

ô

otilde

245

õ

ouml

246

ö

divide

247

÷

oslash

248

ø

ugrave

249

ù

uacute

250

ú

ucirc

251

û

uuml

252

ü

yacute

253

ý

thorn

254

þ

yuml

255

ÿ

Symbols and Greek letter entities

Name

Value

Glyph

fnof

402

ƒ

Alpha

913

Α

Beta

914

Β

Gamma

915

Γ

Delta

916

Δ

Epsilon

917

Ε

Zeta

918

Ζ

Eta

919

Η

Theta

920

Θ

Iota

921

Ι

Kappa

922

Κ

Lambda

923

Λ

Mu

924

Μ

Nu

925

Ν

Xi

926

Ξ

Omicron

927

Ο

Pi

928

Π

Rho

929

Ρ

Sigma

931

Σ

Tau

932

Τ

Upsilon

933

Υ

Phi

934

Φ

Chi

935

Χ

Psi

936

Ψ

Omega

937

Ω

alpha

945

α

beta

946

β

gamma

947

γ

delta

948

δ

epsilon

949

ε

zeta

950

ζ

eta

951

η

theta

952

θ

iota

953

ι

kappa

954

κ

lambda

955

λ

mu

956

μ

nu

957

ν

xi

958

ξ

omicron

959

ο

pi

960

π

rho

961

ρ

sigmaf

962

ς

sigma

963

σ

tau

964

τ

upsilon

965

υ

phi

966

φ

chi

967

χ

psi

968

ψ

omega

969

ω

thetasym

977

ϑ

upsih

978

ϒ

piv

982

ϖ

bull

8226

•

hellip

8230

…

prime

8242

′

Prime

8243

″

oline

8254

‾

frasl

8260

⁄

weierp

8472

℘

image

8465

ℑ

real

8476

ℜ

trade

8482

™

alefsym

8501

ℵ

larr

8592

←

uarr

8593

↑

rarr

8594

→

darr

8595

↓

harr

8596

↔

crarr

8629

↵

lArr

8656

⇐

uArr

8657

⇑

rArr

8658

⇒

dArr

8659

⇓

hArr

8660

⇔

forall

8704

∀

part

8706

∂

exist

8707

∃

empty

8709

∅

nabla

8711

∇

isin

8712

∈

notin

8713

∉

ni

8715

∋

prod

8719

∏

sum

8721

∑

minus

8722

−

lowast

8727

∗

radic

8730

√

prop

8733

∝

infin

8734

∞

ang

8736

∠

and

8743

∧

or

8744

∨

cap

8745

∩

cup

8746

∪

int

8747

∫

there4

8756

∴

sim

8764

∼

cong

8773

≅

asymp

8776

≈

ne

8800

≠

equiv

8801

≡

le

8804

≤

ge

8805

≥

sub

8834

⊂

sup

8835

⊃

nsub

8836

⊄

sube

8838

⊆

supe

8839

⊇

oplus

8853

⊕

otimes

8855

⊗

perp

8869

⊥

sdot

8901

⋅

lceil

8968

⌈

rceil

8969

⌉

lfloor

8970

⌊

rfloor

8971

⌋

loz

9674

◊

spades

9824

♠

clubs

9827

♣

hearts

9829

♥

diams

9830

♦

lang

10216

⟨

rang

10217

⟩

Memory Safety

Memory Safety for a program is defined as it being impossible for the program to corrupt memory. Therefore, the safe subset of D consists only of programming language features that are guaranteed to never result in memory corruption. See this article for a rationale.

Memory-safe code cannot use certain language features, such as:

- Casts that break the type system.

- Modification of pointer values.

- Taking the address of a local variable or function parameter.

Usage

There are three categories of functions from the perspective of memory safety:

- @safe functions

- @trusted functions

- @system functions

@system functions may perform any operation legal from the perspective of the language including inherently memory unsafe operations like returning pointers to expired stackframes. These functions may not be called directly from @safe functions.

@trusted functions have all the capabilities of @system functions but may be called from @safe functions. For this reason they should be very limited in the scope of their use. Typical uses of @trusted functions include wrapping system calls that take buffer pointer and length arguments separately so that @safe` functions may call them with arrays.

@safe functions have a number of restrictions on what they may do and are intended to disallow operations that may cause memory corruption. See @safe functions.

These attributes may be inferred when the compiler has the function body available, such as with templates.

Array bounds checks are necessary to enforce memory safety, so these are enabled (by default) for @safe code even in -release mode.

Scope and Return Parameters

The function parameter attributes return and scope are used to track what happens to low-level pointers passed to functions. Such pointers include: raw pointers, arrays, this, classes, ref parameters, delegate/lazy parameters, and aggregates containing a pointer.

scope ensures that no references to the pointed-to object are retained, in global variables or pointers passed to the function (and recursively to other functions called in the function), as a result of calling the function. Variables in the function body and parameter list that are scope may have their allocations elided as a result.

return indicates that either the return value of the function or the first parameter is a pointer derived from the return parameter or any other parameters also marked return. For constructors, return applies to the (implicitly returned) this reference. For void functions, return applies to the first parameter iff it is ref; this is to support UFCS, property setters and non-member functions (e.g. put used like put(dest, source)).

These attributes may appear after the formal parameter list, in which case they apply either to a method's this parameter, or to a free function's first parameter iff it is ref. return or scope is ignored when applied to a type that is not a low-level pointer.

Note: Checks for scope parameters are currently enabled only for @safe code compiled with the -dip1000 command-line flag.

Limitations

Memory safety does not imply that code is portable, uses only sound programming practices, is free of byte order dependencies, or other bugs. It is focussed only on eliminating memory corruption possibilities.

Application Binary Interface

A D implementation that conforms to the D ABI (Application Binary Interface) will be able to generate libraries, DLLs, etc., that can interoperate with D binaries built by other implementations.

C ABI

The C ABI referred to in this specification means the C Application Binary Interface of the target system. C and D code should be freely linkable together, in particular, D code shall have access to the entire C ABI runtime library.

