11.1. Introduction

This chapter continues our discussion of object-oriented programming (OOP) by introducing inheritance—a form of software reuse in which you create a class that absorbs an existing class’s capabilities, then customizes or enhances them. Software reuse saves time during program development by taking advantage of proven, high-quality software.

When creating a class, instead of writing completely new data members and member functions, you can specify that the new class should inherit the members of an existing class. This existing class is called the base class, and the new class is called the derived class. Other programming languages, such as Java and C#, refer to the base class as the superclass and the derived class as the subclass. A derived class represents a more specialized group of objects.

C++ offers public, protected and private inheritance. In this chapter, we concentrate on public inheritance and briefly explain the other two. With public inheritance, every object of a derived class is also an object of that derived class’s base class. However, base-class objects are not objects of their derived classes. For example, if we have Vehicle as a base class and Car as a derived class, then all Cars are Vehicles, but not all Vehicles are Cars—for example, a Vehicle could also be a Truck or a Boat.

We distinguish between the is-a relationship and the has-a relationship. The is-a relationship represents inheritance. In an is-a relationship, an object of a derived class also can be treated as an object of its base class—for example, a Car is a Vehicle, so any attributes and behaviors of a Vehicle are also attributes and behaviors of a Car. By contrast, the has-a relationship represents composition, which was discussed in Chapter 9. In a has-a relationship, an object contains one or more objects of other classes as members. For example, a Car has many components—it has a steering wheel, has a brake pedal, has a transmission, etc.