9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.3 The typedef specifier [dcl.typedef]

1
#
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.
A typedef-name names the type associated with the identifier ([dcl.decl]) or simple-template-id ([temp.pre]); 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 ([dcl.enum]) 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
]
2
#
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*>;    // error
— 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.
3
#
In a given non-class scope, a typedef specifier can be used to redeclare 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
]
4
#
In a given class scope, a typedef specifier can be used to redeclare 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
]
5
#
If a typedef specifier is used to redeclare 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
]
6
#
In a given scope, a typedef specifier shall not be used to redeclare 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
]
7
#
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
]
8
#
A simple-template-id is only a typedef-name if its template-name names an alias template or a template template-parameter.
[Note
:
A simple-template-id that names a class template specialization is 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 {
  S();
  ~S();
};

typedef struct S T;

S a = T();                      // OK
struct T * p;                   // error
— end example
]
9
#
If the typedef declaration defines an unnamed class or enumeration, the first typedef-name declared by the declaration to be that type is used to denote the type for linkage purposes only ([basic.link]).
[Note
:
A typedef declaration involving a lambda-expression does not itself define the associated closure type, and so the closure type is not given a name for linkage purposes.
— end note
]
[Example
: 
typedef struct { } *ps, S;      // S is the class name for linkage purposes
typedef decltype([]{}) C;       // the closure type has no name for linkage purposes
— end example
]
10
#
An unnamed class with a typedef name for linkage purposes shall not
  • (10.1)
    declare any members other than non-static data members, member enumerations, or member classes,
  • (10.2)
    have any base classes or default member initializers, or
  • (10.3)
    contain a lambda-expression,
and all member classes shall also satisfy these requirements (recursively).
[Example
: 
typedef struct {
  int f() {}
} X;                            // error: struct with typedef name for linkage has member functions
— end example
]