11.2. Base Classes and Derived Classes

Figure 11.1 lists several simple examples of base classes and derived classes. Base classes tend to be more general and derived classes tend to be more specific.

Fig. 11.1. Inheritance examples.

Because every derived-class object is an object of its base class, and one base class can have many derived classes, the set of objects represented by a base class typically is larger than the set of objects represented by any of its derived classes. For example, the base class Vehicle represents all vehicles, including cars, trucks, boats, airplanes, bicycles and so on. By contrast, derived class Car represents a smaller, more specific subset of all vehicles.

Inheritance relationships form class hierarchies. A base class exists in a hierarchical relationship with its derived classes. Although classes can exist independently, once they’re employed in inheritance relationships, they become affiliated with other classes. A class becomes either a base class—supplying members to other classes, a derived class—inheriting its members from other classes, or both.