Supporting different variations for reading file header (see Update)

Question

I am trying read the header for a PGM file, which I found could have the follow variations:

P2
255 255
255

or

P2
255 255 255

or

P2 255 255 255

right now, I got this code:

  std::ifstream file(file_name);
  std::string line_one, line_two, line_three, line_pixels;

  char magicNumber;
  int width, height;

  while(getline(file, line_one)) {
    if(line_one.size() > 0 && line_one.at(0) != '#') {
      magicNumber = line_one.at(1);
      break;
    }
  }

  while(getline(file, line_two)) {
    if(line_two.size() > 0 && line_two.at(0) != '#') {
      std::stringstream ss(line_two);
      ss >> width >> height;
      break;
    }
  }

  while(getline(file, line_three)) {
    if(line_three.size() > 0 && line_three.at(0) != '#') {
      this->max_value = stoi(line_three);
      break;
    }
  }

which works fine for the first variation, but not so much for the other two. I think this could be solve by checking if the end of line was reached after each 'while'/ 'getline' blocks above. Am i right? If so, how to do that?

UPDATE

I try this code, but it is not skipping the lines with comments (starting with #):

  std::ifstream file(file_name);
  std::string line_one, line_two, line_three, line_pixels;

  std::string magicNumber;
  int width, height;

  file >> magicNumber;

  if(file.peek() == '#') {
    std::string comment;
    getline(file, comment);
  }

  file >> width;

  if(file.peek() == '#') {
    std::string comment;
    getline(file, comment);
  }

  file >> height;

  if(file.peek() == '#') {
    std::string comment;
    getline(file, comment);
  }

  file >> this->max_value;

  if(file.peek() == '#') {
    std::string comment;
    getline(file, comment);
  }