#include<iostream> 
using namespace std; 
class Swap {
  public: 
    virtual void swap(int &a, int &b) = 0;
};
class SwapNumbers : public Swap {
  public:  
    void swap(int &a, int &b) {   
      int temp = a; 
      a = b;
      b = temp;
    }
};
int main() 
{ 
  int x = 10;
  int y = 20;
  cout << "Before swapping: x = " << x << ", y = " << y << endl;
  Swap *ptr;
  SwapNumbers s;
  ptr = &s;
  ptr->swap(x, y);
  cout << "After swapping: x = " << x << ", y = " << y << endl;
  return 0;
}