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 ...

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