Question

Say I have a Listener class.

I have class A and class M which both implement the listener class.
Class A has 4 M’s.

Each M has 1 A, but not as an A, as a Listener base class. (So downcasted A because M knows nothing about A).

When A gets a message from 1 of them M’s it needs to be able to know which M sent it.

Therefore, every method in the Listener class has a Listener* parameter.

so if I have something like this:

void A::someListenerMethod(Listener* l, MsgEnum msg)
{
    if(l == m_mInstance[0])
    {
       //will this work if the caller was indeed the M instance in question?
    }

}

So essentially what I’m asking is, do I need to downcast the M’s to Listeners before comparing them?

I read that sometimes C++ will make a separate object for a sub class for multiple inheritance. There is no multiple inheritance in this case, but I want to make sure this will work.

Thanks

Answer 1

It will work as you expect.

In fact, in order to do the comparison, m_mInstance[0] will implicitly be cast to Listener* in order for the types to match. if (l == (Listener*)(m_mInstance[0])) ... would compile to the same target code. This will typically incur no overhead unless you have multiple inheritance, in which case it might incur a minimal overhead.