The use of the binary form of the scope resolution operator is not usually necessary within the class definition itself or when used with member selectors to call member functions. Take a look at this continuation of the lesson example:

widgets  w;
gizmos   g;

g.f();
w.f();
g.gizmos::f();  //legal and redundant
g.widgets::f(); //illegal widgets::f() cannot
                //act on a gizmo