Re: [Boost-users] [iterator] [iterator_facade] Why is advance not defined for bidirectional iterators?

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 mailto:sebastian.redl@getdesigned.at> 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