5.1 The intention of header files

The intention of header files is to publish interfaces (common declarations) that should be made known to consumers. A header file usually includes one or more of the following components:

• function declarations (prototypes): a prototype informs the compiler that ``there exists a function with this name, takes these types of parameters, and return this type.'' However, the function is not defined in a prototype.
• variable declarations: these are prefixed with extern. Each variable declaration informs the compiler that there is a variable with a particular name of a particular type, but does not indicate where it is in memory.
• enum definitions: enumerated types permit the use of symbolic constants instead of literal constants.
• macro definitions: Sometimes, it is necessary to use macro definitions. These definitions are not stored in an object file, therefore they can and must be defined in a header file.
• class or struct definitions. These definitions are not stored in an object file, therefore they can and must be defined in a header file.

The most important thing to remember is that a header file should not include function definitions (with the curly braces), or variable definitions (without extern). Both function and variable definitions are stored in object files. If multiple files have definitions of the same name, the linker complains and no executable will be generated.