Declarations
Declaration: FuncDeclaration VarDeclarations AliasDeclaration 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: BasicType2 Identifier AltDeclaratorSuffixesopt BasicType2 Identifier AltDeclaratorSuffixesopt = Initializer BasicType2opt Identifier AltDeclaratorSuffixes BasicType2opt Identifier AltDeclaratorSuffixes = Initializer Declarator: VarDeclarator AltDeclarator VarDeclarator: BasicType2opt Identifier AltDeclarator: BasicType2opt Identifier AltDeclaratorSuffixes BasicType2opt ( AltDeclaratorX ) BasicType2opt ( AltDeclaratorX ) AltFuncDeclaratorSuffix BasicType2opt ( AltDeclaratorX ) AltDeclaratorSuffixes AltDeclaratorX: BasicType2opt Identifier BasicType2opt Identifier AltFuncDeclaratorSuffix AltDeclarator AltDeclaratorSuffixes: AltDeclaratorSuffix AltDeclaratorSuffix AltDeclaratorSuffixes AltDeclaratorSuffix: [ ] [ AssignExpression ] [ Type ] AltFuncDeclaratorSuffix: Parameters MemberFunctionAttributesopt
Type: TypeCtorsopt BasicType BasicType2opt TypeCtors: TypeCtor TypeCtor TypeCtors TypeCtor: const immutable inout shared BasicType: BasicTypeX . IdentifierList IdentifierList Typeof Typeof . IdentifierList TypeCtor ( Type ) TypeVector BasicTypeX: bool byte ubyte short ushort int uint long ulong char wchar dchar float double real ifloat idouble ireal cfloat cdouble creal void BasicType2: BasicType2X BasicType2opt BasicType2X: * [ ] [ AssignExpression ] [ AssignExpression .. AssignExpression ] [ Type ] delegate Parameters MemberFunctionAttributesopt function Parameters FunctionAttributesopt IdentifierList: Identifier Identifier . IdentifierList TemplateInstance TemplateInstance . IdentifierList Identifier [ AssignExpression ]. 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
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: AutoDeclarationY AutoDeclarationX , AutoDeclarationY AutoDeclarationY: 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 Declarators ; alias StorageClassesopt BasicType FuncDeclarator ; alias AliasDeclarationX ; AliasDeclarationX: AliasDeclarationY AliasDeclarationX , AliasDeclarationY AliasDeclarationY: Identifier TemplateParametersopt = StorageClassesopt Type Identifier TemplateParametersopt = FunctionLiteral
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:
- typeof(this) will generate the type of what this would be in a non-static member function, even if not in a member function.
- Analogously, typeof(super) will generate the type of what super would be in a non-static member function.
- 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.
Type Qualifiers vs. Storage Classes
D draws a distinction between a type qualifer and a storage class.
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 type 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. One example is 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(). }
Due to the fact that some keywords, such as const, can be used both as a type qualifier and a storage class, it may sometimes result in ambiguous-looking code:
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() { return 1; } }
To avoid such confusion, it is recommended that type qualifier syntax with parentheses always be used for return types, and that function storage classes 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; } }