why use a non-member function?

Question

I am learning about the friend keyword in C++ and I am wondering why have a non-member function and use the friend keyword when you can just make the non-member function a member function? I hope I made my question clear enough, thank you!

Answer 1

Because sometimes you need to create an overloaded operator where your class type is on the right-hand-side. This must be implemented as a free function. Classic example:

ostream& operator<<(ostream& str, my_type const& my)
{
    // print out `my` into `str`---requires `friend` if using
    // private members of `my_type`
    return str;
}