c++ reference can not be reassigned

#include <iostream>

using namespace std;

int main () {
   // declare simple variables
   int    i;
   int    j;
   double d;

   // declare reference variables
   int&    r = i;
   double& s = d;
   j = ;
   r = j;
   j = ;

   i = ;
   cout << "Value of i : " << i << endl;
   cout << "Value of i reference : " << r  << endl;

   d = 11.7;
   cout << "Value of d : " << d << endl;
   cout << "Value of d reference : " << s  << endl;

   return ;
}