In Chapter 3, we began by including a class's definition and member-function definitions in one file. We then demonstrated separating this code into two filesa header file for the class definition (i.e., the class's interface) and a source code file for the class's memberfunction definitions (i.e., the class's implementation). Recall that this makes it easier to modify programsas far as clients of a class are concerned, changes in the class's implementation do not affect the client as long as the class's interface originally provided to the client remains unchanged.
Software Engineering Observation 9.6
Clients of a class do not need access to the class's source code in order to use the class. The clients do, however, need to be able to link to the class's object code (i.e., the compiled version of the class). This encourages independent software vendors (ISVs) to provide class libraries for sale or license. The ISVs provide in their products only the header files and the object modules. No proprietary information is revealedas would be the case if source code were provided. The C++ user community benefits by having more ISV-produced class libraries available.
Actually, things are not quite this rosy. Header files do contain some portions of the implementation and hints about others. Inline member functions, for example, need to be in a header file, so that when the compiler compiles a client, the client can include the inline function definition in place. A class's private members are listed in the class definition in the header file, so these members are visible to clients even though the clients may not access the private members. In Chapter 10, we show how to use a "proxy class" to hide even the private data of a class from clients of the class.
Software Engineering Observation 9.7
Information important to the interface to a class should be included in the header file. Information that will be used only internally in the class and will not be needed by clients of the class should be included in the unpublished source file. This is yet another example of the principle of least privilege.