Files that are included using C/C++ preprocessor should have the `.h` suffixes

[karel-brinda]

.cpp is usually used for files that are to be compiled.

This distinction is important for tools such as Make (and also for other developers).

Also, in general, often it's better to use such including rather than compilation as a separate module as the compiler can then better optimize (inline functions/methods, etc.). You this is done very well in the current implementation.