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]
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]