Declarations

Declaration:
    AliasDeclaration
    AggregateDeclaration
    EnumDeclaration
    ImportDeclaration
    Decl

Decl:
    StorageClassesopt BasicType Declarators ;
    StorageClassesopt BasicType Declarator FunctionBody
    AutoDeclaration

Declarators:
    DeclaratorInitializer
    DeclaratorInitializer , DeclaratorIdentifierList

DeclaratorInitializer:
    Declarator
    Declarator TemplateParametersopt = Initializer

DeclaratorIdentifierList:
    DeclaratorIdentifier
    DeclaratorIdentifier , DeclaratorIdentifierList

DeclaratorIdentifier:
    Identifier
    Identifier TemplateParametersopt = Initializer

BasicType:
    BasicTypeX
    . IdentifierList
    IdentifierList
    Typeof
    Typeof . IdentifierList
    TypeCtor ( Type )

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

BasicType2:
    *
    [ ]
    [ AssignExpression ]
    [ AssignExpression .. AssignExpression ]
    [ Type ]
    delegate Parameters MemberFunctionAttributesopt
    function Parameters FunctionAttributesopt

Declarator:
    BasicType2opt ( Declarator ) DeclaratorSuffixesopt
    BasicType2opt Identifier DeclaratorSuffixesopt

DeclaratorSuffixes:
    DeclaratorSuffix
    DeclaratorSuffix DeclaratorSuffixes

DeclaratorSuffix:
    [ ]
    [ AssignExpression ]
    [ Type ]
    Parameters MemberFunctionAttributesopt
    TemplateParameters Parameters MemberFunctionAttributesopt Constraintopt

IdentifierList:
    Identifier
    Identifier . IdentifierList
    TemplateInstance
    TemplateInstance . IdentifierList

StorageClasses:
    StorageClass
    StorageClass StorageClasses

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

TypeCtors:
    TypeCtor
    TypeCtor TypeCtors

TypeCtor:
    const
    immutable
    inout
    shared

Type:
    TypeCtorsopt BasicType
    TypeCtorsopt BasicType Declarator2

Declarator2:
    BasicType2opt DeclaratorSuffixesopt
    BasicType2opt ( Declarator2 ) DeclaratorSuffixesopt

Parameters:
    ( ParameterListopt )

ParameterList:
    Parameter
    Parameter , ParameterList
    ...

Parameter:
    InOutopt BasicType Declarator
    InOutopt BasicType Declarator ...
    InOutopt BasicType Declarator = DefaultInitializerExpression
    InOutopt Type
    InOutopt Type ...

InOut:
    InOutX
    InOut InOutX

InOutX:
    auto
    TypeCtor
    final
    in
    lazy
    out
    ref
    scope

FunctionAttributes:
    FunctionAttribute
    FunctionAttribute FunctionAttributes

FunctionAttribute:
    nothrow
    pure
    Property

MemberFunctionAttributes:
    MemberFunctionAttribute
    MemberFunctionAttribute MemberFunctionAttributes

MemberFunctionAttribute:
    const
    immutable
    inout
    shared
    FunctionAttribute

DefaultInitializerExpression:
    AssignExpression
    SpecialKeyword

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:

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

Arrays read right to left as well:

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

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 array, function pointer and pointer to array declarations are deprecated:

int x[3];          // x is an array of 3 ints
int x[3][5];       // x is an array of 3 arrays of 5 ints
int (*x[5])[3];    // x is an array of 5 pointers to 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

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;  // error, multiple types
int[] x,y; // x and y are arrays of ints
int x[],y; // error, multiple types

Implicit Type Inference

AutoDeclaration:
    StorageClasses AutoDeclarationX ;

AutoDeclarationX:
    Identifier = TemplateParametersopt Initializer
    AutoDeclarationX , 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 = "string"; // s is type immutable(char)[]

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 = ["hello", "world"]; // type is string[], not string[2]

Alias Declarations

AliasDeclaration:
    alias StorageClassesopt BasicType Declarator
    alias AliasDeclarationX ;

AliasDeclarationX:
    Identifier TemplateParametersopt = StorageClassesopt Type
    AliasDeclarationX , Identifier TemplateParametersopt = StorageClassesopt Type

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 string;

alias mylen = string.strlen;
 ...
int len = mylen("hello"); // actually calls string.strlen()

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

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; }
S s;

alias a = s.i; // illegal, s.i is an expression
alias b = S.i; // ok
b = 4;         // sets S.i to 4

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. The primary usefulness of this is to connect with global variable declarations in C files.

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

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

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*]

 writefln("%d", typeof('c').sizeof); // prints 1
 double c = cast(typeof(1.0))j; // cast j to double
}

Expression is not evaluated, just the type of it is generated:

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

There are three 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

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

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.

void foo() {
 int x = void;
 writeln(x);  // will print garbage
}

Therefore, one should only use void initializers as a last resort when optimizing critical code.

Global and Static Initializers

The Initializer for a global or static variable must be evaluatable at compile time. Whether some pointers can be initialized with the addresses of other functions or data is implementation defined. Runtime initialization can be done with static constructors.