Question

Anyone got an idea on how to write an operator for a class that isn’t a member function of the class?

Answer 1

Just make it a free function, or a friend function. A good example of this is operator<<:

class X {
    public:
    int x; 
}

ostream& operator<< (ostream& os, const X& x) {
    os << x.x;
    return os;
}

The benefit of making it a friend function is that you have direct access to private members, whereas a free function must access all members via public methods.