Endianness

The endianness (byte order) of the layout of the data will conform to the endianness of the target machine. The Intel x86 CPUs are little endian meaning that the value 0x0A0B0C0D is stored in memory as: 0D 0C 0B 0A.

Basic Types

bool

8 bit byte with the values 0 for false and 1 for true

byte

8 bit signed value

ubyte

8 bit unsigned value

short

16 bit signed value

ushort

16 bit unsigned value

int

32 bit signed value

uint

32 bit unsigned value

long

64 bit signed value

ulong

64 bit unsigned value

cent

128 bit signed value

ucent

128 bit unsigned value

float

32 bit IEEE 754 floating point value

double

64 bit IEEE 754 floating point value

real

implementation defined floating point value, for x86 it is 80 bit IEEE 754 extended real

Delegates

Delegates are fat pointers with two parts:

Delegate Layout

offset

property

contents

0

.ptr

context pointer

ptrsize

.funcptr

pointer to function

The context pointer can be a class this reference, a struct this pointer, a pointer to a closure (nested functions) or a pointer to an enclosing function's stack frame (nested functions).

Structs

Conforms to the target's C ABI struct layout.

Classes

An object consists of:

Class Object Layout

size

property

contents

ptrsize

.__vptr

pointer to vtable

ptrsize

.__monitor

monitor

ptrsize...

vptrs for any interfaces implemented by this class in left to right, most to least derived, order

...

...

super's non-static fields and super's interface vptrs, from least to most derived

...

named fields

non-static fields

The vtable consists of:

Virtual Function Pointer Table Layout

size

contents

ptrsize

pointer to instance of TypeInfo

ptrsize...

pointers to virtual member functions

Casting a class object to an interface consists of adding the offset of the interface's corresponding vptr to the address of the base of the object. Casting an interface ptr back to the class type it came from involves getting the correct offset to subtract from it from the object.Interface entry at vtbl[0]. Adjustor thunks are created and pointers to them stored in the method entries in the vtbl[] in order to set the this pointer to the start of the object instance corresponding to the implementing method.

An adjustor thunk looks like:

ADD EAX,offset
JMP method

The leftmost side of the inheritance graph of the interfaces all share their vptrs, this is the single inheritance model. Every time the inheritance graph forks (for multiple inheritance) a new vptr is created and stored in the class' instance. Every time a virtual method is overridden, a new vtbl[] must be created with the updated method pointers in it.

The class definition:

class XXXX
{
....
};

Generates the following:

- An instance of Class called ClassXXXX.

- A type called StaticClassXXXX which defines all the static members.

- An instance of StaticClassXXXX called StaticXXXX for the static members.

Interfaces

An interface is a pointer to a pointer to a vtbl[]. The vtbl[0] entry is a pointer to the corresponding instance of the object.Interface class. The rest of the vtbl[1..$] entries are pointers to the virtual functions implemented by that interface, in the order that they were declared.

A COM interface differs from a regular interface in that there is no object.Interface entry in vtbl[0]; the entries vtbl[0..$] are all the virtual function pointers, in the order that they were declared. This matches the COM object layout used by Windows.

A C++ interface differs from a regular interface in that it matches the layout of a C++ class using single inheritance on the target machine.

Arrays

A dynamic array consists of:

Dynamic Array Layout

offset

property

contents

0

.length

array dimension

size_t

.ptr

pointer to array data

A dynamic array is declared as:

type[] array;

whereas a static array is declared as:

type[dimension] array;

Thus, a static array always has the dimension statically available as part of the type, and so it is implemented like in C. Static arrays and Dynamic arrays can be easily converted back and forth to each other.

Associative Arrays

Associative arrays consist of a pointer to an opaque, implementation defined type.

The current implementation is contained in and defined by rt/aaA.d.

Reference Types

D has reference types, but they are implicit. For example, classes are always referred to by reference; this means that class instances can never reside on the stack or be passed as function parameters.

Name Mangling

D accomplishes typesafe linking by mangling a D identifier to include scope and type information.

MangledName:
_D QualifiedName Type
_D QualifiedName Z        // Internal

The Type above is the type of a variable or the return type of a function. This is never a TypeFunction, as the latter can only be bound to a value via a pointer to a function or a delegate.

QualifiedName:
SymbolFunctionName
SymbolFunctionName QualifiedName

SymbolFunctionName:
SymbolName
SymbolName TypeFunctionNoReturn
SymbolName M TypeModifiersopt TypeFunctionNoReturn

The M means that the symbol is a function that requires a this pointer. Class or struct fields are mangled without M. To disambiguate M from being a Parameter with modifier scope, the following type needs to be checked for being a TypeFunction.

SymbolName:
LName
TemplateInstanceName
IdentifierBackRef
0                         // anonymous symbols

Template Instance Names have the types and values of its parameters encoded into it:

TemplateInstanceName:
TemplateID LName TemplateArgs Z

TemplateID:
__T
__U        // for symbols declared inside template constraint

TemplateArgs:
TemplateArg
TemplateArg TemplateArgs

TemplateArg:
TemplateArgX
H TemplateArgX

If a template argument matches a specialized template parameter, the argument is mangled with prefix H.

TemplateArgX:
T Type
V Type Value
S QualifiedName
X Number ExternallyMangledName

ExternallyMangledName can be any series of characters allowed on the current platform, e.g. generated by functions with C++ linkage or annotated with pragma(mangle,...).

Values:
Value
Value Values

Value:
n
i Number
N Number
e HexFloat
c HexFloat c HexFloat
CharWidth Number _ HexDigits
A Number Values
S Number Values
f MangledName

HexFloat:
NAN
INF
NINF
N HexDigits P Exponent
HexDigits P Exponent

Exponent:
N Number
Number

HexDigits:
HexDigit
HexDigit HexDigits

HexDigit:
Digit
A
B
C
D
E
F

CharWidth:
a
w
d

n

is for null arguments.

i Number

is for positive numeric literals (including character literals).

