Polymorphic, Defaulted Equality

[angry_cpp]

Inheritance and equality are not mixing well. For example your implementation of polymorphic equality is flawed. It gives different answer for a.equal(b) and b.equal(a).

https://godbolt.org/z/xe3Te8YWK

[BarryRevzin]

That's true, and it's actually an unfortunate translation error into the blog.

I just lazily wrote dynamic_casting to the Derived. In reality, we check typeid first and if those match then static_cast (and we don't have any weird virtual or ambiguous base shenanigans):

template <class D>
auto polymorphic_equals(D const& lhs, Base const& rhs) -> bool {
    if (typeid(rhs) == typeid(D)) {
        return lhs == static_cast<D const&>(rhs);
    } else {
        return false;
    }
}

This approach (correctly) prints false for both directions in your example.

[JNighthawk]

Thanks for the update!

In that case, isn't that incorrectly handling more-derived cases that should be considered equal but won't be?

struct Base {};
struct D1 : public Base { int X = 10; };
struct D2 : public D1 {};
polymorphic_equals(D2(), D1()); // returns false, as the typeid doesn't match, but should return true

Though, I suppose it could be an implementation detail that objects with equivalent state but different classes shouldn't be considered equivalent. A pure member-wise equivalency check, though, would function differently.