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

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).