Normally in C++, class declarations (definitions from your other topic :+) )go into header files, while the definition of their functions go into .cpp files. Inside some .cpp file, you want to use a particular function or object from a class, then include the header file in which it belongs.
If you don't have any classes, there is nothing stopping you from gathering related functions together and doing the same thing. The fact that they are related will probably imply that they go into classes though.