Re: File.byLine ought to return dups?

[Steven Schveighoffer]

On Fri, 04 Jun 2010 15:27:03 -0400, Graham Fawcett <fawc...@uwindsor.ca>  
wrote:

Hi folks,
I expected the following program to print the lines of a file in
reverse:

    // bad.d
    import std.stdio;
    import std.range;
   void main() {
      auto f = File("bad.d");
      foreach(char[] line; retro(array(f.byLine())))
        writeln(line);
    }

However, this produces very unusual output: fragments of the same
lines are printed repeatedly. I suspect it's because the byLine()
'generator' is not dup'ing the arrays it reads from the file.

This works as expected:

    // good.d
    import std.stdio;
    import std.range;
   Retro!(char[][]) retroLines(File f) {
     char[][] lines;
      foreach(line; f.byLine())
        lines ~= line.dup;         // note the .dup
      return retro(lines);
     }
   void main() {
      auto f = File("good.d");
      foreach(line; retroLines(f))
        writeln(line);
    }

If you remove the '.dup', then this behaves badly as well.

So is this a bug in File.byLine, or am I just using it badly? :)

The latter.  File is re-using the buffer for each line, so you are seeing  
the data get overwritten.  This is for performance reasons.  Not everyone  
wants incur heap allocations for every line of a file ;)  As you showed,  
it's possible to get the desired behavior if you need it.  The reverse  
would be impossible.

Now, that being said, a nice addition would be to create a duper range  
that allows you to do one expression:

foreach(char[] line; retro(array(duper(f.byLine()))))

-Steve