Question

I have question that bothers me for few days.

Abstract class is a special type of class that we cannot instantiate, right?. (Which is denoted/specified by giving a “= 0” to at least one method declaration, which looks like an afterthought).

What are the extra benefits that the abstract class mechanism brings to C++, that a ‘normal’ base class cannot achieve?

Answer 1

According to the wikibooks section on abstract classes:

It’s a way of forcing a contract between the class designer and the users of that class. If we wish to create a concrete class (a class that can be instantiated) from an abstract class we must declare and define a matching member function for each abstract member function of the base class.

As mentioned, it’s a way of defining an interface to which derived classes must adhere. Their example of the Vehicle abstract class is very apropos: you’d never have just a Vehicle in real life, you’d have a Ford Explorer or a Toyota Prius, but those both conform to (for the sake of argument) a base set of functionality that being a Vehicle might define. But, you can’t just go to the Vehicle dealership and drive a Vehicle off the lot. Thus, you’d never want to be able to construct and use a base Vehicle object where you’d really want a specialized, derived object.