9 Declarations [dcl.dcl]

9.2 Specifiers [dcl.spec]

9.2.1 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
:
See [temp.expl.spec] and [temp.explicit] for restrictions in explicit specializations and explicit instantiations, respectively.
— end note
]
2
#
[Note
:
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
:
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
: 
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
#
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
: 
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
: 
class X {
  mutable const int* p;         // OK
  mutable int* const q;         // error
};
— end example
]
9
#
[Note
:
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
]