N Number

is for negative numeric literals.

e HexFloat

is for real and imaginary floating point literals.

c HexFloat c HexFloat

is for complex floating point literals.

CharWidth Number _ HexDigits

CharWidth is whether the characters are 1 byte (a), 2 bytes (w) or 4 bytes (d) in size. Number is the number of characters in the string. The HexDigits are the hex data for the string.

A Number Values

An array or asssociative array literal. Number is the length of the array. Value is repeated Number times for a normal array, and 2 * Number times for an associative array.

S Number Values

A struct literal. Value is repeated Number times.

Name:
Namestart
Namestart Namechars

Namestart:
_
Alpha

Namechar:
Namestart
Digit

Namechars:
Namechar
Namechar Namechars

A Name is a standard D identifier.

LName:
Number Name

Number:
Digit
Digit Number

Digit:
0
1
2
3
4
5
6
7
8
9

An LName is a name preceded by a Number giving the number of characters in the Name.

Back references

Any LName or non-basic Type (i.e. any type that does not encode as a fixed one or two character sequence) that has been emitted to the mangled symbol before will not be emitted again, but is referenced by a special sequence encoding the relative position of the original occurrence in the mangled symbol name.

Numbers in back references are encoded with base 26 by upper case letters A - Z for higher digits but lower case letters a - z for the last digit.

TypeBackRef:
Q NumberBackRef

IdentifierBackRef:
Q NumberBackRef

NumberBackRef:
lower-case-letter
upper-case-letter NumberBackRef

To distinguish between the type of the back reference a look-up of the back referenced character is necessary: An identifier back reference always points to a digit 0 to 9, while a type back reference always points to a letter.

Type Mangling

Types are mangled using a simple linear scheme:

Type:
TypeModifiersopt TypeX
TypeBackRef

TypeX:
TypeArray
TypeStaticArray
TypeAssocArray
TypePointer
TypeFunction
TypeIdent
TypeClass
TypeStruct
TypeEnum
TypeTypedef
TypeDelegate
TypeVoid
TypeByte
TypeUbyte
TypeShort
TypeUshort
TypeInt
TypeUint
TypeLong
TypeUlong
TypeCent
TypeUcent
TypeFloat
TypeDouble
TypeReal
TypeIfloat
TypeIdouble
TypeIreal
TypeCfloat
TypeCdouble
TypeCreal
TypeBool
TypeChar
TypeWchar
TypeDchar
TypeNull
TypeTuple
TypeVector

TypeModifiers:
Const
Wild
Wild Const
Shared
Shared Const
Shared Wild
Shared Wild Const
Immutable

Shared:
O

Const:
x

Immutable:
y

Wild:
Ng

TypeArray:
A Type

TypeStaticArray:
G Number Type

TypeAssocArray:
H Type Type

TypePointer:
P Type

TypeVector:
Nh Type

TypeFunction:
TypeFunctionNoReturn Type

TypeFunctionNoReturn:
CallConvention FuncAttrsopt Parametersopt ParamClose

CallConvention:
F       // D
U       // C
W       // Windows
R       // C++
Y       // Objective-C

FuncAttrs:
FuncAttr
FuncAttr FuncAttrs

FuncAttr:
FuncAttrPure
FuncAttrNothrow
FuncAttrRef
FuncAttrProperty
FuncAttrNogc
FuncAttrReturn
FuncAttrScope
FuncAttrTrusted
FuncAttrSafe
FuncAttrLive

Function attributes are emitted in the order as listed above.

FuncAttrPure:
Na

FuncAttrNogc:
Ni

FuncAttrNothrow:
Nb

FuncAttrProperty:
Nd

FuncAttrRef:
Nc

FuncAttrReturn:
Nj

FuncAttrScope:
Nl

FuncAttrTrusted:
Ne

FuncAttrSafe:
Nf

FuncAttrLive:
Nm

Parameters:
Parameter
Parameter Parameters

Parameter:
Parameter2
M Parameter2     // scope
Nk Parameter2    // return

Parameter2:
Type
I Type     // in
J Type     // out
K Type     // ref
L Type     // lazy

ParamClose:
X     // variadic T t...) style
Y     // variadic T t,...) style
Z     // not variadic

TypeIdent:
I QualifiedName

TypeClass:
C QualifiedName

TypeStruct:
S QualifiedName

TypeEnum:
E QualifiedName

TypeTypedef:
T QualifiedName

TypeDelegate:
D TypeModifiersopt TypeFunction

TypeVoid:
v

TypeByte:
g

TypeUbyte:
h

TypeShort:
s

TypeUshort:
t

TypeInt:
i

TypeUint:
k

TypeLong:
l

TypeUlong:
m

TypeCent:
zi

TypeUcent:
zk

TypeFloat:
f

TypeDouble:
d

TypeReal:
e

TypeIfloat:
o

TypeIdouble:
p

TypeIreal:
j

TypeCfloat:
q

TypeCdouble:
r

TypeCreal:
c

TypeBool:
b

TypeChar:
a

TypeWchar:
u

TypeDchar:
w

TypeNull:
n

TypeTuple:
B Parameters Z

Function Calling Conventions

The extern (C) and extern (D) calling convention matches the C calling convention used by the supported C compiler on the host system. Except that the extern (D) calling convention for Windows x86 is described here.

Register Conventions

- EAX, ECX, EDX are scratch registers and can be destroyed by a function.

- EBX, ESI, EDI, EBP must be preserved across function calls.

- EFLAGS is assumed destroyed across function calls, except for the direction flag which must be forward.

- The FPU stack must be empty when calling a function.

- The FPU control word must be preserved across function calls.

- Floating point return values are returned on the FPU stack. These must be cleaned off by the caller, even if they are not used.

Return Value

- The types bool, byte, ubyte, short, ushort, int, uint, pointer, Object, and interfaces are returned in EAX.

- long and ulong are returned in EDX,EAX, where EDX gets the most significant half.

- float, double, real, ifloat, idouble, ireal are returned in ST0.

