On Fri, 04 Jun 2010 15:35:06 -0400, Graham Fawcett <[email protected]>
wrote:
Hi folks,
I expected this program to print the lines of a file in reverse order:
// bad.d
import std.stdio;
import std.range;
void main() {
auto f = File("bad.d");
foreach(char[] line; retro(array(f.byLine())))
writeln(line);
}
...but instead it prints a jumble of line fragments. I suspect that
it's because byLine() is not dup'ing the arrays it's reading from the
file?
This version works:
// 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);
}
..but if you remove the '.dup' then it prints a similar mess.
So, is there a bug in byLine(), or am I just using it wrong?
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
Copied from my .announce reply :)
