Overloading increment and decrement operators (C++ only)

You overload the prefix increment operator ++ with either a nonmember function operator that has one argument of class type or a reference to class type, or with a member function operator that has no arguments.

In the following example, the increment operator is overloaded in both ways:

class X {
public:

  // member prefix ++x
  void operator++() { }
};

class Y { };

// non-member prefix ++y
void operator++(Y&) { }

int main() {
  X x;
  Y y;

  // calls x.operator++()
  ++x;

  // explicit call, like ++x
  x.operator++();

  // calls operator++(y)
  ++y;

  // explicit call, like ++y
  operator++(y);
}

The postfix increment operator ++ can be overloaded for a class type by declaring a nonmember function operator operator++() with two arguments, the first having class type and the second having type int. Alternatively, you can declare a member function operator operator++() with one argument having type int. The compiler uses the int argument to distinguish between the prefix and postfix increment operators. For implicit calls, the default value is zero.

For example:

class X {
public:

  // member postfix x++
  void operator++(int) { };
};

class Y { };

// nonmember postfix y++
void operator++(Y&, int) { };

int main() {
  X x;
  Y y;

  // calls x.operator++(0)
  // default argument of zero is supplied by compiler
  x++;
  // explicit call to member postfix x++
  x.operator++(0);

  // calls operator++(y, 0)
  y++;

  // explicit call to non-member postfix y++
  operator++(y, 0);
}

The prefix and postfix decrement operators follow the same rules as their increment counterparts.

Related information

• Increment operator ++
• Decrement operator --