12.3.2. Aiming Derived-Class Pointers at Base-Class Objects

In Section 12.3.1, we assigned the address of a derived-class object to a base-class pointer and explained that the C++ compiler allows this assignment, because a derived-class object is a base-class object. We take the opposite approach in Fig. 12.2, as we aim a derived-class pointer at a base-class object. [Note: This program reuses the final versions of classes CommissionEmployee and BasePlusCommissionEmployee from Section 11.3.5.] Lines 8–9 of Fig. 12.2 create a CommissionEmployee object, and line 10 creates a BasePlusCommissionEmployee pointer. Line 14 attempts to assign the address of base-class object commissionEmployee to derived-class pointer basePlusCommissionEmployeePtr, but the compiler generates an error. The compiler prevents this assignment, because a CommissionEmployee is not a BasePlusCommissionEmployee.

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Fig. 12.2. Aiming a derived-class pointer at a base-class object.

Consider the consequences if the compiler were to allow this assignment. Through a BasePlusCommissionEmployee pointer, we can invoke every BasePlusCommissionEmployee member function, including setBaseSalary, for the object to which the pointer points (i.e., the base-class object commissionEmployee). However, the CommissionEmployee object does not provide a setBaseSalary member function, nor does it provide a baseSalary data member to set. This could lead to problems, because member function setBaseSalary would assume that there is a baseSalary data member to set at its “usual location” in a BasePlusCommissionEmployee object. This memory does not belong to the CommissionEmployee object, so member function setBaseSalary might overwrite other important data in memory, possibly data that belongs to a different object.