- cfloat, cdouble, creal are returned in ST1,ST0 where ST1 is the real part and ST0 is the imaginary part.

- Dynamic arrays are returned with the pointer in EDX and the length in EAX.

- Associative arrays are returned in EAX.

- References are returned as pointers in EAX.

- Delegates are returned with the pointer to the function in EDX and the context pointer in EAX.

- 1, 2 and 4 byte structs and static arrays are returned in EAX.

- 8 byte structs and static arrays are returned in EDX,EAX, where EDX gets the most significant half.

- For other sized structs and static arrays, the return value is stored through a hidden pointer passed as an argument to the function.

- Constructors return the this pointer in EAX.

Parameters

The parameters to the non-variadic function:

foo(a1, a2, ..., an);

are passed as follows:

a1

a2

...

an

hidden

this

where hidden is present if needed to return a struct value, and this is present if needed as the this pointer for a member function or the context pointer for a nested function.

The last parameter is passed in EAX rather than being pushed on the stack if the following conditions are met:

- It fits in EAX.

- It is not a 3 byte struct.

- It is not a floating point type.

Parameters are always pushed as multiples of 4 bytes, rounding upwards, so the stack is always aligned on 4 byte boundaries. They are pushed most significant first. out and ref are passed as pointers. Static arrays are passed as pointers to their first element. On Windows, a real is pushed as a 10 byte quantity, a creal is pushed as a 20 byte quantity. On Linux, a real is pushed as a 12 byte quantity, a creal is pushed as two 12 byte quantities. The extra two bytes of pad occupy the ‘most significant’ position.

The callee cleans the stack.

The parameters to the variadic function:

void foo(int p1, int p2, int[] p3...)
foo(a1, a2, ..., an);

are passed as follows:

p1

p2

a3

hidden

this

The variadic part is converted to a dynamic array and the rest is the same as for non-variadic functions.

The parameters to the variadic function:

void foo(int p1, int p2, ...)
foo(a1, a2, a3, ..., an);

are passed as follows:

an

...

a3

a2

a1

_arguments

hidden

this

The caller is expected to clean the stack. _argptr is not passed, it is computed by the callee.

Exception Handling

Windows

Conforms to the Microsoft Windows Structured Exception Handling conventions.

Linux, FreeBSD and OS X

Uses static address range/handler tables. It is not compatible with the ELF/Mach-O exception handling tables. The stack is walked assuming it uses the EBP/RBP stack frame convention. The EBP/RBP convention must be used for every function that has an associated EH (Exception Handler) table.

For each function that has exception handlers, an EH table entry is generated.

EH Table Entry

field

description

void*

pointer to start of function

DHandlerTable*

pointer to corresponding EH data

uint

size in bytes of the function

The EH table entries are placed into the following special segments, which are concatenated by the linker.

EH Table Segment

Operating System

Segment Name

Windows

FI

Linux

.deh_eh

FreeBSD

.deh_eh

OS X

__deh_eh, __DATA

The rest of the EH data can be placed anywhere, it is immutable.

DHandlerTable

field

description

void*

pointer to start of function

uint

offset of ESP/RSP from EBP/RBP

uint

offset from start of function to return code

uint

number of entries in DHandlerInfo[]

DHandlerInfo[]

array of handler information

DHandlerInfo

field

description

uint

offset from function address to start of guarded section

uint

offset of end of guarded section

int

previous table index

uint

if != 0 offset to DCatchInfo data from start of table

void*

if not null, pointer to finally code to execute

DCatchInfo

field

description

uint

number of entries in DCatchBlock[]

DCatchBlock[]

array of catch information

DCatchBlock

field

description

ClassInfo

catch type

uint

offset from EBP/RBP to catch variable

void*, catch handler code

Garbage Collection

The interface to this is found in Druntime's gc/gcinterface.d.

Runtime Helper Functions

These are found in Druntime's rt/.

Module Initialization and Termination

All the static constructors for a module are aggregated into a single function, and a pointer to that function is inserted into the ctor member of the ModuleInfo instance for that module.

All the static destructors for a module are aggregated into a single function, and a pointer to that function is inserted into the dtor member of the ModuleInfo instance for that module.

Unit Testing

All the unit tests for a module are aggregated into a single function, and a pointer to that function is inserted into the unitTest member of the ModuleInfo instance for that module.

Symbolic Debugging

D has types that are not represented in existing C or C++ debuggers. These are dynamic arrays, associative arrays, and delegates. Representing these types as structs causes problems because function calling conventions for structs are often different than that for these types, which causes C/C++ debuggers to misrepresent things. For these debuggers, they are represented as a C type which does match the calling conventions for the type.

Types for C Debuggers

D type

C representation

dynamic array

unsigned long long

associative array

void*

delegate

long long

dchar

unsigned long

For debuggers that can be modified to accept new types, the following extensions help them fully support the types.

Codeview Debugger Extensions

The D dchar type is represented by the special primitive type 0x78.

D makes use of the Codeview OEM generic type record indicated by LF_OEM (0x0015). The format is:

Codeview OEM Extensions for D

field size

2

2

2

2

2

2

D Type

Leaf Index

OEM Identifier

recOEM

num indices

type index

type index

dynamic array

LF_OEM

OEM

1

2

@index

@element

associative array

LF_OEM

OEM

2

2

@key

@element

delegate

LF_OEM

OEM

3

2

@this

@function

where:

OEM

0x42

index

type index of array index

key

type index of key

element

type index of array element

this

type index of context pointer

function

type index of function

These extensions can be pretty-printed by obj2asm. The Ddbg debugger supports them.

Vector Extensions

CPUs often support specialized vector types and vector operations (a.k.a. media instructions). Vector types are a fixed array of floating or integer types, and vector operations operate simultaneously on them.

Specialized Vector types provide access to them.

The VectorBaseType must be a Static Array. The VectorElementType is the unqualified element type of the static array. The dimension of the static array is the number of elements in the vector.

Implementation Defined: Which vector types are supported depends on the target. The implementation is expected to only support the vector types that are implemented in the target's hardware.

