Including a Header That Contains a User-Defined Class

A header such as GradeBook.h (Fig. 3.9) cannot be used as a complete program, because it does not contain a main function. To test class GradeBook (defined in Fig. 3.9), you must write a separate source-code file containing a main function (such as Fig. 3.10) that instantiates and uses objects of the class.

The compiler doesn’t know what a GradeBook is because it’s a user-defined type. In fact, the compiler doesn’t even know the classes in the C++ Standard Library. To help it understand how to use a class, we must explicitly provide the compiler with the class’s definition—that’s why, for example, to use type string, a program must include the <string> header. This enables the compiler to determine the amount of memory that it must reserve for each string object and ensure that a program calls a string’s member functions correctly.

To create GradeBook objects gradeBook1 and gradeBook2 in lines 11–12 of Fig. 3.10, the compiler must know the size of a GradeBook object. While objects conceptually contain data members and member functions, C++ objects actually contain only data. The compiler creates only one copy of the class’s member functions and shares that copy among all the class’s objects. Each object, of course, needs its own data members, because their contents can vary among objects (such as two different BankAccount objects having two different balances). The member-function code, however, is not modifiable, so it can be shared among all objects of the class. Therefore, the size of an object depends on the amount of memory required to store the class’s data members. By including GradeBook.h in line 4, we give the compiler access to the information it needs (Fig. 3.9, line 37) to determine the size of a GradeBook object and to determine whether objects of the class are used correctly (in lines 11–12 and 15–16 of Fig. 3.10).

Line 4 instructs the C++ preprocessor to replace the directive with a copy of the contents of GradeBook.h (i.e., the GradeBook class definition) before the program is compiled. When the source-code file fig03_10.cpp is compiled, it now contains the GradeBook class definition (because of the #include), and the compiler is able to determine how to create GradeBook objects and see that their member functions are called correctly. Now that the class definition is in a header (without a main function), we can include that header in any program that needs to reuse our GradeBook class.