Re: [boost] [iterator][range] BoostIteratorTraversalConcepts-aware boost::advance/distance

Subject: Re: [boost] [iterator][range] BoostIteratorTraversalConcepts-aware boost::advance/distance
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2017-06-29 20:44:59

On 28/06/2017 22:14, Andrzej Krzemienski via Boost wrote:
> 2017-06-28 22:03 GMT+02:00 Michel Morin via Boost <boost_at_[hidden]>:
> 
>> Hi,
>>
>> The Boost iterator traversal concepts have not been adopted by the C++
>> Standard.
>> Because of this, some of RandomAccessTraversalIterators (Boost's concepts)
>> are
>> treated as InputIterators (Standard's concepts) by the stdlib.
>>
>> IMHO, Boost.Iterator should provide BoostIteratorTraversalConcepts-aware
>> `boost::advance` and `boost::distance` to avoid the inefficiencies.
> 
> 
> Not just inefficiencies. Using `prev()` may simply cause UB. See here:
> https://akrzemi1.wordpress.com/2017/01/02/not-detecting-bugs/

I would like to confirm something in your post. I think the standard 
requires iterator_category to be one of the standard types, not 
something derived from them:

Draft N4659 (2017/03)  27.4.2 Standard iterator tags

For every iterator of type Iterator, 
iterator_traits<Iterator>::iterator_category shall be defined to be the 
most specific category tag that describes the iterator’s behavior

If this is correct, in your article is_BidirectionalIterator() should be 
defined using is_same instead of is_base_of, and Boost.Iterator's 
iterator_category definitions might not compatible with the standard. Or 
am I missing something?

Best,

Ion