efficient input range buffering

[Dmitry Olshansky]

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.

> 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
...

-- 
Dmitry Olshansky