9.1. Introduction

This chapter takes a deeper look at classes. We use an integrated Time class case study and other examples to demonstrate several class construction capabilities. We begin with a Time class that reviews several of the features presented in preceding chapters. The example also demonstrates using an include guard in headers to prevent header code from being included in the same source code file more than once.

We demonstrate how client code can access a class’s public members via 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 write an object’s data members. 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.

We show how to pass arguments to constructors and show how default arguments can be used in constructors to enable client code to initialize objects using a variety of arguments. Next, we discuss a special member function called a destructor that’s part of every class and is used to perform “termination housekeeping” on an object before it’s destroyed. We demonstrate the order in which constructors and destructors are called.

We show that returning a reference or pointer to private data breaks the encapsulation of a class, allowing client code to directly access an object’s data. We use default memberwise assignment to assign an object of a class to another object of the same class.

We use const objects and const member functions to prevent modifications of objects and enforce the principle of least privilege. We discuss composition—a form of reuse in which a class can have objects of other classes as members. Next, we use friendship to specify that a nonmember function can also access a class’s non-public members—a technique that’s often used in operator overloading (Chapter 10) for performance reasons. We discuss the this pointer, which is an implicit argument in all calls to a class’s non-static member functions, allowing them to access the correct object’s data members and non-static member functions. We motivate the need for static class members and show how to use them in your own classes.