12.3. Relationships Among Objects in an Inheritance Hierarchy

Section 11.3 created an employee class hierarchy, in which class BasePlusCommissionEmployee inherited from class CommissionEmployee. The Chapter 11 examples manipulated CommissionEmployee and BasePlusCommissionEmployee objects by using the objects’ names to invoke their member functions. We now examine the relationships among classes in a hierarchy more closely. The next several sections present a series of examples that demonstrate how base-class and derived-class pointers can be aimed at base-class and derived-class objects, and how those pointers can be used to invoke member functions that manipulate those objects.

• In Section 12.3.1, we assign the address of a derived-class object to a base-class pointer, then show that invoking a function via the base-class pointer invokes the base-class functionality in the derived-class object—i.e., the type of the handle determines which function is called.

• In Section 12.3.2, we assign the address of a base-class object to a derived-class pointer, which results in a compilation error. We discuss the error message and investigate why the compiler does not allow such an assignment.

• In Section 12.3.3, we assign the address of a derived-class object to a base-class pointer, then examine how the base-class pointer can be used to invoke only the base-class functionality—when we attempt to invoke derived-class member functions through the base-class pointer, compilation errors occur.

• Finally, in Section 12.3.4, we demonstrate how to get polymorphic behavior from base-class pointers aimed at derived-class objects. We introduce virtual functions and polymorphism by declaring a base-class function as virtual. We then assign the address of a derived-class object to the base-class pointer and use that pointer to invoke derived-class functionality—precisely the capability we need to achieve polymorphic behavior.

A key concept in these examples is to demonstrate that with public inheritance an object of a derived class can be treated as an object of its base class. This enables various interesting manipulations. For example, a program can create an array of base-class pointers that point to objects of many derived-class types. Despite the fact that the derived-class objects are of different types, the compiler allows this because each derived-class object is an object of its base class. However, we cannot treat a base-class object as an object of any of its derived classes. For example, a CommissionEmployee is not a BasePlusCommissionEmployee in the hierarchy defined in Chapter 11—a CommissionEmployee does not have a baseSalary data member and does not have member functions setBaseSalary and getBaseSalary. The is-a relationship applies only from a derived class to its direct and indirect base classes.