[over.inc]

The user-defined function called operator++ implements the prefix and postfix ++ operator.

If this function is a non-static member function with no parameters, or a non-member function with one parameter, it defines the prefix increment operator ++ for objects of that type.

If the function is a non-static member function with one parameter (which shall be of type int) or a non-member function with two parameters (the second of which shall be of type int), it defines the postfix increment operator ++ for objects of that type.

When the postfix increment is called as a result of using the ++ operator, the int argument will have value zero.133

[ Example

struct X {
  X&   operator++();            // prefix ++a
  X    operator++(int);         // postfix a++
};

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

void f(X a, Y b) {
  ++a;                          // a.operator++();
  a++;                          // a.operator++(0);
  ++b;                          // operator++(b);
  b++;                          // operator++(b, 0);

  a.operator++();               // explicit call: like ++a;
  a.operator++(0);              // explicit call: like a++;
  operator++(b);                // explicit call: like ++b;
  operator++(b, 0);             // explicit call: like b++;
}

— end example

]

133)

Calling operator++ explicitly, as in expressions like a.operator++(2), has no special properties: The argument to operator++ is 2.

11 Overloading [over]

11.5 Overloaded operators [over.oper]

11.5.7 Increment and decrement [over.inc]