Declarations

Grammar

Declaration can be used inside a function body for a DeclarationStatement, as well as outside a function as it is included in DeclDef.

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

Aggregates

AggregateDeclaration:
    ClassDeclaration
    InterfaceDeclaration
    StructDeclaration
    UnionDeclaration

Variable Declarations

VarDeclarations:
    StorageClassesopt BasicType TypeSuffixesopt IdentifierInitializers ;
    AutoDeclaration

IdentifierInitializers: 
    IdentifierInitializer
    IdentifierInitializer , IdentifierInitializers

IdentifierInitializer: 
    Identifier
    Identifier TemplateParametersopt = Initializer

Declarator: 
    TypeSuffixesopt Identifier

See also:

• Declaration Syntax
• AutoDeclaration
• Variable Templates

Storage Classes

See Type Classes vs. Storage Classes.

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

Initializers

Initializer:
    VoidInitializer
    NonVoidInitializer

NonVoidInitializer:
    AssignExpression
    ArrayLiteral
    StructInitializer

See also VoidInitializer.

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

See Pointers, Arrays and TypeSuffix.

Function Pointers

Function Pointers 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 not supported. The following C declarations are for comparison only:

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

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

This is in contrast to C:

int x, *y;  // x is an int, y is a pointer to int
int x[], y; // x is an array/pointer, y is an int

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 TypeSuffixesopt Identifiers ;
    alias StorageClassesopt BasicType FuncDeclarator ;
    alias AliasAssignments ;

Identifiers:
    Identifier
    Identifier , Identifiers

AliasAssignments:
    AliasAssignment
    AliasAssignments , AliasAssignment

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

An AliasDeclaration creates a symbol name that refers to a type or another symbol. That name can then be used anywhere that the target may appear. The following can be aliased:

• Types
  • Function Types (with default arguments)
• Variables
• Manifest Constants
• Modules
• Packages
• Functions
• Overload Sets
• Function Literals
• Templates
• Template Instantiations
• Other Alias Declarations

Type Aliases

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

Type aliases can sometimes look indistinguishable from other symbol aliases:

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

Symbol Aliases

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

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 imported module or package into the current scope:

static import string;
...
alias strlen = string.strlen;

Aliasing an Overload Set

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

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

class C : B
{
    // declaring an overload hides any base class overloads
    int foo(int a) { return 3; }
    // redeclare hidden overload
    alias foo = B.foo;
}

void main()
{
    import std.stdio;

    C c = new C();
    c.foo(1, 2u).writeln;   // calls B.foo
    c.foo(1).writeln;       // calls C.foo
}

Aliasing Variables

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`
assert(S.i == 2);
assert(S.j == 4);

Aliasing a Function Type

Function types can be aliased:

alias Fun = int(string);
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));

Type aliases can be used to call a function with different default arguments, change an argument from required to default or vice versa:

import std.stdio : writeln;

void fun(int v = 6)
{
    writeln("v: ", v);
}

void main()
{
    fun();  // prints v: 6

    alias Foo = void function(int=7);
    Foo foo = &fun;
    foo();  // prints v: 7
    foo(8); // prints v: 8
}

import std.stdio : writefln;

void main()
{
    fun(4);          // prints a: 4, b: 6, c: 7

    Bar bar = &fun;
    //bar(4);           // compilation error, because the `Bar` alias
                        // requires an explicit 2nd argument
    bar(4, 5);          // prints a: 4, b: 5, c: 9
    bar(4, 5, 6);       // prints a: 4, b: 5, c: 6

    Baz baz = &fun;
    baz();              // prints a: 2, b: 3, c: 4
}

alias Bar = void function(int, int, int=9);
alias Baz = void function(int=2, int=3, int=4);

void fun(int a, int b = 6, int c = 7)
{
    writefln("a: %d, b: %d, c: %d", a, b, c);
}

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.

import std.meta : AliasSeq;

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.

import std.meta : AliasSeq;

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

enum size(T) = T.sizeof;
static assert(staticMap!(size, char, wchar, dchar) == AliasSeq!(1, 2, 4));

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.

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;

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. Void initializers for variables with a type that may contain unsafe values (such as types with pointers) are not allowed in @safe code.

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

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

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.

Type Qualifiers vs. Storage Classes

Type qualifers and storage classes are distinct concepts.

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

ref Storage Class

A parameter declared with ref is passed by reference:

void func(ref int i)
{
    i++; // modifications to i 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().
}

Methods Returning a Qualified Type

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*
        return null;
    }

    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;
}

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; }
}