17. Covariant virtual functions

• Suppose class A declares member function A::f( ) as a virtual function, and class B derives from A.

• Ordinarily, return types are allowed to vary between overrides of f( ).

• However, for virtual functions the rules are more complex,
  that is, B may override f( ) with a different return type, if:

  1. The function B::f( ) returns a reference or pointer to a class of some type T,
     and A::f( ) returns a pointer or a reference to an unambiguous direct or indirect base class of T.

  2. The const qualification of the pointer or reference returned by B::f( ) has the same,
     or less const qualification of the pointer or reference returned by A::f( ).

  3. The return type of B::f( ) must be complete at the point of declaration of B::f( ),
     or it can be of type B.