C++ Primer Plus, Fourth Edition

Overloaded Operators: Member Versus Nonmember Functions

For many operators, you have a choice between using member functions or nonmember functions to implement operator overloading. Typically, the nonmember version would be a friend function so that it can access directly the private data for a class. For example, consider the addition operator for the Time class. It had this prototype in the Time class declaration:

Time operator+(const Time & t) const;                     // member version

Instead, the class could have used the following prototype:

friend Time operator+(const Time & t1, const Time & t2);  // non-member version

The addition operator requires two operands. For the member function version, one is passed implicitly via the this pointer and the ...