#include <deque>
#include <fstream>
#include <iostream>
#include <string>
using namespace std;

int main()
  {
  deque <string> text;
  string s;

  // load the file
  ifstream inf( filename );  // You must supply the proper filename
  if (!inf) ...              // Check for errors here
  while (getline( inf, s ))
    text.push_back( s );
  if (!inf.eof()) ...        // and here
  inf.close();

  // modify the text
  text.insert(
    text.begin() + linenumber,  // again, you must supply the line number
    string( "This is a new line of text" )
    );

  // rewrite the new text to file
  ofstream outf( filename );
  if (!outf) ...
  for (deque <string> ::iterator
       line  = text.begin();
       line != text.end();
       ++line)
    outf << *line << '\n';
  outf.close();

  return 0;
  }