Abstract classes - virtual and pure virtual functions

An abstract class is a class that cannot be instantiated and therefore cannot be made into an object.

Tip

Some terminology we might like to learn here is concrete class. A concrete class is any class that isn't abstract. In other words, all the classes we have written so far have been concrete classes and can be instantiated into usable objects.

So, it's code that will never be used, then? But that's like paying an architect to design your home and then never building it!

If we, or the designer of a class, want to force its users to inherit it before using their class, they can make a class abstract. Then, we cannot make an object from it; therefore, we must extend it first and make an object from the sub-class.

To do so, we can make a function pure virtual and not provide any definition. Then that function must be overridden (re-written) in any class that extends it.

Let's look at an example; it will help. We make a class...