c++ ios::app and ios::ate

 

First let's assure, that I understand what you want to do: You want to open an existing file, append some data, then go back to the position where you started to append and write some other data at that position; was that correct so far? If yes, then you first have to know, that you cannot insert new data into the middle of a file, but only overwrite existing data. Here I wrote an example that will do so:

#include <fstream>

using namespace std;

int main()...{
  const char* filename = "test.txt";
  
  //just build a little sample file
  ofstream sampleFile(filename);
  sampleFile << "This is a sample content ";
  sampleFile.close();

  //declare a new file object for appending and "overwriting"...
  fstream file(filename, ios::ate | ios::out | ios::in);

  //...mark the current position...
  long changePos = file.tellp();
  file << "This text will be changed ";

  //..append something else
  file << "Append some other text";

  //...overwrite previous contents
  file.seekp(changePos);
  file << "changed text ";
  file.close();
  return 0;
}