12 Classes [class]

12.2 Class members [class.mem]

12.2.3 Static members [class.static]

A static member s of class X may be referred to using the qualified-id expression X​::​s; it is not necessary to use the class member access syntax to refer to a static member.
A static member may be referred to using the class member access syntax, in which case the object expression is evaluated.
[Example
:
struct process {
  static void reschedule();
};
process& g();

void f() {
  process::reschedule();        // OK: no object necessary
  g().reschedule();             // g() is called
}
end example
]
A static member may be referred to directly in the scope of its class or in the scope of a class derived from its class; in this case, the static member is referred to as if a qualified-id expression was used, with the nested-name-specifier of the qualified-id naming the class scope from which the static member is referenced.
[Example
:
int g();
struct X {
  static int g();
};
struct Y : X {
  static int i;
};
int Y::i = g();                 // equivalent to Y​::​g();
end example
]
If an unqualified-id is used in the definition of a static member following the member's declarator-id, and name lookup finds that the unqualified-id refers to a static member, enumerator, or nested type of the member's class (or of a base class of the member's class), the unqualified-id is transformed into a qualified-id expression in which the nested-name-specifier names the class scope from which the member is referenced.
[Note
:
See [expr.prim] for restrictions on the use of non-static data members and non-static member functions.
end note
]
Static members obey the usual class member access rules.
When used in the declaration of a class member, the static specifier shall only be used in the member declarations that appear within the member-specification of the class definition.
[Note
:
It cannot be specified in member declarations that appear in namespace scope.
end note
]

12.2.3.1 Static member functions [class.static.mfct]

[Note
:
The rules described in [class.mfct] apply to static member functions.
end note
]
[Note
:
A static member function does not have a this pointer.
end note
]
A static member function shall not be virtual.
There shall not be a static and a non-static member function with the same name and the same parameter types ([over.load]).
A static member function shall not be declared const, volatile, or const volatile.

12.2.3.2 Static data members [class.static.data]

A static data member is not part of the subobjects of a class.
If a static data member is declared thread_­local there is one copy of the member per thread.
If a static data member is not declared thread_­local there is one copy of the data member that is shared by all the objects of the class.
The declaration of a non-inline static data member in its class definition is not a definition and may be of an incomplete type other than cv void.
The definition for a static data member that is not defined inline in the class definition shall appear in a namespace scope enclosing the member's class definition.
In the definition at namespace scope, the name of the static data member shall be qualified by its class name using the ​::​ operator.
The initializer expression in the definition of a static data member is in the scope of its class ([basic.scope.class]).
[Example
:
class process {
  static process* run_chain;
  static process* running;
};

process* process::running = get_main();
process* process::run_chain = running;
The static data member run_­chain of class process is defined in global scope; the notation process​::​run_­chain specifies that the member run_­chain is a member of class process and in the scope of class process.
In the static data member definition, the initializer expression refers to the static data member running of class process.
end example
]
[Note
:
Once the static data member has been defined, it exists even if no objects of its class have been created.
[Example
:
In the example above, run_­chain and running exist even if no objects of class process are created by the program.
end example
]
end note
]
If a non-volatile non-inline const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression ([expr.const]).
The member shall still be defined in a namespace scope if it is odr-used in the program and the namespace scope definition shall not contain an initializer.
An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.
If the member is declared with the constexpr specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see [depr.static_constexpr]).
Declarations of other static data members shall not specify a brace-or-equal-initializer.
[Note
:
There shall be exactly one definition of a static data member that is odr-used in a program; no diagnostic is required.
end note
]
Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.
[Note
:
Static data members of a class in namespace scope have the linkage of that class.
A local class cannot have static data members.
end note
]
Static data members are initialized and destroyed exactly like non-local variables ([basic.start.static], [basic.start.dynamic], [basic.start.term]).
A static data member shall not be mutable.