C++ Abstract class can't have a method with a parameter of that class

Question

I created this .h file

#pragma once

namespace Core
{
    class IComparableObject
    {
    public:
            virtual int CompareTo(IComparableObject obj)=0;
    };
}

But compiler doesn't like IComparableObject obj param if the method is virtual pure, while

virtual int CompareTo(IComparableObject obj) {}

It's ok, however I want it as virtual pure. How can I manage to do it? Is it possible?

Answer 1

You are trying to pass obj by value. You cannot pass an abstract class instance by value, because no abstract class can ever be instantiated (directly). To do what you want, you have to pass obj by reference, for example like so:

virtual int CompareTo(IComparableObject const &obj)=0;

It works when you give an implementation for CompareTo because then the class is not abstract any longer. But be aware that slicing occurs! You don't want to pass obj by value.