#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
 
int main(int argc, char *argv[])
{
  fstream inout("test.txt", ios::in | ios::out | ios::binary);
 
  if(!inout) {
    cout << "Cannot open input file.\n";
    return 1;
  }
 
  long e, i, j;
  char c1, c2;
  e = 5;
 
  for(i=0, j=e; i<j; i++, j--) {
    inout.seekg(i, ios::beg);
    inout.get(c1);
    inout.seekg(j, ios::beg);
    inout.get(c2);
 
    inout.seekp(i, ios::beg);
    inout.put(c2);
    inout.seekp(j, ios::beg);
    inout.put(c1);
  }
 
  inout.close();
  return 0;
}