Shape Class Hierarchy

Now consider the Shape inheritance hierarchy in Fig. 11.3. This hierarchy begins with base class Shape. Classes TwoDimensionalShape and ThreeDimensionalShape derive from base class Shape—a Shape is a TwoDimensionalShape or is a ThreeDimensionalShape. The third level of this hierarchy contains more specific types of TwoDimensionalShapes and ThreeDimensionalShapes. As in Fig. 11.2, we can follow the arrows from the bottom of the diagram upwards to the topmost base class in this hierarchy to identify several is-a relationships. For instance, a Triangle is a TwoDimensionalShape and is a Shape, while a Sphere is a ThreeDimensionalShape and is a Shape.

Fig. 11.3. Inheritance hierarchy for Shapes.

To specify that class TwoDimensionalShape (Fig. 11.3) is derived from (or inherits from) class Shape, class TwoDimensionalShape’s definition could begin as follows:

This is an example of public inheritance, the most commonly used form. We’ll also discuss private inheritance and protected inheritance (Section 11.5). With all forms of inheritance, private members of a base class are not accessible directly from that class’s derived classes, but these private base-class members are still inherited (i.e., they’re still considered parts of the derived classes). With public inheritance, all other base-class members retain their original member access when they become members of the derived class (e.g., public members of the base class become public members of the derived class, and, as we’ll soon see, protected members of the base class become protected members of the derived class). Through inherited base-class member functions, the derived class can manipulate private members of the base class (if these inherited member functions provide such functionality in the base class). Note that friend functions are not inherited.

Inheritance is not appropriate for every class relationship. In Chapter 9, we discussed the has-a relationship, in which classes have members that are objects of other classes. Such relationships create classes by composition of existing classes. For example, given the classes Employee, BirthDate and TelephoneNumber, it’s improper to say that an Employee is a BirthDate or that an Employee is a TelephoneNumber. However, it is appropriate to say that an Employee has a BirthDate and that an Employee has a TelephoneNumber.

It’s possible to treat base-class objects and derived-class objects similarly; their commonalities are expressed in the members of the base class. Objects of all classes derived from a common base class can be treated as objects of that base class (i.e., such objects have an is-a relationship with the base class). In Chapter 12, we consider many examples that take advantage of this relationship.