I'm working with functions.
Is it good practice, that I write the function in another .cpp file, and I include it in the main one?
Like this : #include "lehel.cpp".
Is this ok, or should I write the functions directly in the main.cpp file?
A good practice is to separate functionality into separate Software Units so they can be reused and so that a change to one unit has little effect on other units.
lehel.cpp is included by
main, this means that any changes in
lehel.cpp will force a compilation of
main. However, if
lehel.cpp is compiled separately and linked in, then a change to
lehel.cpp does not force
main to be recompiled; only linked together.
IMHO, header files should contain information on how to use the functions. The source files should contain the implementations of the functions. The functions in a source file should be related by a theme. Also, keeping the size of the source files small will reduce the quantity of injected defects.