[boost] Re: iterator_adaptor distance question

From: David Abrahams (dave_at_[hidden])
Date: 2004-04-12 14:56:56

"Neal D. Becker" <ndbecker2_at_[hidden]> writes:

> Is the preference for the former case over the latter intentional?  

Not particularly; it just worked out that way.  I guess in the
interest of efficiency it'd be better to take the one that doesn't
involve a unary minus.

> This
> caused some surprise when I tried to re-implement my
> cycle_iterator_adapator (a kind of circular buffer adaptor).
>
> std::copy (b, a, ...)
> will do:
>   template<typename _RandomAccessIter, typename _OutputIter>
>     inline _OutputIter
>     __copy(_RandomAccessIter __first, _RandomAccessIter __last,
>            _OutputIter __result,
>            random_access_iterator_tag)
>     {
>       typedef typename iterator_traits<_RandomAccessIter>::difference_type
>           _Distance;
>       for (_Distance __n = __last - __first; __n > 0; --__n) {
>         *__result = *__first;
>         ++__first;
>         ++__result;
>       }
>       return __result;
>     }
>
> As I implemented cycle_iterator_adaptor distance_to, it was expecting that
> given iterators [a,b], you probably want a positive distance from a
> to b,

Correct, if b is reachable by applying increments to a.

> but the way iterator_adaptors is implemented, it wants a negative
> distance. 

Huh?  What do you mean "it wants"?

> Not hard to workaround, but it seemed odd that the default case was
> inverted, and I wondered if that was really intentional.

Now I'm completely confused.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com