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




Variable Declarations

    StorageClassesopt BasicType TypeSuffixesopt IdentifierInitializers ;
IdentifierInitializers: IdentifierInitializer IdentifierInitializer , IdentifierInitializers
IdentifierInitializer: Identifier Identifier TemplateParametersopt = Initializer
Declarator: TypeSuffixesopt Identifier

See also:

Storage Classes

See Type Classes vs. Storage Classes.

    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

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
  • In D types are straightforward to read from right to left, unlike in C where parentheses are sometimes required and the type is read iteratively using the clockwise/spiral rule.
  • For a C function pointer declaration a (*b)(c); a C parser needs to attempt a type lookup in order to parse it unambiguously - it could be a call to a function called a which returns a function pointer, which is immediately called. D function pointer syntax is unambiguous, avoiding the need for types to be forward declared.

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


When no Initializer is given, a variable is set to the default .init value for its type.


NonVoidInitializer: ArrayInitializer StructInitializer AssignExpression

A variable can be initialized with a NonVoidInitializer. An ArrayInitializer or StructInitializer takes precedence over an expression initializer.

struct S { int i; }

S s = {}; // struct initializer, not a function literal expression
S[] a = [{2}]; // array initializer holding a struct initializer
//a = [{2}]; // invalid array literal expression

Void Initialization


Normally a variable will be initialized. However, if a variable initializer is void, 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.

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 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.

Implicit Type Inference

    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]

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.

Alias Declarations

Note: New code should use the AliasAssignments form only.
    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:

Type Aliases

alias myint =;

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 =; // is it a type or a symbol?
Best Practices: Other than when aliasing simple basic type names, type alias names should be Capitalized.

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 =;
version (linux)
    alias myfoo =;

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 =;

void main()
    import std.stdio;

    C c = new C();, 2u).writeln;   // calls;       // calls

Aliasing Variables

Variables can be aliased, expressions cannot:

int i = 0;
alias a = i; // OK
alias b = a; // alias a variable alias
assert(i == 2);

//alias c = i * 2; // error
//alias d = i + i; // error

Members of an aggregate can be aliased, however non-static field aliases cannot be accessed outside their parent type.

struct S
    static int i = 0;
    int j;
    alias a = j; // OK

    void inc() { a++; }

alias a = S.i; // OK
assert(S.i == 1);

alias b = S.j; // allowed
static assert(b.offsetof == 0);
//b++;   // error, no instance of S
//S.a++; // error, no instance of S

S s = S(5);;
assert(s.j == 6);
//alias c = s.j; // scheduled for deprecation

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

    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
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.
import std.meta : AliasSeq;

static if (0) // recursive method for comparison
    template Reverse(T...)
        static if (T.length == 0)
            alias Reverse = AliasSeq!();
            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

    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.

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.

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;
    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).
    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;
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; }