Updating the end of the file in c++ fstream -

i wrote code:

#include <fstream> #include <iostream>  using namespace std;  struct man {     int  id;     char name[20]; };  void add(); void update(); void print();  int main() {     int n;      cout << "1-add, 2-update, 3-print, 5-exit" << endl;     cin >> n;      while (n != 5)     {         switch (n)         {             case 1: add(); break;             case 2: update(); break;             case 3: print(); break;         }          cout << "1-add, 2-update, 3-print, 5-exit" << endl;         cin >> n;     }     return 0; }  void add() {     fstream file;      file.open("data.dat", ios::in | ios::out | ios::binary);      if (file.is_open())     {         int  id;         man  man;         bool didfound = false;          cout << "id  : ";         cin >> id;          file.read((char*)&man, sizeof(man));          while (!file.eof() && !didfound)         {             if (man.id == id)             {                 cout << "already exist" << endl;                 didfound = true;             }              file.read((char*)&man, sizeof(man));         }          if (!didfound)         {             man.id = id;             cout << "name: ";             cin >> man.name;              file.clear();             file.seekp(0, ios::end);             file.write((char*)&man, sizeof(man));         }     } }  void update() {     fstream file;      file.open("data.dat", ios::in | ios::out | ios::binary);      if (file.is_open())     {         int  id;         man  man;         bool didfound = false;          cout << "id  : ";         cin >> id;          file.read((char*)&man, sizeof(man));          while (!file.eof() && !didfound)         {             if (man.id == id)             {                 cout << "name: ";                 cin >> man.name;                  file.seekp((int)file.tellg() - sizeof(man), ios::beg);                 file.write((char*)&man, sizeof(man));                  didfound = true;             }              file.read((char*)&man, sizeof(man));         }          file.close();          if (!didfound)         {             cout << "cant update none existing man" << endl;         }     } }  void print() {     fstream file;      file.open("data.dat", ios::in | ios::binary);      if (file.is_open())     {         int  id;         man  man;         bool didfound = false;          cout << "id\tname" << endl;          file.read((char*)&man, sizeof(man));          while (!file.eof())         {             cout << man.id << '\t' << man.name << endl;              file.read((char*)&man, sizeof(man));         }          file.close();     } } 

but have problem in update function: when update last man in file, when reach file.read file writes value of last man(in file before writting) end of file (after updated man)

i added after file.write , seems solve it:

file.seekg(file.tellp(), ios::beg); 

can explain why?

(yes can solved else)

somewhat arbitrarily, required perform seek in between read , write. it's not spelled out in standard, c++ standard mentions c++ fstream has same properties c stdio streams respect validity of stream operations, , c standard mentions positioning command required between reading , writing (or vice versa).

some platforms relax requirement. gcc after version 4.5 or 4.6, modified basic_filebuf eliminate byzantine rule.

by way, file.seekg( 0, ios::cur ) safer.