Swap two variables without using third and in one line

Couple of weeks back I was interviewing a fresh graduate. Even though they are taught programming, I am not sure if they take it seriously and learn or practice it well. One of the questions I asked was to swap 2 numbers without using a temp variable.

Looking back now, I think it may be a bigger challenge to ask to swap numbers without using a temp variable and in one line. Below are my three different approaches but I would advise you to try it yourself before looking at the answer.

//Program to swap 2 numbers without using 3rd variable and in one line
//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

void approach1(int& a, int& b)
{
  cout<<"\nApproach 1"<<endl;
  a^=b^=a^=b;
}

void approach2(int& a, int& b)
{
  cout<<"\nApproach 2"<<endl;
  //b=(a+b)-(a=b); - This should work but doesnt, why?
  a =((a = a + b) - (b = a - b)); 
}

void approach3(int& a, int& b)
{
  cout<<"\nApproach 3"<<endl;
  a = ((a = a * b) / (b = a / b));
}



int main()
{
  int a = 13, b = 29;
  cout<<"\nOriginal"<<endl;
  cout<<"a = "<<a<<", b = "<<b<<endl;

  approach1(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  a = 13, b = 29;
  approach2(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  a = 13, b = 29;
  approach3(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  return 0;
}

The output is as follows:

Agreed that the above would be applicable only for integers.