references vs pointers

struct P{
  int* x;
  P(int& px):x(&px){}
};

struct R{
  int& x;
  R(int& px):x(px){}
};


int main(){
  int i;
  P p1(i),p2(i);
  R r1(i),r2(i);

  p1=p2; //OK
  r1=r2; //error: non-static reference member ‘int& R::x’, can’t use default assignment operator
                  note: synthesized method ‘R& R::operator=(const R&)’ first required here
 
}