9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.4 The typedef specifier [dcl.typedef]

1
#
Declarations containing the decl-specifier typedef declare identifiers that can be used later for naming fundamental ([basic.fundamental]) or compound ([basic.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 ([dcl.fct]) 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 1:
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 is not looked up; it 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 2: 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
#
A simple-template-id is only a typedef-name if its template-name names an alias template or a template template-parameter.
[Note 1:
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 ([dcl.type.elab]), a class definition, a constructor declaration, or a destructor declaration, the program is ill-formed.
— end note]
[Example 3: struct S { S(); ~S(); }; typedef struct S T; S a = T(); // OK struct T * p; // error — end example]
4
#
An unnamed class or enumeration C defined in a typedef declaration has the first typedef-name declared by the declaration to be of type C as its typedef name for linkage purposes ([basic.link]).
[Note 2:
A typedef declaration involving a lambda-expression does not itself define the associated closure type, and so the closure type is not given a typedef name for linkage purposes.
— end note]
[Example 4: typedef struct { } *ps, S; // S is the typedef name for linkage purposes typedef decltype([]{}) C; // the closure type has no typedef name for linkage purposes — end example]
5
#
An unnamed class with a typedef name for linkage purposes shall not
  • (5.1)
    declare any members other than non-static data members, member enumerations, or member classes,
  • (5.2)
    have any base classes or default member initializers, or
  • (5.3)
    contain a lambda-expression,
and all member classes shall also satisfy these requirements (recursively).
[Example 5: typedef struct { int f() {} } X; // error: struct with typedef name for linkage has member functions — end example]