What is an overloaded operator in C++?

Question

I realize this is a basic question but I have searched online, been to cplusplus.com, read through my book, and I can't seem to grasp the concept of overloaded operators. A specific example from cplusplus.com is:

// vectors: overloading operators example
#include <iostream>
using namespace std;

class CVector {
  public:
    int x,y;
    CVector () {};
    CVector (int,int);
    CVector operator + (CVector);
};

CVector::CVector (int a, int b) {
  x = a;
  y = b;
}

CVector CVector::operator+ (CVector param) {
  CVector temp;
  temp.x = x + param.x;
  temp.y = y + param.y;
  return (temp);
}

int main () {
  CVector a (3,1);
  CVector b (1,2);
  CVector c;
  c = a + b;
  cout << c.x << "," << c.y;
  return 0;
}

From http://www.cplusplus.com/doc/tutorial/classes2/ but reading through it I'm still not understanding them at all. I just need a basic example of the point of the overloaded operator (which I assume is the "CVector CVector::operator+ (CVector param)").

There's also this example from wikipedia:

 Time operator+(const Time& lhs, const Time& rhs)
 {
   Time temp = lhs;
   temp.seconds += rhs.seconds;
   if (temp.seconds >= 60)
   {
     temp.seconds -= 60;
     temp.minutes++;
   }
   temp.minutes += rhs.minutes;
   if (temp.minutes >= 60)
   {
     temp.minutes -= 60;
     temp.hours++;
   }
   temp.hours += rhs.hours;
   return temp;
 }

From "http://en.wikipedia.org/wiki/Operator_overloading"

The current assignment I'm working on I need to overload a ++ and a -- operator.

Thanks in advance for the information and sorry about the somewhat vague question, unfortunately I'm just not sure on it at all.

Answer 1

Operator overloading is the technique that C++ provides to let you define how the operators in the language can be applied to non-built in objects.

In you example for the Time class operator overload for the + operator:

Time operator+(const Time& lhs, const Time& rhs);

With that overload, you can now perform addition operations on Time objects in a 'natural' fashion:

Time t1 = some_time_initializer;
Time t2 = some_other_time_initializer;

Time t3 = t1 + t2;    // calls operator+( t1, t2)

The overload for an operator is just a function with the special name "operator" followed by the symbol for the operator being overloaded. Most operators can be overloaded - ones that cannot are:

.  .*  :: and ?:

You can call the function directly by name, but usually don't (the point of operator overloading is to be able to use the operators normally).

The overloaded function that gets called is determined by normal overload resolution on the arguments to the operator - that's how the compiler knows to call the operator+() that uses the Time argument types from the example above.

One additional thing to be aware of when overloading the ++ and -- increment and decrement operators is that there are two versions of each - the prefix and the postfix forms. The postfix version of these operators takes an extra int parameter (which is passed 0 and has no purpose other than to differentiate between the two types of operator). The C++ standard has the following examples:

class X {
public:
    X&   operator++();      //prefix ++a
    X    operator++(int);   //postfix a++
};

class Y { };

Y&   operator++(Y&);        //prefix ++b
Y    operator++(Y&, int);   //postfix b++

You should also be aware that the overloaded operators do not have to perform operations that are similar to the built in operators - being more or less normal functions they can do whatever you want. For example, the standard library's IO stream interface uses the shift operators for output and input to/from streams - which is really nothing like bit shifting. However, if you try to be too fancy with your operator overloads, you'll cause much confusion for people who try to follow your code (maybe even you when you look at your code later).

Use operator overloading with care.