Force all classes to implement / override a 'pure virtual' method in multi-level inheritance hierarchy

Question

In C++ why the pure virtual method mandates its compulsory overriding only to its immediate children (for object creation), but not to

Answer 1

What you're basically asking for is to require that the most derived class implement the functiom. And my question is: why? About the only time I can imagine this to be relevant is a function like clone() or another(), which returns a new instance of the same type. And that's what you really want to enforce, that the new instance has the same type; even there, where the function is actually implemented is irrelevant. And you can enforce that:

class Base
{
    virtual Base* doClone() const = 0;
public:
    Base* clone() const
    {
        Base* results = doClone();
        assert( typeid(*results) == typeid(*this) );
        return results;
    }
}

(In practice, I've never found people forgetting to override clone to be a real problem, so I've never bothered with something like the above. It's a generally useful technique, however, anytime you want to enforce post-conditions.)