The Compilation and Linking Process

The diagram in Fig. 3.14 shows the compilation and linking process that results in an executable GradeBook application that can be used by instructors. Often a class’s interface and implementation will be created and compiled by one programmer and used by a separate programmer who implements the client code that uses the class. So, the diagram shows what’s required by both the class-implementation programmer and the client-code programmer. The dashed lines in the diagram show the pieces required by the class-implementation programmer, the client-code programmer and the GradeBook application user, respectively. [Note: Figure 3.14 is not a UML diagram.]

A class-implementation programmer responsible for creating a reusable GradeBook class creates the header GradeBook.h and the source-code file GradeBook.cpp that #includes the header, then compiles the source-code file to create GradeBook’s object code. To hide the class’s member-function implementation details, the class-implementation programmer would provide the client-code programmer with the header GradeBook.h (which specifies the class’s interface and data members) and the GradeBook object code (i.e., the machine code instructions that represent GradeBook’s member functions). The client-code programmer is not given GradeBook.cpp, so the client remains unaware of how GradeBook’s member functions are implemented.

The client code programmer needs to know only GradeBook’s interface to use the class and must be able to link its object code. Since the interface of the class is part of the class definition in the GradeBook.h header, the client-code programmer must have access to this file and must #include it in the client’s source-code file. When the client code is compiled, the compiler uses the class definition in GradeBook.h to ensure that the main function creates and manipulates objects of class GradeBook correctly.

To create the executable GradeBook application, the last step is to link

1. the object code for the main function (i.e., the client code),

2. the object code for class GradeBook’s member-function implementations and

3. the C++ Standard Library object code for the C++ classes (e.g., string) used by the class-implementation programmer and the client-code programmer.

The linker’s output is the executable GradeBook application that instructors can use to manage their students’ grades. Compilers and IDEs typically invoke the linker for you after compiling your code.

For further information on compiling multiple-source-file programs, see your compiler’s documentation. We provide links to various C++ compilers in our C++ Resource Center at www.deitel.com/cplusplus/.