Base class pointer vs inherited class pointer?


Suppose I have a class Dog that inherits from a class Animal. What is the difference between these two lines of code?

    Animal *a = new Dog();
    Dog *d = new Dog();

In one, the pointer is for the base class, and in the other, the pointer is for the derived class. But when would this distinction become important? For polymorphism, either one would work exactly the same, right?


It makes no real difference at run time, as the two instances are the same. The only difference is at compile time, where you could call for example d->bark() but not a->bark(), even if a actually contains a dog. The compiler considers the variable to be an animal and only that.

By : Shtong

The difference is important when you try to call Dog's methods that are not Animal's method. In the first case (pointer to Animal) you have to cast the pointer to Dog first. Another difference is if you happen to overload non-virtual method. Then either Animal::non_virtual_method() (pointer to Animal) or Dog::non_virtual_method(pointer to Dog) will be called.

By : a1ex07

You must always remember there are 2 parts in every class, the data and the interface.

Your code truly created 2 Dog objects on the heap. Which means the data is of Dog. This object is of size the sum of all data members Dog Animal the vtable pointer.

The ponters a and d (lvalues) differ as from a interface point of view. Which determines how you can treat them code wise. So even though Animal* a is really a Dog, you could not access a->Bark() even if Dog::Bark() existed. d->Bark() would have worked fine.

Adding the vtable back into the picture, assuming the interface of Animal had Animal::Move a generic Move() and that Dog really overwriten with a Dog::Move() { like a dog }.

Even if you had Animal a* and performed a->Move() thanks to the vtable you would actually Move() { like a dog }. This happens because Animal::Move() was a (virtual) function pointer re-pointed to Dog's::Move() while constructing Dog().

This video can help you solving your question :)
By: admin