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. It has the same semantics as if it were introduced by the typedef specifier. In particular, it does not define a new type and it shall not appear in the type-id. [ Example:
using handler_t = void (*)(int); extern handler_t ignore; extern void (*ignore)(int); using cell = pair<void*, cell*>;
— end example ]
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; typedef struct B B; typedef A A; };
— 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; }
struct S { };
— 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;
— 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 { /* ... */ };
— 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;
— end example ]
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