9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.2 Storage class specifiers [dcl.stc]

1
#
The storage class specifiers are
storage-class-specifier:
static
thread_­local
extern
mutable
At most one storage-class-specifier shall appear in a given decl-specifier-seq, except that thread_­local may appear with static or extern.
If thread_­local appears in any declaration of a variable it shall be present in all declarations of that entity.
If a storage-class-specifier appears in a decl-specifier-seq, there can be no typedef specifier in the same decl-specifier-seq and the init-declarator-list or member-declarator-list of the declaration shall not be empty (except for an anonymous union declared in a named namespace or in the global namespace, which shall be declared static ([class.union.anon])).
The storage-class-specifier applies to the name declared by each init-declarator in the list and not to any names declared by other specifiers.
[Note 1:
See [temp.expl.spec] and [temp.explicit] for restrictions in explicit specializations and explicit instantiations, respectively.
— end note]
2
#
[Note 2:
A variable declared without a storage-class-specifier at block scope or declared as a function parameter has automatic storage duration by default.
— end note]
3
#
The thread_­local specifier indicates that the named entity has thread storage duration.
It shall be applied only to the declaration of a variable of namespace or block scope, to a structured binding declaration ([dcl.struct.bind]), or to the declaration of a static data member.
When thread_­local is applied to a variable of block scope the storage-class-specifier static is implied if no other storage-class-specifier appears in the decl-specifier-seq.
4
#
The static specifier shall be applied only to the declaration of a variable or function, to a structured binding declaration ([dcl.struct.bind]), or to the declaration of an anonymous union ([class.union.anon]).
There can be no static function declarations within a block, nor any static function parameters.
A static specifier used in the declaration of a variable declares the variable to have static storage duration, unless accompanied by the thread_­local specifier, which declares the variable to have thread storage duration.
A static specifier can be used in declarations of class members; [class.static] describes its effect.
For the linkage of a name declared with a static specifier, see [basic.link].
5
#
The extern specifier shall be applied only to the declaration of a variable or function.
The extern specifier shall not be used in the declaration of a class member or function parameter.
For the linkage of a name declared with an extern specifier, see [basic.link].
[Note 3:
The extern keyword can also be used in explicit-instantiations and linkage-specifications, but it is not a storage-class-specifier in such contexts.
— end note]
6
#
The linkages implied by successive declarations for a given entity shall agree.
That is, within a given scope, each declaration declaring the same variable name or the same overloading of a function name shall imply the same linkage.
[Example 1: static char* f(); // f() has internal linkage char* f() // f() still has internal linkage { /* ... */ } char* g(); // g() has external linkage static char* g() // error: inconsistent linkage { /* ... */ } void h(); inline void h(); // external linkage inline void l(); void l(); // external linkage inline void m(); extern void m(); // external linkage static void n(); inline void n(); // internal linkage static int a; // a has internal linkage int a; // error: two definitions static int b; // b has internal linkage extern int b; // b still has internal linkage int c; // c has external linkage static int c; // error: inconsistent linkage extern int d; // d has external linkage static int d; // error: inconsistent linkage — end example]
7
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The name of a declared but undefined class can be used in an extern declaration.
Such a declaration can only be used in ways that do not require a complete class type.
[Example 2: struct S; extern S a; extern S f(); extern void g(S); void h() { g(a); // error: S is incomplete f(); // error: S is incomplete } — end example]
8
#
The mutable specifier shall appear only in the declaration of a non-static data member whose type is neither const-qualified nor a reference type.
[Example 3: class X { mutable const int* p; // OK mutable int* const q; // error }; — end example]
9
#
[Note 4:
The mutable specifier on a class data member nullifies a const specifier applied to the containing class object and permits modification of the mutable class member even though the rest of the object is const ([basic.type.qualifier], [dcl.type.cv]).
— end note]