Fs lab program for variable lenght record using RRN

#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;
class student
{
  public:
      string USN;
      string Name;
      string Branch;
      int Semester;
      string buffer;
      int count;

      int rrn_list[100];

      void read_data();
      void pack();
      void write_to_file();
      void unpack();
      void create_rrn();
      void search_by_rrn(int);     
};
void student::read_data()
{
      cout<<"\nUsn:";
      cin>>USN;
      cout<<"\nName:";
      cin>>Name;
      cout<<"\nBranch:";
      cin>>Branch;
      cout<<"\nSemster:";
      cin>>Semester;
}

void student::pack()
{
      string sem;
      stringstream out;
      out << Semester;
      sem = out.str(); 
      buffer.erase();
      buffer=USN+'|'+Name+'|'+Branch+'|'+sem+'$'+'\n';           
}    

void student::write_to_file()
{
      int pos;   
      fstream file;
      file.open("1.txt",ios::out|ios::app);
      pos=file.tellp();
      file<<buffer;
      file.close();
      rrn_list[++count]=pos;
}

void student::unpack()
{
     
      string sem;
      int ch=1,i=0;
      USN.erase();     
      while (buffer[i]!='|')
        USN+=buffer[i++];          
     
      Name.erase();
      i++;
      while (buffer[i]!='|')
        Name+=buffer[i++];   
     
      Branch.erase();  
      i++;
      while (buffer[i]!='|')
        Branch+=buffer[i++];
     
      sem.erase();     
      i++;
      while (buffer[i]!='$')
        sem+=buffer[i++];
      istringstream out(sem);
      out>>Semester;
           
}

void student::create_rrn()
{
      ifstream file;
      int pos;
      count=-1;
      file.open("1.txt",ios::in);
      while (!file.eof())
      {
            pos=file.tellg();      
            buffer.erase();        
            getline(file,buffer);
            rrn_list[++count]=pos;       
      }
      file.close();    
}

void student::search_by_rrn(int rrn)
{
      int pos=-1;
      fstream file;
      if (rrn>count)    cout<<"\n Not Found";
      else{
      buffer.erase();  
      file.open("1.txt");
      pos=rrn_list[rrn];
      file.seekp(pos,ios::beg);
      getline(file,buffer);
      cout<<"\n"<<buffer<<"\n" ;
      }
}

     
int main()
{
      int choice,rrn;
      student s1;
     
      s1.create_rrn();
     
      while(1){
      cout <<"\nMain Menu\n 1.Add  \n\n 2.Search 3.EXIT\n\nEnter the choice:";
      cin>>choice;     
      switch (choice)
      {
            case 1: cout<<"Data\n";
                  s1.read_data();
                  s1.pack();             
                  s1.write_to_file();
                  break;
            case 2: cout <<"\n\nEnter the RRN";
                  cin>>rrn;
                  s1.search_by_rrn(rrn);
                  break;
            case 3: return 0;
            default:cout<<"\n\nWrong Choice";
                  break;
      }}
}