A cool hack exposed in the C++ compiler (works with GCC, at least).

swap.cpp

#include <iostream>

using namespace std;

int main()
{
   int x;
   int y;

   cout << "Enter x: ";
   cin  >> x;

   cout << "Enter y: ";
   cin  >> y;

   cout << "x: " << x << endl
        << "y: " << y << endl;

   cout << endl << "x = x + y - (y = x)" << endl << endl;

   x = x + y - (y = x);

   cout << "x: " << x << endl
        << "y: " << y << endl;
   
   return 0;
}