9.1. Introduction

In the preceding chapters, we introduced many basic terms and concepts of C++ objectoriented programming. We also discussed our program development methodology: We selected appropriate attributes and behaviors for each class and specified the manner in which objects of our classes collaborated with objects of C++ Standard Library classes to accomplish each program's overall goals.

In this chapter, we take a deeper look at classes. We use an integrated Time class case study in both this chapter (three examples) and Chapter 10, Classes: A Deeper Look, Part 2 (two examples) to demonstrate several class construction capabilities. We begin with a Time class that reviews several of the features presented in the preceding chapters. The example also demonstrates an important C++ software engineering concept—using a "preprocessor wrapper" in header files to prevent the code in the header from being included into the same source code file more than once. Since a class can be defined only once, using such preprocessor directives prevents multiple definition errors.

Next, we discuss class scope and the relationships among members of a class. We also demonstrate how client code can access a class's public members via three types of "handles"—the name of an object, a reference to an object or a pointer to an object. As you'll see, object names and references can be used with the dot (.) member selection operator to access a public member, and pointers can be used with the arrow (->) member selection operator.

We discuss access functions that can read or display data in an object. A common use of access functions is to test the truth or falsity of conditions—such functions are known as predicate functions. We also demonstrate the notion of a utility function (also called a helper function)—a private member function that supports the operation of the class's public member functions, but is not intended for use by clients of the class.

The second example of the Time class case study demonstrates how to pass arguments to constructors and shows how to use default arguments in a constructor to enable client code to initialize objects of a class using a variety of arguments. Next, we discuss a special member function called a destructor that is part of every class and is used to perform "termination housekeeping" on an object before the object is destroyed. We then demonstrate the order in which constructors and destructors are called, because your programs' correctness depends on using properly initialized objects that have not yet been destroyed.

Our last example of the Time class case study in this chapter shows a dangerous programming practice in which a member function returns a reference to private data. We discuss how this breaks the encapsulation of a class and allows client code to directly access an object's data. This last example shows that objects of the same class can be assigned to one another using default memberwise assignment, which copies the data members in the object on the right side of the assignment into the corresponding data members of the object on the left side of the assignment. The chapter concludes with a discussion of software reusability.