c++ - Writing on last line with fstream -

i trying replace string in text file. replace() function.

here code:

#include <iostream> #include <fstream> #include <cstring> using namespace std; int main() {  fstream file("c:\\users\\amir hossein\\desktop\\temp1\\test1.txt");  char *sub; sub = new char[20]; sub = "amir"; char *replace1; replace1 = new char[20]; replace1 = "reza"; char *buffer; buffer = new char[100];   int i, temp; streampos flag; int c=0,mark; bool flagforwrite=0;   if (file.is_open()){     while (!file.eof()) {         flag = file.tellg();         file.getline(buffer, 100);    //we'll search sub inside of buffer, if true, we'll replace replace string         flagforwrite=0;         (i=0;buffer[i];i++){             mark=i; c=0;             while (sub[c] && sub[c]== buffer[mark]){                 c++; mark++;             }              if (!sub[c]){   //make changes in line                 (int j=i,count=0;count<strlen(replace1);j++,count++) buffer[j] = replace1[count]; //until here, we've replace1d sub                 flagforwrite=1;             }          }          if (flagforwrite){   //write line ( after whole changes in line have been made!!                 file.seekp(flag);                 file << buffer << "\n";   // endl here super important!! if don't put it, i'll mess                 if(file.bad()) cout << "error\n";         }     } }  else cout << "error!!\n"; file.close();   delete[] sub; delete[] replace1; delete[] buffer;   return 0; } 

i want replace "amir" "reza".

my text file includes 4 lines:

hi name amir , friend's name amir too!
hi name amir , friend's name amir too!
hi name amir , friend's name amir too!
hi name amir , friend's name amir too!

when run program this.

hi name reza , friend's name reza too!
hi name reza , friend's name reza too!
hi name reza , friend's name reza too!
hi name amir , friend's name amir too!

what wrong last line?

i think problem here:

if (flagforwrite){                      file.seekp(flag);                     file << buffer << "\n";                     if(file.bad()) cout << "error\n";             } 

why flag referring line?

i'm using gnu gcc compiler.

i found problem! caused eof()! instead of using while(!file.eof()) used while(file.getline(buffer,100))

and moved file.tellg() end, , defined line this: streampos line=file.tellg();