Typedef declaration

 

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The typedef declaration provides a way to create an alias that can be used anywhere in place of a (possibly complex) type name.

Syntax

typedef type_declaration;

Explanation

The declaration that follows the keyword typedef is otherwise usual simple declaration (except that other type specifiers, e.g. static, cannot be used). It may declare one or many identifiers on the same line (e.g. int and a pointer to int), it may declare array and function types, pointers and references, class types, etc. Every identifier introduced in this declaration becomes a typedef-name rather than an object that it would become if the keyword typedef was removed.

The typedef-names are aliases for existing types, and are not declarations of new types. Typedef cannot be used to change the meaning of an existing type name (including a typedef-name). Once declared, a typedef-name may only be redeclared to refer to the same type again. Typedef names are only in effect in the scope where they are visible: different functions or class declarations may define identically-named types with different meaning.

Keywords

typedef

Example

// simple typedef
typedef unsigned long ulong;
 
// the following two objects have the same type
unsigned long l1;
ulong l2;
 
// more complicated typedef
typedef int int_t, *intp_t, (&fp)(int, ulong), arr_t[10];
 
// the following two objects have the same type
int a1[10];
arr_t a2;
 
// common C idiom to avoid having to write "struct S"
typedef struct {int a; int b;} S;
 
// the following two objects have the same type
struct {int a; int b;} s1;
S s2;
 
// error: conflicting type specifier
// typedef static unsigned int uint;
 
// std::add_const, like many other metafunctions, use member typedefs
template< class T>
struct add_const {
    typedef const T type;
};

See also

type aliases provide the same functionality as typedefs using a different syntax, and are also applicable to template names.