I also separate them, but not strictly on the extension, but on the access of the file.
Suppose you have a module that manages customer information and uses 2 classes to do this: Customer, CustomerValidityChecker.
Also suppose that other parts in your application only need to know about the Customer class, and that the CustomerValidityChecker is only used by the Customer class to perform some checking.
Based on these assumptions I store the files like this:
Public folder (or include folder):
Private folder (or source folder):
- customer.cpp
- customervaliditychecker.h
- customervaliditychecker.cpp
That way, it becomes immediately clear for callers of your module which parts are accessible (public) and which parts aren't.
