C++ Header Files, Code Separation

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Header files should contain class and function declarations.

Source files contain class and function definitions.

It is standard practice (i.e. read easier) to have one declaration per header file and one definition per source file, though for small (read simpler helper) objects you sometimes group them with related more substantial objects.

Example: Class Menu

Menu.h:     Contains the Menu declaration.
Menu.cpp:   Contains the Menu definition.

The reason header files contain the declarations is so that you can include them from multiple source files and thus each source file has exactly the same definition of each class and function.

Consider it this way:
If you did not have header files then you would need to have the class and/or function declarations (without) definitions in every source file, this means a copy of the same declaration in every file. Thus if you modify a class you need to make the same modification in every file. By the use of a header file you have the declaration in one place and thus only one object to modify.

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