Comparing objects using bool operator==

Question

So, after reading some SO questions and answers, i still doesn\'t understand why use

friend bool operator==( BaseClass const &left, BaseClass const &righ         


        

Answer 1

The biggest problem I see with virtual bool operator==( BaseClass const &right ) is that, without multiple dynamic dispatch*, the following assertion will fail.

class Derived1 : public BaseClass {
    bool operator==( BaseClass const &right ) override {
        return true;
    }
}

class Derived2 : public BaseClass {
    bool operator==( BaseClass const &right ) override {
        return false;
    }
}

Derived1 d1;
Derived2 d2;
assert((d1 == d2) == (d2 == d1));

The friend function can be made to handle this, but to be honest, it's a lot of trouble to get something like this right without multiple dispatch. Even with multiple dispatch, proper semantics for such a polymorphic equality operator may not be easy to come up with.

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_{* multiple dynamic dispatch is the ability to do dynamic dispatch on more than one argument, instead of just on this.}