10.11. Operators as Member vs. Non-Member Functions
Whether an operator function is implemented as a member function or as a non-member function, the operator is still used the same way in expressions. So which is best?
When an operator function is implemented as a member function, the leftmost (or only) operand must be an object (or a reference to an object) of the operator’s class. If the left operand must be an object of a different class or a fundamental type, this operator function must be implemented as a non-member function (as we did in Section 10.5 when overloading
>> as the stream insertion and stream extraction operators, respectively). A non-member operator function can be made a
friend of a class if that function must access ...