Writting to file in callback function

[JorgeNevada]

Hey everyone! I want to do a basic thing but for some strange reason there is something not working.

I need to write to a file but I'm handling all the code in a callback function, which in turn is declared in a class. Here is the architecture of my code:

int main (int argc, char **argv)
{
   
  ImageConverter Aclass;
  while(1); //infinite loop
  return 0;
}
 
class ImageConverter
{
//some variables ..
   
public:
 
ImageConverter ():
  
  {
 
    
 
  }
 
  ~ImageConverter ()
  {
 
  }
 
  void imageCb () //this function gets called when I receive an image frm server
  {
 
    .... //some calculations
     
    fstream outfile ("directory/train.txt");
      
    outfile.open ("directory/train.txt", fstream::in | fstream::out | fstream::app);
     
    outfile<<"P,"<<SomeValues<<endl;
     
    outfile.close();
    
   ....
 
 
  }
 
};

But this doesn't work, the file gets created but it allways overwrites itself which is not what I need to do.

Can anyone please tell me what I am doing wrong? Is it because i am constantly declaring outfile?

Thanks