Operator functions can be member functions or global functions; global functions are often made friends for performance reasons. Member functions use the this pointer implicitly to obtain one of their class object arguments (the left operand for binary operators). Arguments for both operands of a binary operator must be explicitly listed in a global function call.
Operators That Must Be Overloaded as Member Functions
When overloading (), , -> or any of the assignment operators, the operator overloading function must be declared as a class member. For the other operators, the operator overloading functions can be class members or global functions.
Operators as Member Functions and Global Functions
Whether an operator function is implemented as a member function or as a global function, the operator is still used the same way in expressions. So which implementation 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 global function (as we will do in Section 11.5 when overloading << and >> as the stream insertion and stream extraction operators, respectively). A global 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.
Why Overloaded Stream Insertion and Stream Extraction Operators Are Overloaded as Global Functions
The overloaded stream insertion operator (<<) is used in an expression in which the left operand has type ostream &, as in cout << classObject. To use the operator in this manner where the right operand is an object of a user-defined class, it must be overloaded as a global function. To be a member function, operator << would have to be a member of the ostream class. This is not possible for user-defined classes, since we are not allowed to modify C++ Standard Library classes. Similarly, the overloaded stream extraction operator (>>) is used in an expression in which the left operand has type istream &, as in cin >> classObject, and the right operand is an object of a user-defined class, so it, too, must be a global function. Also, each of these overloaded operator functions may require access to the private data members of the class object being output or input, so these overloaded operator functions can be made friend functions of the class for performance reasons.
Performance Tip 11.1
It is possible to overload an operator as a global, non-friend function, but such a function requiring access to a class's private or protected data would need to use set or get functions provided in that class's public interface. The overhead of calling these functions could cause poor performance, so these functions can be inlined to improve performance.
Another reason why one might choose a global function to overload an operator is to enable the operator to be commutative. For example, suppose we have an object, number, of type long int, and an object bigInteger1, of class HugeInteger (a class in which integers may be arbitrarily large rather than being limited by the machine word size of the underlying hardware; class HugeInteger is developed in the chapter exercises). The addition operator (+) produces a temporary HugeInteger object as the sum of a HugeInteger and a long int (as in the expression bigInteger1 + number), or as the sum of a long int and a HugeInteger (as in the expression number + bigInteger1). Thus, we require the addition operator to be commutative (exactly as it is with two fundamental-type operands). The problem is that the class object must appear on the left of the addition operator if that operator is to be overloaded as a member function. So, we overload the operator as a global function to allow the HugeInteger to appear on the right of the addition. The operator+ function, which deals with the HugeInteger on the left, can still be a member function.