Best Practices: Use the declarations in core.simd instead of the language Vector grammar.

core.simd

Vector types and operations are introduced by importing core.simd:

import core.simd;

Implementation Defined:

These types and operations will be the ones defined for the architecture the compiler is targeting. If a particular CPU family has varying support for vector types, an additional runtime check may be necessary. The compiler does not emit runtime checks; those must be done by the programmer.

Depending on the architecture, compiler flags may be required to activate support for SIMD types.

The types defined will all follow the naming convention:

typeNN

where type is the vector element type and NN is the number of those elements in the vector type. The type names will not be keywords.
Properties

Vector types have the property:

Vector Type Properties

Property

Description

.array

Returns static array representation

Vectors support the following properties based on the vector element type. The value produced is that of a vector of the same type with each element set to the value corresponding to the property value for the element type.

Integral Vector Type Properties

Property

Description

.min

minimum value

.max

maximum value

Floating Point Vector Type Properties

Property

Description

.epsilon

smallest increment to the value 1

.infinity

infinity value

.max

largest representable value that is not infinity

.min_normal

smallest representable value that is not 0

.nan

NaN value

Conversions

Vector types of the same size can be implicitly converted among each other, this is done as a reinterpret cast (a type paint). Vector types can be cast to their VectorBaseType.

Integers and floating point values can be implicitly converted to their vector equivalents:

int4 v = 7;
v = 3 + v;   // add 3 to each element in v

Accessing Individual Vector Elements

They cannot be accessed directly, but can be when converted to an array type:

int4 v;
(cast(int*)&v)[3] = 2;   // set 3rd element of the 4 int vector
(cast(int[4])v)[3] = 2;  // set 3rd element of the 4 int vector
v.array[3] = 2;          // set 3rd element of the 4 int vector
v.ptr[3] = 2;            // set 3rd element of the 4 int vector

Conditional Compilation

If vector extensions are implemented, the version identifier D_SIMD is set.

Whether a type exists or not can be tested at compile time with an IsExpression:

static if (is(typeNN))
... yes, it is supported ...
else
... nope, use workaround ...

Whether a particular operation on a type is supported can be tested at compile time with:

float4 a,b;
static if (__traits(compiles, a+b))
... yes, it is supported ...
else
... nope, use workaround ...

For runtime testing to see if certain vector instructions are available, see the functions in core.cpuid.

A typical workaround would be to use array vector operations instead:

float4 a,b;
static if (__traits(compiles, a/b))
c = a / b;
else
c[] = a[] / b[];

X86 And X86_64 Vector Extension Implementation

Implementation Defined:

The following describes the specific implementation of the vector types for the X86 and X86_64 architectures.

The vector extensions are currently implemented for the OS X 32 bit target, and all 64 bit targets.

core.simd defines the following types:

Vector Types

Type Name

Description

gcc Equivalent

void16

16 bytes of untyped data

no equivalent

byte16

16 bytes

signed char __attribute__((vector_size(16)))

ubyte16

16 ubytes

unsigned char __attribute__((vector_size(16)))

short8

8 shorts

short __attribute__((vector_size(16)))

ushort8

8 ushorts

ushort __attribute__((vector_size(16)))

int4

4 ints

int __attribute__((vector_size(16)))

uint4

4 uints

unsigned __attribute__((vector_size(16)))

long2

2 longs

long __attribute__((vector_size(16)))

ulong2

2 ulongs

unsigned long __attribute__((vector_size(16)))

float4

4 floats

float __attribute__((vector_size(16)))

double2

2 doubles

double __attribute__((vector_size(16)))

void32

32 bytes of untyped data

no equivalent

byte32

32 bytes

signed char __attribute__((vector_size(32)))

ubyte32

32 ubytes

unsigned char __attribute__((vector_size(32)))

short16

16 shorts

short __attribute__((vector_size(32)))

ushort16

16 ushorts

ushort __attribute__((vector_size(32)))

int8

8 ints

int __attribute__((vector_size(32)))

uint8

8 uints

unsigned __attribute__((vector_size(32)))

long4

4 longs

long __attribute__((vector_size(32)))

ulong4

4 ulongs

unsigned long __attribute__((vector_size(32)))

float8

8 floats

float __attribute__((vector_size(32)))

double4

4 doubles

double __attribute__((vector_size(32)))

Note: for 32 bit gcc, it's long long instead of long.

