Question

In C++, I have a class FooA and a class FooB that both are child classes of a base class Foo. FooB has to implement interface Bar. If I only have a Foo pointer to a FooB and wish to use FooB’s Bar methods, I would have to do a cast. However, that seems to violate polymorphism.

I’m wondering if there’s a better approach to allow FooB to implement the Bar interface without forcing all Foo derived classes to also implement Bar.

Answer 1

Redesign your classes. Chances are inheritance isn’t what you need here.

Rationale: If you have a Foo pointer somewhere, then its owner shouldn’t need to know whether it points to Foo object or FooB or any other derived object – it only uses Foo functionality, ignorant of the implementation. (That’s how polymorphism works in general.)

In other words: If you have a Foo pointer and its user depends on functionality only in FooB, then that pointer should be of type FooB in the first place.