12.5. Abstract Classes and Pure virtual Functions

When we think of a class as a type, we assume that programs will create objects of that type. However, there are cases in which it’s useful to define classes from which you never intend to instantiate any objects. Such classes are called abstract classes. Because these classes normally are used as base classes in inheritance hierarchies, we refer to them as abstract base classes. These classes cannot be used to instantiate objects, because, as we’ll soon see, abstract classes are incomplete—derived classes must define the “missing pieces” before objects of these classes can be instantiated. We build programs with abstract classes in Section 12.6.

An abstract class is a base class from which other classes can inherit. Classes that can be used to instantiate objects are called concrete classes. Such classes define or inherit implementations for every member function they declare. We could have an abstract base class TwoDimensionalShape and derive such concrete classes as Square, Circle and Triangle. We could also have an abstract base class ThreeDimensionalShape and derive such concrete classes as Cube, Sphere and Cylinder. Abstract base classes are too generic to define real objects; we need to be more specific before we can think of instantiating objects. For example, if someone tells you to “draw the two-dimensional shape,” what shape would you draw? Concrete classes provide the specifics that make it possible to instantiate objects.

An inheritance hierarchy does not need to contain any abstract classes, but many object-oriented systems have class hierarchies headed by abstract base classes. In some cases, abstract classes constitute the top few levels of the hierarchy. A good example of this is the shape hierarchy in Fig. 11.3, which begins with abstract base class Shape. On the next level of the hierarchy we have two more abstract base classes—TwoDimensionalShape and ThreeDimensionalShape. The next level of the hierarchy defines concrete classes for two-dimensional shapes (namely, Circle, Square and Triangle) and for three-dimensional shapes (namely, Sphere, Cube and Tetrahedron).