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Last modified Sep 12, 2002.
Declarations
Declaration:
typedef Decl
alias Decl
Decl
Decl:
const Decl
static Decl
final Decl
synchronized Decl
deprecated Decl
BasicType BasicType2 Declarators ;
BasicType BasicType2 FunctionDeclarator
Declarators:
Declarator
Declarator , Declarators
Declaration Syntax
Declaration syntax generally reads left to right:
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, when lexically next to each other, read right to left:
int[3] x; // x is an array of 3 ints
int[3][5] x; // x is an array of 3 arrays of 5 ints
int[3]*[5] x; // x is an array of 5 pointers to arrays of 3 ints
Pointers to functions are declared as subdeclarations:
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
C-style array declarations, where the [] appear to the right of the
identifier, may be used as an alternative:
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
In a declaration declaring multiple declarations, 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
Type Defining
Strong types can be introduced with the typedef. Strong types are semantically a
distinct type to the type checking system, for function overloading, and for the debugger.
typedef int myint;
void foo(int x) { . }
void foo(myint m) { . }
.
myint b;
foo(b); // calls foo(myint)
Typedefs can specify a default initializer different from the
default initializer of the underlying type:
typedef int myint = 7;
myint m; // initialized to 7
Type Aliasing
It's sometimes convenient to use an alias for a type, such as a shorthand for typing
out a long, complex type like a pointer to a function. In D, this is done with the
alias declaration:
alias abc.Foo.bar 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 int myint;
void foo(int x) { . }
void foo(myint m) { . } error, multiply defined function foo
Type aliases are equivalent to the C typedef.
Alias Declarations
A symbol can be declared as an alias of another symbol.
For example:
import string;
alias string.strlen mylen;
...
int len = mylen("hello"); // actually calls string.strlen()
The following alias declarations are valid:
template Foo2(T) { alias T t; }
instance Foo2(int) t1; // a TemplateAliasDeclaration
alias instance Foo2(int).t t2;
alias t1.t t3;
alias t2 t4;
alias instance Foo2(int) t5;
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
t5.t v5; // v5 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 win32.foo myfoo;
}
version (linux)
{
alias linux.bar myfoo;
}
Aliasing can be used to 'import' a symbol from an import into the
current scope:
alias string.strlen strlen;
Note: Type aliases can sometimes look indistinguishable from
alias declarations:
alias foo.bar abc; // is it a type or a symbol?
The distinction is made in the semantic analysis pass.
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