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)?
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6`operator==` should be implemented as a free-standing functions, otherwise it is not symmetrical in its arguments. – Philipp Jul 28 '10 at 12:25
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3There is a good discussion of double dispatch in More Effective C++. – Jul 28 '10 at 12:27
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It is possible for a `Derived1` to be equal to a `Derived2`? Please give more info. – curiousguy Nov 23 '11 at 08:46
2 Answers
7
- Implement operator== as a free standing function.
- 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
- Use RTTI to dynamic_cast<> base to Derived1
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.
Lou Franco
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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
rodrigob
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2You can (and probably should) do it without dynamic_cast using virtual functions. – Jul 28 '10 at 13:00
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Well it depends is the issue about base1 receiving the proper call or being able to tell that the input argument is Derived2 instead of just Base ? The first will be solved using virtual function (I agree), the second, using dynamic_cast – rodrigob Jul 28 '10 at 13:38