2.16 What goes in a .h file? What goes in a .cpp file?

The short answer is that a .h file contains shared declarations, a .cpp file contains definitions and local declarations.

It’s important that you understand the difference between declarations and definitions.

• A .h file is intended to be #included from many different .cpp files that make up a single program. In fact, the earliest stage of compilation, the preprocessor, actually replaces each #include by the full contents of the included file. Consequently, a .h file may be processed many times during the compilation of a single program, and should contain should contain only declarations.
• A .cpp file is intended to be compiled once for any given build of the program. So the .cpp file can have any declarations that it doesn’t need to share with other parts of the program, and it can have definitions. But the main purpose of a .cpp file is to contain definitions that must only be compiled once. The most common example of this would be function bodies.

Never, ever, ever name a .cpp file in an #include. That defeats the whole purpose of a C++ program structure. .h files are #included; .cpp files are compiled.

A typical program will consist of many .cpp files.

Usually, each class or group of utility functions will have their definitions in a separate .cpp file that defines everything declared in the corresponding .h file. The .h file can then be #included by many different parts of the program that use those classes or functions, and the .cpp file can be separately compiled once, then linked together with the output of other .cpp files to form the complete program.

Splitting the program into pieces like this helps, among other things, divide the responsibility for who can change what and reduces the amount of compilation that must take place after a change to a function body.