operator== with double dispatch in C++

Question

How should one implement

operator==(const Base& base)

to compare subclasses s.t. the calls would be properly dispatched when called as

Base* base1 = new Derived1();
Base* base2 = new Derived2();
base1->operator==(*base2)?

Answer 1

1. Implement operator== as a free standing function.
2. Have it call a virtual method on one of the arguments (e.g. IsEqual())

That gets you to the point where you have

Derived1::IsEqual(const Base& base)

Called. From here you have some options

1. Use RTTI to dynamic_cast<> base to Derived1

2. If the number of derived is small and finite, you can implement

   virtual bool Base::IsEqualToDerived(const Derived1& d) {return false};
   virtual bool Base::IsEqualToDerived(const Derived2& d) {return false};
   

as virtual methods. In Derived1, you override and compare for real.

Answer 2

This seems like a generic C++ question on classes and type rather than a specific question on the operator== . Up to my knowledge, in the particular example you are giving there is no other way but to use dynamic_cast