change a file by offset : fstream « File Stream « C++ Tutorial

1. C++ Tutorial
2. File Stream
3. fstream

#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;

int main(int argc, char *argv[])
{

  fstream out("test.txt", ios::in | ios::out | ios::binary);
  if(!out) {
    cout << "Cannot open file.";
    return 1;
  }

  out.seekp(12, ios::beg);

  out.put('a');
  out.close();

  return 0;
}