Wrap-Up

Answers to Self Review Exercises

12.1	a) Inheritance. b) protected. c) is-a or inheritance. d) has-a or composition or aggregation. e) hierarchical. f) public. g) private. h) multiple inheritance. i) constructor. j) public, protected. k) protected, protected.
12.2	a) True. b) False. A has-a relationship is implemented via composition. An is-a relationship is implemented via inheritance. c) False. This is an example of a has-a relationship. Class Car has an is-a relationship with class Vehicle. d) True. e) True.

Exercises

12.3	Many programs written with inheritance could be written with composition instead, and vice versa. Rewrite class BasePlusCommissionEmployee of the CommissionEmployeeBasePlusCommissionEmployee hierarchy to use composition rather than inheritance. After you do this, assess the relative merits of the two approaches for designing classes CommissionEmployee and BasePlusCommissionEmployee, as well as for object-oriented programs in general. Which approach is more natural? Why?
12.4	Discuss the ways in which inheritance promotes software reuse, saves time during program development and helps prevent errors.
12.5	Some programmers prefer not to use protected access because they believe it breaks the encapsulation of the base class. Discuss the relative merits of using protected access vs. using private access in base classes.
12.6	Draw an inheritance hierarchy for students at a university similar to the hierarchy shown in Fig. 12.2. Use Student as the base class of the hierarchy, then include classes UndergraduateStudent and GraduateStudent that derive from Student. Continue to extend the hierarchy as deep (i.e., as many levels) as possible. For example, Freshman, Sophomore, Junior and Senior might derive from UndergraduateStudent, and DoctoralStudent and MastersStudent might derive from GraduateStudent. After drawing the hierarchy, discuss the relationships that exist between the classes. [Note: You do not need to write any code for this exercise.]
12.7	The world of shapes is much richer than the shapes included in the inheritance hierarchy of Fig. 12.3. Write down all the shapes you can think ofboth two-dimensional and three-dimensionaland form them into a more complete Shape hierarchy with as many levels as possible. Your hierarchy should have base class Shape from which class TwoDimensionalShape and class ThreeDimensionalShape are derived. [Note: You do not need to write any code for this exercise.] We will use this hierarchy in the exercises of Chapter 13 to process a set of distinct shapes as objects of base-class Shape. (This technique, called polymorphism, is the subject of Chapter 13.)
12.8	Draw an inheritance hierarchy for classes Quadrilateral, TRapezoid, Parallelogram, Rectangle and Square. Use Quadrilateral as the base class of the hierarchy. Make the hierarchy as deep as possible.
12.9	(Package Inheritance Hierarchy) Package-delivery services, such as FedEx®, DHL® and UPS®, offer a number of different shipping options, each with specific costs associated. Create an inheritance hierarchy to represent various types of packages. Use Package as the base class of the hierarchy, then include classes TwoDayPackage and OvernightPackage that derive from Package. Base class Package should include data members representing the name, address, city, state and ZIP code for both the sender and the recipient of the package, in addition to data members that store the weight (in ounces) and cost per ounce to ship the package. Package's constructor should initialize these data members. Ensure that the weight and cost per ounce contain positive values. Package should provide a public member function calculateCost that returns a double indicating the cost associated with shipping the package. Package's calculateCost function should determine the cost by multiplying the weight by the cost per ounce. Derived class TwoDayPackage should inherit the functionality of base class Package, but also include a data member that represents a flat fee that the shipping company charges for two-day-delivery service. TwoDayPackage's constructor should receive a value to initialize this data member. TwoDayPackage should redefine member function calculateCost so that it computes the shipping cost by adding the flat fee to the weight-based cost calculated by base class Package's calculateCost function. Class OvernightPackage should inherit directly from class Package and contain an additional data member representing an additional fee per ounce charged for overnight-delivery service. OvernightPackage should redefine member function calculateCost so that it adds the additional fee per ounce to the standard cost per ounce before calculating the shipping cost. Write a test program that creates objects of each type of Package and tests member function calculateCost.
12.10	(Account Inheritance Hierarchy) Create an inheritance hierarchy that a bank might use to represent customers' bank accounts. All customers at this bank can deposit (i.e., credit) money into their accounts and withdraw (i.e., debit) money from their accounts. More specific types of accounts also exist. Savings accounts, for instance, earn interest on the money they hold. Checking accounts, on the other hand, charge a fee per transaction (i.e., credit or debit). Create an inheritance hierarchy containing base class Account and derived classes SavingsAccount and CheckingAccount that inherit from class Account. Base class Account should include one data member of type double to represent the account balance. The class should provide a constructor that receives an initial balance and uses it to initialize the data member. The constructor should validate the initial balance to ensure that it is greater than or equal to 0.0. If not, the balance should be set to 0.0 and the constructor should display an error message, indicating that the initial balance was invalid. The class should provide three member functions. Member function credit should add an amount to the current balance. Member function debit should withdraw money from the Account and ensure that the debit amount does not exceed the Account's balance. If it does, the balance should be left unchanged and the function should print the message "Debit amount exceeded account balance." Member function getBalance should return the current balance. Derived class SavingsAccount should inherit the functionality of an Account, but also include a data member of type double indicating the interest rate (percentage) assigned to the Account. SavingsAccount's constructor should receive the initial balance, as well as an initial value for the SavingsAccount's interest rate. SavingsAccount should provide a public member function calculateInterest that returns a double indicating the amount of interest earned by an account. Member function calculateInterest should determine this amount by multiplying the interest rate by the account balance. [Note: SavingsAccount should inherit member functions credit and debit as is without redefining them.] Derived class CheckingAccount should inherit from base class Account and include an additional data member of type double that represents the fee charged per transaction. CheckingAccount's constructor should receive the initial balance, as well as a parameter indicating a fee amount. Class CheckingAccount should redefine member functions credit and debit so that they subtract the fee from the account balance whenever either transaction is performed successfully. CheckingAccount's versions of these functions should invoke the base-class Account version to perform the updates to an account balance. CheckingAccount's debit function should charge a fee only if money is actually withdrawn (i.e., the debit amount does not exceed the account balance). [Hint: Define Account's debit function so that it returns a bool indicating whether money was withdrawn. Then use the return value to determine whether a fee should be charged.] After defining the classes in this hierarchy, write a program that creates objects of each class and tests their member functions. Add interest to the SavingsAccount object by first invoking its calculateInterest function, then passing the returned interest amount to the object's credit function.