Supported 128-bit Vector Operators

Operator

void16

byte16

ubyte16

short8

ushort8

int4

uint4

long2

ulong2

float4

double2

=

×

×

×

×

×

×

×

×

×

×

×

+

–

×

×

×

×

×

×

×

×

×

×

-

–

×

×

×

×

×

×

×

×

×

×

*

–

–

–

×

×

–

–

–

–

×

×

/

–

–

–

–

–

–

–

–

–

×

×

&

–

×

×

×

×

×

×

×

×

–

–

|

–

×

×

×

×

×

×

×

×

–

–

^

–

×

×

×

×

×

×

×

×

–

–

+=

–

×

×

×

×

×

×

×

×

×

×

-=

–

×

×

×

×

×

×

×

×

×

×

*=

–

–

–

×

×

–

–

–

–

×

×

/=

–

–

–

–

–

–

–

–

–

×

×

&=

–

×

×

×

×

×

×

×

×

–

–

|=

–

×

×

×

×

×

×

×

×

–

–

^=

–

×

×

×

×

×

×

×

×

–

–

unary~

–

×

×

×

×

×

×

×

×

–

–

unary+

–

×

×

×

×

×

×

×

×

×

×

unary-

–

×

×

×

×

×

×

×

×

×

×

Supported 256-bit Vector Operators

Operator

void32

byte32

ubyte32

short16

ushort16

int8

uint8

long4

ulong4

float8

double4

=

×

×

×

×

×

×

×

×

×

×

×

+

–

×

×

×

×

×

×

×

×

×

×

-

–

×

×

×

×

×

×

×

×

×

×

*

–

–

–

–

–

–

–

–

–

×

×

/

–

–

–

–

–

–

–

–

–

×

×

&

–

×

×

×

×

×

×

×

×

–

–

|

–

×

×

×

×

×

×

×

×

–

–

^

–

×

×

×

×

×

×

×

×

–

–

+=

–

×

×

×

×

×

×

×

×

×

×

-=

–

×

×

×

×

×

×

×

×

×

×

*=

–

–

–

–

–

–

–

–

–

×

×

/=

–

–

–

–

–

–

–

–

–

×

×

&=

–

×

×

×

×

×

×

×

×

–

–

|=

–

×

×

×

×

×

×

×

×

–

–

^=

–

×

×

×

×

×

×

×

×

–

–

unary~

–

×

×

×

×

×

×

×

×

–

–

unary+

–

×

×

×

×

×

×

×

×

×

×

unary-

–

×

×

×

×

×

×

×

×

×

×

Operators not listed are not supported at all.

Vector Operation Intrinsics

See core.simd for the supported intrinsics.

Better C

It is straightforward to link C functions and libraries into D programs. But linking D functions and libraries into C programs is not straightforward.

D programs generally require:

1. The D runtime library to be linked in, because many features of the core language require runtime library support.

2. The main() function to be written in D, to ensure that the required runtime library support is properly initialized.

To link D functions and libraries into C programs, it's necessary to only require the C runtime library to be linked in. This is accomplished by defining a subset of D that fits this requirement, called BetterC.

Implementation Defined: BetterC is typically enabled by setting the -betterC command line flag for the implementation.

When BetterC is enabled, the predefined version D_BetterC can be used for conditional compilation.

An entire program can be written in BetterC by supplying a C main() function:

extern(C) void main()
{
import core.stdc.stdio : printf;
printf("Hello betterC\n");
}

> dmd -betterC hello.d && ./hello
Hello betterC

Limiting a program to this subset of runtime features is useful when targeting constrained environments where the use of such features is not practical or possible.

BetterC makes embedding D libraries in existing larger projects easier by:

1. Simplifying the process of integration at the build-system level

2. Removing the need to ensure that Druntime is properly initialized on calls to the library, for situations when an initialization step is not performed or would be difficult to insert before the library is used.

3. Mixing memory management strategies (GC + manual memory management) can be tricky, hence removing D's GC from the equation may be worthwhile sometimes.

BetterC and ImportC are very different. ImportC is an actual C compiler. BetterC is a subset of D that relies only on the existence of the C Standard library.

Retained Features

Nearly the full language remains available. Highlights include:

1. Unrestricted use of compile-time features

2. Full metaprogramming facilities

3. Nested functions, nested structs, delegates and lambdas

4. Member functions, constructors, destructors, operating overloading, etc.

5. The full module system

6. Array slicing, and array bounds checking

7. RAII (yes, it can work without exceptions)

8. scope(exit)

9. Memory safety protections

10. Interfacing with C++

11. COM classes and C++ classes

12. assert failures are directed to the C runtime library

13. switch with strings

14. final switch

15. unittest

16. printf format validation

Running unittests in -betterC
While, testing can be done without the -betterC flag, it is sometimes desirable to run the testsuite in -betterC too. unittest blocks can be listed with the getUnitTests trait:

unittest
{
assert(0);
}

extern(C) void main()
{
static foreach(u; __traits(getUnitTests, __traits(parent, main)))
u();
}

> dmd -betterC -unittest -run test.d
dmd_runpezoXK: foo.d:3: Assertion `0' failed.

However, in -betterC assert expressions don't use Druntime's assert and are directed to assert of the C runtime library instead.
Unavailable Features

D features not available with BetterC:

1. Garbage Collection

2. TypeInfo and ModuleInfo

3. Classes

4. Built-in threading (e.g. core.thread)

5. Dynamic arrays (though slices of static arrays work) and associative arrays

6. Exceptions

7. synchronized and core.sync

8. Static module constructors or destructors

ImportC

Note: This document is under construction. Please excuse the dust and noise.

ImportC is a C compiler embedded into the D implementation. Its purpose is to enable direct importation of C files, without needing to manually prepare a D file corresponding to the the declarations in the C file. It enables directly compiling C files into modules, and linking them in with D code to form an executable. It can be used as a C compiler to compile and link 100% C programs.

This is not a C reference manual nor programming tutorial. It describes the specifics of the dialect of C that ImportC is, and how to use it effectively.

ImportC and BetterC are very different. ImportC is an actual C compiler. BetterC is a subset of D that relies only on the existence of the C Standard library.

ImportC Dialect

There are many versions of C. ImportC is designed to be an implementation of ISO/IEC 9899:2011, which will hereafter be referred to as C11. References to the C11 Standard will be C11 followed by the paragraph number. Prior versions, such as C99, C89, and K+R C, are not supported.

Adjustment to the ImportC dialect is made to match the behavior of the C compiler that the D compiler is matched to. For example, on Win32 D is matched to the Digital Mars C compiler, and can be matched to the Visual C compiler using the -m32mscoff switch. Win64 D is matched to the Visual C compiler. On Posix targets, the matching C compiler is Gnu C or Clang C.

Further adjustment is made to take advantage of some of the D implementation's capabilities.

This is all covered in the rest of this document.

Invoking ImportC

The ImportC compiler can be invoked in two ways:

- directly via the command line

- indirectly via importing a C file

ImportC Files on the Command Line

Importing C Files

Preprocessor

ImportC does not have a preprocessor. It is designed to compile C files after they have been first run through the C preprocessor.

Digital Mars C Preprocessor sppn.exe

sppn.exe runs on Win32 and is invoked as:

sppn file.c

and the preprocessed output is written to file.i.

Gnu C Preprocessor

The Gnu C Preprocessor can be invoked as:

gcc -E file.c > file.i

Clang C Preprocessor

The Clang Preprocessor can be invoked as:

clang -E file.c > file.i

dmpp C Preprocessor

The dmpp C Preprocessor can be invoked as:

dmpp file.c

and the preprocessed output is written to file.i.

