10 Declarations [dcl.dcl]

10.1 Specifiers [dcl.spec]

10.1.3 The typedef specifier [dcl.typedef]

Declarations containing the decl-specifier typedef declare identifiers that can be used later for naming fundamental or compound types. The typedef specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier, and it shall not be used in the decl-specifier-seq of a parameter-declaration nor in the decl-specifier-seq of a function-definition ([dcl.fct.def]). If a typedef specifier appears in a declaration without a declarator, the program is ill-formed.


A name declared with the typedef specifier becomes a typedef-name. Within the scope of its declaration, a typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier in the way described in Clause [dcl.decl]. A typedef-name is thus a synonym for another type. A typedef-name does not introduce a new type the way a class declaration ([class.name]) or enum declaration does. [Example: After

typedef int MILES, *KLICKSP;

the constructions

MILES distance;
extern KLICKSP metricp;

are all correct declarations; the type of distance is int and that of metricp is “pointer to int”. end example]

A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. Such a typedef-name has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type. [Example:

using handler_t = void (*)(int);
extern handler_t ignore;
extern void (*ignore)(int);         // redeclare ignore
using cell = pair<void*, cell*>;    // ill-formed

end example] The defining-type-specifier-seq of the defining-type-id shall not define a class or enumeration if the alias-declaration is the declaration of a template-declaration.

In a given non-class scope, a typedef specifier can be used to redefine the name of any type declared in that scope to refer to the type to which it already refers. [Example:

typedef struct s { /* ... */ } s;
typedef int I;
typedef int I;
typedef I I;

end example]

In a given class scope, a typedef specifier can be used to redefine any class-name declared in that scope that is not also a typedef-name to refer to the type to which it already refers. [Example:

struct S {
  typedef struct A { } A;       // OK
  typedef struct B B;           // OK
  typedef A A;                  // error

end example]

If a typedef specifier is used to redefine in a given scope an entity that can be referenced using an elaborated-type-specifier, the entity can continue to be referenced by an elaborated-type-specifier or as an enumeration or class name in an enumeration or class definition respectively. [Example:

struct S;
typedef struct S S;
int main() {
  struct S* p;                  // OK
struct S { };                   // OK

end example]

In a given scope, a typedef specifier shall not be used to redefine the name of any type declared in that scope to refer to a different type. [Example:

class complex { /* ... */ };
typedef int complex;            // error: redefinition

end example]

Similarly, in a given scope, a class or enumeration shall not be declared with the same name as a typedef-name that is declared in that scope and refers to a type other than the class or enumeration itself. [Example:

typedef int complex;
class complex { /* ... */ };    // error: redefinition

end example]

[Note: A typedef-name that names a class type, or a cv-qualified version thereof, is also a class-name ([class.name]). If a typedef-name is used to identify the subject of an elaborated-type-specifier, a class definition, a constructor declaration, or a destructor declaration, the program is ill-formed. end note] [Example:

struct S {

typedef struct S T;

S a = T();                      // OK
struct T * p;                   // error

end example]

If the typedef declaration defines an unnamed class (or enum), the first typedef-name declared by the declaration to be that class type (or enum type) is used to denote the class type (or enum type) for linkage purposes only ([basic.link]). [Example:

typedef struct { } *ps, S;      // S is the class name for linkage purposes

end example]