typedef specifier

< cpp‎ | language
  • typedef - creates an alias that can be used anywhere in place of a (possibly complex) type name.


[edit] Explanation

The typedef specifier, when used in a declaration's decl-specifier-seq, specifies that the declaration is a typedef declaration, and declares typedef-names rather than functions or objects. 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, which is a synonym for the type of the object or function that it would become if the keyword typedef were removed.

The typedef specifier cannot be combined with any other specifier except for type-specifiers.

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.

The typedef specifier may not appear in a declaration that does not contain a declarator.

typedef struct X {}; // ill-formed
(since C++17)

[edit] Keywords


[edit] Notes

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

[edit] 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, *pS;
// the following two objects have the same type
pS ps1;
S* ps2;
// error: storage-class-specifier cannot appear in a typedef declaration
// typedef static unsigned int uint;
// typedef can be used anywhere in the decl-specifier-seq
long unsigned typedef int long ullong;
// more conventionally spelled "typedef unsigned long long int ullong;"
// std::add_const, like many other metafunctions, use member typedefs
template< class T>
struct add_const {
    typedef const T type;
typedef struct Node {
    struct listNode* next; // declares a new (incomplete) struct type named listNode
} listNode; // error: conflicts with the previously declared struct name

[edit] See also

C documentation for Typedef declaration