Question

I’m trying to use an overloaded “>>” to scan input from a file.

The problem is, I have no idea how to deal with end of file.
In this case, my file is composed of a number, followed by several chars

Ex:

9rl

8d

6ff

istream &operator>>(istream &is, Move &move)
{
  char c;
  int i = 0;

  c = is.get();

  if (!isalnum(c))
      return;

  move.setNum(c); // I convert the char to an int, but I'l edit it out

  while ( (c = is.get()) != '\n')
  {
    move.setDirection(i, c); //sets character c into in array at index i
    i++;

  } // while chars are not newline

  return is;
} // operator >>

The test for the character being alpha numeric worked when I had this as a regular function, but doesn’t work here as it expects an input stream to be returned. I’ve tried returning NULL as well. Suggestions?

EDIT: this is being called in a while loop, So i’m trying to figure out some way to have this trigger some flag so that I can break out of the loop. In my previous function, I had this return a boolean, returning true if successful or false if the character was not alphanumeric

Answer 1

Return is. Callers should check the stream for errors.

Be sure to set error bits as appropriate:

std::istream &operator>>(std::istream &is, Move &move)
{
  char c;
  int i = 0;

  c = is.get();
  if (is.eof())
    return is;
  else if (c < '0' || c > '9') {
    is.setstate(std::ios::badbit);
    return is;
  }
  else
    move.setNum(c-'0');

  while ( (c = is.get()) != '\n' && is)
    move.setDirection(i++, c);

  if (c != '\n')
    is.setstate(std::ios::badbit);
  return is;
}

Use it as in the following:

int main(int argc, char **argv)
{
  std::stringstream s;

  s << "9rl\n"
    << "8d\n"
    << "6ff\n";
  s.seekg(0);

  Move m;
  while (s >> m)
    std::cout << m;

  if (s.bad())
    std::cerr << argv[0] << ": extraction failed\n";

  return 0;
}

Notice that the code uses the instance m only after successful extraction.