RFC on range design for D2

[Steven Schveighoffer]

"Sergey Gromov" wrote
> You don't mention here which iterator usage pattern you are trying to
> model with ranges.  I can think of at least two.
>
> 1.  You use a single bidirectional 'center' iterator, center == 5.  As
> one would naturally do with iterators.  Note then that whenever you use
> your center for, say, backward iteration, you reconstruct the actual
> range by calling list.begin.  You do it on each iteration.  No wonder it
> stays valid even if you remove the first element in the meantime: you're
> constructing your range from scratch anyway.  If you want to model this
> pattern with ranges---no problem, keep an empty 'center' range, center
> == (5,5), and reconstruct backward iteration range,
>
> reverse = all.before(center);
>
> whenever you need to iterate, then
>
> center = reverse.end;
>
> This 'center' range, being slightly less efficient, stays valid and
> becomes invalid in exactly the same conditions as your classical
> iterator.

This is exactly the pattern I use.  I agree that your example would solve 
the problem, I hadn't thought of an empty range to be a cursor, that is 
clever!

The only missing piece to your solution is that I must construct the range 
after the center range in order to access the value to see where I need to 
go.

What I see as the biggest downside is the cumbersome and verbose code of 
moving the 'iterator' around, as every time I want to move forward, I 
construct a new range, and every time I want to move backwards I construct a 
new range (and construct a new 'center' afterwards).  So a 'move back one' 
looks like:

auto before = all.before(center);
if(!before.isEmpty)
  center = before.pop.end;

And to move forward it's:
auto after = all.after(center);
if(!after.isEmpty)
  center = after.next.begin;

To get the value there, I have to do:
all.after(center).left // or whatever gets decided as the 'get first value 
of range' member

or if opStar is used:

*all.after(center);

I much prefer:

forward:
if(center != list.end)
    ++center;

reverse:
if(center != list.begin)
   --center;

get value:
*center;

Especially without all the extra overhead

I see both methods as being just as open to mistakes, the first more-so, and 
more difficult to comprehend (at least for me).

-Steve