title: I want to write some data into a .txt file using c++ / opencv
body: <p>I want to save some information about every frame in a video to a <code>.txt</code> file.</p>
  
  <p>The information could be like the current frame number, the position of the feature points etc,</p>
  
  <p>I could only find ways to write it into a XML/YML file. Is it possible to write these into a .txt file? IF so, how ?</p>
  
  <p>Thanks in advance!</p>