FAQ 16.09 How should static member functions be called?

Explicitly name the class using ::.

For documentation purposes, calls to static member functions should be coded as Classname::staticMember() rather than as object.staticMember() or ptr->staticMember(). The :: is a reminder that the member function is statically bound (see FAQ 21.09) and that the member function is attached to the class rather than to an individual object of the class.

Calling a static member function Classname::f() from another member function of class Classname is an exception to this rule. In this case, the call can be simply f(), since the meaning is usually clear in this context. For example, when Classname::f() is a protected: static member function of the class, simply write f() rather than Classname::f().