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 << and >> 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 private or protected members of that class directly.

Operator member functions of a specific class are called (implicitly by the compiler) only when the left operand of a binary operator is specifically an object of that class, or when the single operand of a unary operator is an object of that class.