C++ Primer, Fifth Edition

Virtual Functions in a Derived Class

When a derived class overrides a virtual function, it may, but is not required to, repeat the virtual keyword. Once a function is declared as virtual, it remains virtual in all the derived classes.

A derived-class function that overrides an inherited virtual function must have exactly the same parameter type(s) as the base-class function that it overrides.

With one exception, the return type of a virtual in the derived class also must match the return type of the function from the base class. The exception applies to virtuals that return a reference (or pointer) to types that are themselves related by inheritance. That is, if D is derived from B, then a base class virtual can return a B* and the version in ...