Using #include and header files

Well, I've checked up references and the tutorials. There seems no trace of this, but pardon me if i'm wrong.

I'm working on my little stepper motor control project, and it has became large enough that I need to split it in several files.

Basically, I want something like include or require in php. That simply reads a file and spits it where the function is.
I went with #include, and pasted a bunch of my functions in another file. Then, at the place where i cut them, i wrote #include "functions.cpp"

However, the code doesn't work anymore and i get a undeclared for each function.

How exactly are header files and include used?

Use them the way you would use include_once or require_once in PHP: to include a set of functions or classes that you have written for use in the current source file.

Basically, if no templates are involved, place the function prototypes and/or class declarations in the header file. Place their implementation in some source file. Include the header (not the source file) in your other files. To get the include_once effect, use inclusion guards for the header files.

If templates are involved, then just place the entire function/class in the header file.

This is a really odd mechanism, you know. I got it working.. but for example

If function func() isolated in other.h requires a string, I'll include <string> in other.h, so that its implementation can have access to the string type. Now, if my main program also requires a string, I'll include <string> again. The compiler does its job on other.h, then main. Next step, linking the two.

Doesn't this end up taking more space?

not significantly with header guards.

linking doesn't work on source files, just object files and things like that.

edit: I misunderstood your question.

if string is required in the implementation, then include it in the source file (cpp etc), but not in the header file unless it is part of the function prototype or something.