uses both seekp( ) and seekg( ) to reverse the first characters in a file. : file pointer « File Stream « C++ Tutorial

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2. File Stream
3. file pointer

#include <iostream>
#include <fstream>
#include <cstdlib>
using namespace std;
   
int main(int argc, char *argv[])
{
  if(argc!=3) {
    cout << "Usage: Reverse <filename> <num>\n";
    return 1;
  }
   
  fstream inout(argv[1], ios::in | ios::out | ios::binary);
   
  if(!inout) {
    cout << "Cannot open input file.\n";
    return 1;
  }
   
  long e, i, j;
  char c1, c2;
  e = atol(argv[2]);
   
  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;
}