On Sun, 02 Oct 2011 16:46:38 +0200, Dmitry Olshansky
<[email protected]> wrote:
On 02.10.2011 18:12, Martin Nowak wrote:
I've written a wrapper to promote input ranges to buffered forward
ranges.
It allows to write range agnostic lexers/parsers with infinite
lookahead.
Buffering is done through a singly linked list of memory blocks that are
reference counted.
Each saved range holds a reference to all future blocks.
Blocks are recycled when being no longer used.
https://gist.github.com/1257196
Like it, but one thing catches my eye - why use GC for blocks when the
whole thing is already refcounted? Straight malloc/free would be a
better fit. Certainly it may use an allocator when we have them.
I've added that, didn't before as it's always quite a lot of boilerplate
to get this right.
There is a major issue with the somewhat broken
implicit-save-through-copy concept.
A lot of copies in function parameters, foreach loops etc. will also
create
references and thus can be easily responsible for inefficiencies.
While the concept of ForwardRange is quite elegant, I found myself
wondering on occasion that InputRange with restore could be way more
efficient. Consider:
auto r = range();
auto save_it = r; ///at least full bitwise copy and/or refCounting
if(do_stuff(r))
...
else
r = save_it;//another one
...
Compared to:
auto save_pt = r.savepoint(); //different object, minimal overhead
if(do_stuff(r))
...
else
r.restore(save_pt);//range knows how to do this efficiently
...
Looks interesting and clearly has some advantages.
I personally tend to alway write .save when I intend so and
thought that way more phobos functions should take ranges by auto ref
(i.e. actually advance them).
But I recapture from an earlier discussion that most people liked the
implicit save
and some proposed to ditch save at all.
How would restore allow to actually duplicate the range at different
positions.
I think a pretty common use case for forward ranges is backtracking:
auto start = range.save;
popWhile!pred(range);
doSomeThing(start, range);
Will I need to be able to do this?
auto range2 = range;
range.restore(start);
doSomeThing(range, range2);
