program membalik 4 digit angka

#include <cstdlib>
#include <iostream>

using namespace std;
class balik{
      friend istream& operator>>(istream&, balik&);
      friend ostream& operator<<(ostream&, balik&);
     
      public:
             balik();
             void proses();
            
      private:
              int nilai;
              int a,b,c,d,e,f;
      };
     
balik::balik(){
        cout<<"membalik nilai 4 digit\n\n";
        }
       
istream& operator>>(istream& in, balik& mlebu){
         cout<<"masukkan nilai  : ";
         in>>mlebu.nilai;
         return in;
         }
        
ostream& operator<<(ostream& out, balik& metu){
         if(metu.nilai>9999 || metu.nilai<=999) {
                   out<<"range nilai salah"<<endl;
                   return out;
                   }
         else {
         out<<"setelah dibalik : "<<metu.f<<metu.e<<metu.c<<metu.a<<endl<<endl;
         return out;
         }
         }
        
void balik::proses(){
     a = nilai / 1000;
     b = nilai % 1000;
     c = b / 100;
     d = b % 100;
     e = d / 10;
     f = d % 10;
     
     }
int main(int argc, char *argv[])
{
    balik njo;
    cin>>njo;
    njo.proses();
    cout<<njo;
   
    system("PAUSE");
    return EXIT_SUCCESS;
}