When should I use non-member operator overloading?

Question

Should I be using non-member overload or member overload? How do I tell which I should use?

Answer 1

If you're thinking about something like operator< or swap, there's a rule of thumb ( though it's not terribly strict or mandatory):

If the function only requires access to the public interface of your class, make it a free non-member function. Otherwise make it a member function. (Alternatively you may consider a friend free function.)

Note that for overloads of operators you will need at least one of the operands to be a user-defined type.

The design advantage of a free function is that you can make it a template and get M + N complexity rather than M * N if you were to implement a version of the operator for each class for which it's applicable. This may or may not be relevant to your situation.

See also Nawaz's very fine answer on the subject.