Predefined Macros

ImportC does not predefine any macros.

Best Practices: To distinguish an ImportC compile vs some other C compiler, define __IMPORTC__ to be 1 when the preprocessor is run.

Preprocessor Directives

Nevertheless, ImportC supports these preprocessor directives:

Line control

C11 6.10.4

Linemarker

linemarker directives are normally embedded in the output of C preprocessors.

Limitations

Exception Handling

ImportC is assumed to never throw exceptions. setjmp and longjmp are not supported.

Const

C11 specifies that const only applies locally. const in ImportC applies transitively, meaning that although

int *const p;

means in C11 that p is a const pointer to int, in ImportC it means p is a const pointer to a const int.
Volatile

The volatile type-qualifier (C11 6.7.3) is ignored. Use of volatile to implement shared memory access is unlikely to work anyway, _Atomic is for that. To use volatile as a device register, call a function to do it that is compiled separately, or use inline assembler.

Restrict

The restrict type-qualifier (C11 6.7.3) is ignored.

_Atomic

The _Atomic type-qualifier (C11 6.7.3) is ignored. To do atomic operations, use an externally compiled function for that, or the inline assembler.

Compatible Types

Compatible Types (C11 6.7.2) are identical types in ImportC.

Same only Different Types

On some platforms, C long and unsigned long are the same size as int and unsigned int, respectively. On other platforms, C long and unsigned long are the same size as long long and unsigned long long. long double and long double _Complex can be same size as double and double _Complex. In ImportC, these types that are the same size and signed-ness are treated as the same types.

_Generic

Generic selection expressions (C11 6.5.1.1) differ from ImportC. The types in Same only Different Types are indistinguishable in the type-name parts of generic-association. Instead of giving an error for duplicate types per C11 6.5.1.1-2, ImportC will select the first compatible type-name in the generic-assoc-list.

Extensions

Forward References

Any declarations in scope can be accessed, not just declarations that lexically precede a reference.

Compile Time Function Execution

Evaluating constant expressions includes executing functions in the same manner as D's CTFE can.

Function Inlining

Functions for which the function body is present can be inlined by ImportC as well as by the D code that calls them.

Register Storage Class

Objects with register storage class are treated as auto declarations.

Objects with register storage class may have their address taken. C11 6.3.2.1-2

Arrays can have register storage class, and may be enregistered by the compiler. C11 6.3.2.1-3

Gnu and Clang Extensions

gcc and clang are presumed to have the same behavior w.r.t. extensions, so gcc as used here refers to both.

__attribute__((noreturn))

__attribute__((noreturn)) marks a function as never returning. gcc set this as an attribute of the function, it is not part of the function's type. In D, a function that never returns has the return type noreturn. The difference can be seen with the code:

attribute((noreturn)) int foo();
size_t x = sizeof(foo());

This code is accepted by gcc, but makes no sense for D. Hence, although it works in ImportC, it is not representable as D code, meaning one must use judgement in creating a .di file to interface with C noreturn functions.

Furthermore, the D compiler takes advantage of noreturn functions by issuing compile time errors for unreachable code. Such unreachable code, however, is valid C11, and the ImportC compiler will accept it.

Best Practices: C code that uses the noreturn attribute should at the very least set the return type to void.

Visual C Extensions

Digital Mars C Extensions

ImportC from D's Point of View

There is no one-to-one mapping of C constructs to D constructs, although it is very close. What follows is a description of how the D side views the C declarations that are imported.

Module Name

The module name assigned to the ImportC file is the filename stripped of its path and extension. This is just like the default module name assigned to a D module that does not have a module declaration.

extern (C)

All C symbols are extern (C).

Enums

The C enum:

enum E { A, B = 2 };

appears to D code as:

enum E : int { A, B = 2 }
alias A = E.A;
alias B = E.B;

The .min and .max properties are available:

static assert(E.min == 0 && E.max == 2);

Wrapping C Code

Many difficulties with adapting C code to ImportC can be done without editing the C code itself. Wrap the C code in another C file and then

#include

it. Consider the following problematic C file file.c:

void func(int *__restrict p);
int S;
struct S { int a, b; };

The problems are that

__restrict

is not a type qualifier recognized by ImportC (or C11), and the struct S is hidden from D by the declaration

int S;

. To wrap file.c with a fix, create the file file_ic.c with the contents:

#define __restrict restrict
#include "file.c"
typedef struct S S_t;

Then, import file_ic; instead of import file;, and use S_t when

struct S

is desired.
Warnings

Many suspicious C constructs normally cause warnings to be emitted by default by typical compilers, such as:

int *p = 3; // Warning: integer implicitly converted to pointer

ImportC does not emit warnings. The presumption is the user will be importing existing C code developed using another C compiler, and it is written as intended. If C11 says it is legal, ImportC accepts it.

ImportC++

ImportC will not compile C++ code. For that, use dpp.

Other Solutions

dpp by Atila Neves

dpp code

dpp Article

From the Article:

dpp is a compiler wrapper that will parse a D source file with the .dpp extension and expand in place any #include directives it encounters, translating all of the C or C++ symbols to D, and then pass the result to a D compiler (DMD by default).

Like DStep, dpp relies on libclang.

DStep by Jacob Carlborg

DStep code

DStep Article

From the Article:

DStep is a tool for automatically generating D bindings for C and Objective-C libraries. This is implemented by processing C or Objective-C header files and outputting D modules. DStep uses the Clang compiler as a library (libclang) to process the header files.

htod by Walter Bright

htod converts a C .h file to a D source file, suitable for importing into D code. htod is built from the front end of the Digital Mars C and C++ compiler. It works just like a C or C++ compiler except that its output is source code for a D module rather than object code.

How ImportC Works

This is a description of how ImportC is implemented, intended to remove the mystery of various design choices.

Live Functions
Experimental, Subject to Change
If a memory object has only one pointer to it, that pointer is the owner of the memory object. With the single owner, it becomes straightforward to manage the memory for the object. It also becomes trivial to synchronize access to that memory object among multiple threads, because it can only be accessed by the thread that controls that single pointer.

