11 Classes [class]

11.4 Class members [class.mem]

11.4.9 Static members [class.static] General [class.static.general]

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 ([expr.ref]) 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 1: 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 ([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 2: int g(); struct X { static int g(); }; struct Y : X { static int i; }; int Y::i = g(); // equivalent to Y​::​g(); — end example]
Static members obey the usual class member access rules ([class.access]).
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 1:
It cannot be specified in member declarations that appear in namespace scope.
— end note]