c++ - Modifying dynamic data like string in binary files -

i have binary file contains records of student each record instance of class student defined as:

class student {    string idno, sname;    int books_issued;    vector<string> bname;    public:       void save(fstream &of);       void load(fstream &inf);       void modify_student()       {            cout<<"id no:\t"<<idno<<"\n";            cout <<"modify student's name:\n";            cout<<"enter name:\t";            cin.ignore();            getline(cin, sname); }       //some more code here }; 

i writing record in file using save member function:

void save(fstream &of) {     unsigned len = idno.size();     of.write((char *)&len, sizeof(len));     of.write(idno.c_str(), len);      len = sname.size();     of.write((char *)&len, sizeof(len));     of.write(sname.c_str(), len);      of.write((char *)&books_issued, sizeof(books_issued));      len = bname.size();     of.write((char *)&len, sizeof(len));     int = 0;     while(len>0)     {         unsigned strlen = bname[i].size();         of.write((char *)&strlen, sizeof(strlen));         of.write(bname[i].c_str(), strlen);         i++;         len--;     } } 

and reading records file using load function returns size of record read:

 unsigned int load(fstream& inf)  {     unsigned finallength = 0;     unsigned len;     inf.read((char *)&len, sizeof(len));     if(len>0)     {         char *buf = new char[len+1];         inf.read(buf, len);         buf[len] = '\0';         idno= buf;         delete[] buf;     }     finallength+=(len+sizeof(len));      inf.read((char *)&len, sizeof(len));     if(len>0)     {         char *buf = new char[len+1];         inf.read(buf, len);         buf[len] = '\0';         sname = buf;         delete[] buf;     }     finallength+=(len+sizeof(len));      inf.read((char *)&books_issued, sizeof(books_issued     finallength+=(sizeof(books_issued));      bname.erase(bname.begin(), bname.end());     inf.read((char *)&len, sizeof(len));     finallength+=(len);     while(len>0)     {         unsigned strlen;         inf.read((char *)&strlen, sizeof(strlen));         if(strlen>0)         {             string str;             char *buf = new char[strlen+1];             inf.read(buf, strlen);             buf[strlen] = '\0';             str = buf;             bname.push_back(str);             delete[] buf;         }         finallength+=(strlen+sizeof(strlen));         len--;     }      return finallength; } 

the read , write operations performed successfully, while trying modify records, shows unexpected behaviour:

 void modify_specific_student()  {      string studentid;      cout<<"enter id no:\t";      cin>>studentid;      fstream fp;      fp.open("student.dat", ios::binary|ios::in|ios::out);      student st;      int found = 0;      if(fp.is_open())      {          while(!fp.eof())          {              unsigned int len = st.load(fp);              cout<<"length: "<<len<<"\n";              if(!fp.eof())              {                   if(st.searchbyidno(studentid))                   {                      st.show_personal_details();                      st.modify_student();                      unsigned int pos = -1*len;                      fp.seekp(pos,ios::cur);                      st.save(fp);                      cout<<"record updated\n";                      found = 1;                   }             }          }      }     fp.close();      if(!found)     cout<<"record not found\n";  } 

in view, problem arises because new string may of different size , require subsequent data(and records) shifted. if cause, how correct it, or if not,achieve modification of records?