No, we don't. There is no need to seperately declare and implement a function in C++. The declaration you show can be put in a file included (as you describe in the section on #include), but it's only needed if you want to use it in another place than where it's defined.
I don't see the point in this article, given that in the introducion you state you chose Python and C++ because both are object oriented, and then don't cover any object oriented aspects. Including wrongs like the one I pointed out doesn't make it any better.
Sorry if this is a bit harsh, but some things simply make no sense.