In which file do we put non-member function in C++?

Question

What is the normal practice when it comes to non-member function in C++? Do we put them in main.cpp or header file or class implementation file, or do we make a separate .cpp fi

Answer 1

I would say you should not treat non-member functions differently to classes and member functions and other symbols.

You should create a distinct header file .h and a corresponding source file .cpp for each logical component (module) of your application.

All public symbols should be declared/defined in the header file (whether they be non-member functions or otherwise) and all non-public symbols and all required definitions should go in the source file.

In short, group according to logical program components, rather than by the type of symbol/function.

Answer 2

Your class should have its own .cpp file. Non-member functions should go in other files (all together or grouped according to similarity). That's the convention here in North America, but conventions differ. Prototype just needs go into header file so you can include it wherever you use it.

Answer 3

General idea in pseudo code:

if (it will be used in other cpp files) 
    put the declaration in a header file. 
    implement it in a header or a cpp file.
else 
    if (only need it in some functions in a header file)
        if (it's a function more than N line )  // please define this N in your mind
            declare it in the same header and implement it in a cpp file
        else
             put it in the same header file
    else // used in cpp only
        put it in the cpp file

As long as it compiles, you should consider readability (easy for anyone to read) and accessibility (easy for anyone to find and debug).