Insert a line in a text file using fstream

You can use std::vector<string> to read each line in the text file.  While you're reading the line into the std::vector<string>, you can insert a line.
Then you can write to the text file, but writeing the std::vector<string> to it.

When you read it, use the global getline function to read the text file line by line.
Something like the following:
     vector<string> lines;
     fstream in("test2.cpp");
     while (!in.eof())
     {
          string line;
          getline(in,line);
          lines.push_back(line);
     }
     in.close();

#include <fstream>
#include <vector>
#include <string>

using namespace std;

int main()
{
   vector<string> FilDat;
   fstream Fil("filename.txt");

   // Read in all the data
  while (!Fil.eof()))
  {
     string Lin;

     getline(Fil,Lin);
     FilDat.push_back(Lin);
  }

  // Insert a new line after the 3rd line.
  FilDat.insert(FilDat.begin()+2,"This is a new line");

  // Write out the data.
  const int LinCnt = FilDat.size();  

  Fil.seekp(0);
  for (int i = 0; i < LinCnt; ++i)
      Fil << FilDat[i] << endl;

   return 0;
}

Thanks. I thought it would be this way, but I hopped you guys would have come with a better solution. Thanks nayway for clarifying the things for me.

Every word processor in the world works in one of these ways.   Most are now memory based, readig all the file in and manipulating it in memory, but when memory was tight they sed to be mostly file based.