Fs lab program for fixed length record

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

      void read_data();
      void pack();
      void write_to_file();
      void unpack();
      int search(string);
      int delete_from_file(string);
      void modify(string);
};
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,temp;
      stringstream out;
      out << Semester;
      sem = out.str();
      temp.erase();    
      temp+=USN+'|'+Name+'|'+Branch+'|'+sem;
      for(;temp.size()<100;) temp+='$';
      buffer+=temp+'\n';
     
}    

void student::write_to_file()
{
      fstream file;
      file.open("1.txt",ios::out|ios::app);
      file<<buffer;
      file.close();
}

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;
           
}

int student::search(string key)
{
      ifstream file;
      int flag=0, pos=0;
      file.open("1.txt",ios::in);
      while (!file.eof())
      {
            buffer.erase();        
            getline(file,buffer);
            unpack();
            if (key==USN) {cout<<"\nFound the key. The record is  "<<buffer;pos=file.tellg(); flag=1;return pos;}
      }
      file.close();    
      if (!flag) {cout<<"\n Not Found \n\n"; return pos;}
     
}

int student::delete_from_file(string key)
{
      fstream file;
      char del_mark='*',t;
      int pos,flag=0;
      pos=search(key);
      if (pos){  
            file.open("1.txt");
            pos-=101; //skip the $$$$$$ and \n characters
            file.seekp(pos,ios::beg);
            file.put(del_mark);                      
            flag=1;
            }
      file.close();    
      if (!flag) return 0;
      else return 1;
           
}          

void student::modify(string key)
{
      int choice;
      if (delete_from_file(key)){
      cout<<"\n What to modify?";
      cin>>choice;
      switch(choice)
      {
            case 1: cout<<"\nUSN:"; cin>>USN; break;
            case 2:     cout<<"\nName:";cin>>Name;break;
            case 3:     cout<<"\nBranch:";cin>>Branch;break;
            case 4:     cout<<"\nSemster:";cin>>Semester;break;
            default: cout <<"Wrong Choice";
      }
      buffer.erase();
      pack();
      write_to_file();
      }
}
     
int main()
{
      int count,choice,len,i;
      student s1;
      string key;
      cout <<"\nMain Menu\n 1.Add \n\n 2.Delete \n\n 3.Modify \n\n 4.Search \n\nEnter the choice:";
      cin>>choice;
      switch (choice)
      {
            case 1: cout<<"\n\nhow many records to insert?\n";
                  cin>>count;
                  for (i=0;i<count;i++)
                  {
                        cout<<"Data\n";
                        s1.read_data();
                        s1.pack();             
                  }
                  s1.write_to_file();
                  break;
            case 2: cout <<"\n\nEnter the key";
                  cin>>key;
                  i=s1.delete_from_file(key);
                  break;
            case 3: cout <<"\n\nEnter the key";
                  cin>>key;
                  s1.modify(key);
                  break;
            case 4: cout <<"\n\nEnter the key";
                  cin>>key;
                  i=s1.search(key);
                  break;
            default: cout<<"\n\nWrong Choice";
      }
      return 0;  
}