File.byLine should return dups?

[Graham Fawcett]

Hi Steven,

On Fri, 04 Jun 2010 15:43:18 -0400, Steven Schveighoffer wrote:

> On Fri, 04 Jun 2010 15:35:06 -0400, Graham Fawcett <fawcett at uwindsor.ca>
> 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.
> 

Thanks for the response. Yes, it makes mode to favour performance for
the common case, and as you say it's not hard to resolve this issue.

> 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()))))

Yes -- duper (and iduper) for the win! Great idea.

> -Steve
> 
> Copied from my .announce reply :)

Sorry again for the .announce posting. :P

Graham