This can be generalized to a graph of memory objects interconnected by pointers, where only a single pointer connects to that graph from elsewhere. That single pointer becomes the owner of all the memory objects in that graph.

When the owner of the graph is no longer needed, then the graph of memory objects it points to is no longer needed and can be safely disposed of. If the owner itself is no longer in use (i.e. is no longer live) and the owned memory objects are not disposed of, an error can be diagnosed.

Hence, the following errors can be statically detected:

int* allocate();    // allocate a memory object
void release(int*); // deallocate a memory object

@live void test()
{
auto p = allocate();
}   // error: p is not disposed of

@live void test()
{
auto p = allocate();
release(p);
release(p); // error: p was already disposed of
}

@live void test()
{
int* p = void;
release(p); // error, p does not have a defined value
}

@live void test()
{
auto p = allocate();
p = allocate(); // error: p was not disposed of
release(p);
}

Functions with the @live attribute enable diagnosing these sorts of errors by tracking the status of owner pointers.

Ownership/Borrowing

Tracking the ownership status of a pointer can be safely extended by adding the capability of temporarilly borrowing ownership of a pointer from the owner. The owner can no longer use the pointer as long as the borrower is still using the pointer value (i.e. is live). Once the borrower is no longer live, the owner and resume using it. Only one borrower can be live at any point.

Multiple borrower pointers can simultaneously exist if all of them are pointers to read only (const or immutable) data, i.e. none of them can modify the memory object(s) pointed to.

This is collectively called an Ownership/Borrowing system. It can be state as:

At any point in the program, for each memory object, there is exactly one live mutable pointer to it or all the live pointers to it are read-only.

Function declarations annotated with the @live attribute are checked for compliance with the Ownership/Borrowing rules. The checks are run after other semantic processing is complete. The checking does not influence code generation.

Whether a pointer is allocated memory using the GC or some other storage allocator is immaterial to OB, they are not distinguished and are handled identically.

Class references are assumed to be allocated using either the GC or are allocated on the stack as scope classes, and are not tracked.

If @live functions call non-@live functions, those called functions are expected to present an @live compatible interface, although it is not checked. if non-@live functions call @live functions, arguments passed are expected to follow @live conventions.

It will not detect attempts to dereference null pointers or possibly null pointers. This is unworkable because there is no current method of annotating a type as a non-null pointer.

Tracked Pointers

The only pointers that are tracked are those declared in the @live function as this, function parameters or local variables. Variables from other functions are not tracked, even @live ones, as the analysis of interactions with other functions depends entirely on that function signature, not its internals. Parameters that are const are not tracked.

Pointer States

Each tracked pointer is in one of the following states:

Undefined

The pointer is in an invalid state. Dereferencing such a pointer is an error.

Owner

The owner is the sole pointer to a memory object graph. An Owner pointer normally does not have a scope attribute. If a pointer with the scope attribute is initialized with an expression not derived from a tracked pointer, it is an Owner. If an Owner pointer is assigned to another Owner pointer, the former enters the Undefined state.

Borrowed

A Borrowed pointer is one that temporarily becomes the sole pointer to a memory object graph. It enters that state via assignment from an owner pointer, and stays in that state until its last use. A Borrowed pointer must have the scope attribute and must be a pointer to mutable.

Readonly

A Readonly pointer acquires its value from an Owner. While the Readonly pointer is live, only Readonly pointers can be acquired from that Owner. A Readonly pointer must have the scope attribute and also must not be a pointer to mutable.

Lifetimes

The lifetime of a Borrowed or Readonly pointer value starts when it is assigned a value from an Owner or another Borrowed pointer, and ends at the last read of that value.

This is also known as Non-Lexical Lifetimes.

Pointer State Transitions

A pointer changes its state when one of these operations is done to it:

- storage is allocated for it (such as a local variable on the stack), which places the pointer in the Undefined state

- initialization (treated as assignment)

- assignment - the source and target pointers change state based on what states they are in and their types and storage classes

- passed to an out function parameter (changes state after the function returns), treated the same as initialization

- passed by ref to a function parameter, treated as an assignment to a Borrow or a Readonly depending on the storage class and type of the parameter

- returned from a function

- it is passed by value to a function parameter, which is treated as an assignment to that parameter.

- it is implicitly passed by ref as a closure variable to a nested function

- the address of the pointer is taken, which is treated as assignment to whoever receives the address

- the address of any part of the memory object graph is taken, which is treated as assignment to whoever receives that address

- a pointer value is read from any part of the memory object graph, which is treated as assignment to whoever receives that pointer

- merging of control flow reconciles the state of each variable based on the states they have from each edge

Borrowers can be Owners

Borrowers are considered Owners if they are initialized from other than a pointer.

@live void uhoh()
{
scope p = malloc();  // p is considered an Owner
scope const pc = malloc(); // pc is not considered an Owner
} // dangling pointer pc is not detected on exit

Exceptions

The analysis assumes no exceptions are thrown.

@live void leaky()
{
auto p = malloc();
pitcher();  // throws exception, p leaks
free(p);
}

One solution is to use scope(exit):

@live void waterTight()
{
auto p = malloc();
scope(exit) free(p);
pitcher();
}

or use RAII objects or call only nothrow functions.

Lazy Parameters

Lazy parameters are not considered.

Mixing Memory Pools

Conflation of different memory pools:

void* xmalloc(size_t);
void xfree(void*);

void* ymalloc(size_t);
void yfree(void*);

auto p = xmalloc(20);
yfree(p);  // should call xfree() instead

is not detected.

This can be mitigated by using type-specific pools:

U* umalloc();
void ufree(U*);

V* vmalloc();
void vfree(V*);

auto p = umalloc();
vfree(p);  // type mismatch

and perhaps disabling implicit conversions to void* in @live functions.

Variadic Function Arguments

Arguments to variadic functions (such as printf) are considered to be consumed.