int  main()  {
      int  x  =  1,  y  =  3;
      void  (*funcpoint)(int&,  int&);
      funcpoint  =  &swap;
      swap(a,  b);
      std::cout  <<  a  <<  '  '  <<  b  <<  endl;
      (*funcpoint)(a,  b);
      std::cout  <<  a  <<  '  '  <<  b  <<  endl;
  }
Outputs:
     3  1
     1  3