Subject: Re: [Boost-users] [iterator] [iterator_facade] Why is advance not defined for bidirectional iterators?
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2011-08-22 10:53:31
On 22.08.2011 13:09, Ovanes Markarian wrote:
> Sebastian,
>
> thanks for your answer.
>
> On Mon, Aug 22, 2011 at 11:40 AM, Sebastian Redl
> <sebastian.redl_at_[hidden] <mailto:sebastian.redl_at_[hidden]>>
> wrote:
>
> On 22.08.2011 10:51, Ovanes Markarian wrote:
>
> [...]
>
>
> This mismatch is why the split of traversal and access categories
> was introduced in Boost.Iterator, but it didn't make it into the
> new standard, and it was never in the old to begin with, so
> std::advance is not aware of it. What you basically need is a
> traversal-aware std::advance.
>
> I will try to fix this with a proper reference type. Or what do you
> mean with traversal-aware std::advance?
std::advance will switch implementations based on the standard iterator
tags, i.e. std::input_iterator_tag, std::forward_iterator_tag,
std::bidirectional_iterator_tag and std::random_access_iterator_tag.
(Actually, forward isn't treated any different than input in
std::advance.) These categories conflate traversal and access. The
problem is that to be a forward iterator, the associated type
'reference' must be a true reference. So whenever that isn't the case,
the iterator is an input_iterator, no matter how you can navigate it.
Boost.Iterator separates the concepts, and so it has
boost::one_pass_traversal_tag, boost::forward_traversal_tag,
boost::bidirectional_traversal_tag, and
boost::random_access_traversal_tag. However, you would need an advance
algorithm that is aware of these tags. No std::advance I know is, so
basically, you can't use it. (I'm not sure if you're allowed to
specialize std::advance.)
Sebastian