Re: [boost] [iterator] Combining traversal_tags with iterator_tags

Subject: Re: [boost] [iterator] Combining traversal_tags with iterator_tags
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-01-07 16:56:19

on Sat Jan 07 2012, Ronald Garcia <rxg-AT-cs.cmu.edu> wrote:

> Hi,
>
> In MultiArray I would like use iterator_facade to create an iterator
> that can satisfy random_access_traversal, input_iterator_tag, and
> output_iterator_tag.  I see that in the file
> boost/iterator/detail/facade_iterator_category.hpp, at line 138 there is a class in the boost::detail namespace declared
>
> template <class Category, class Traversal>
> struct iterator_category_with_traversal
>
> So it looks like passing:
> boost::detail::iterator_category_with_traversal<boost::detail::input_output_iterator_tag,boost::random_traversal_tag>
>
> as the CategoryOrTraversal would be what I want.  However, two
> components above are in the detail namespace, so I am not sure if they
> are meant to be used by other libraries.  Is this a safe operation?

Using things in detail namespaces is never really "safe" unless you own
the surrounding namespace.

> Side Notes:
> In my searches, I found a few buggy bits in the docs and code comments:
>
> - In boost/iterator/detail/facade_iterator_category.hpp, line 76, there's a comment:
> // If writability has been disabled per the above metafunction, the
> // result will not be convertible to output_iterator_tag.
>
> This comment corresponds to behavior that was removed in changeset:
> https://svn.boost.org/trac/boost/changeset/21683
>
> Now the result will never be convertible to output_iterator_tag.
>
> - In the documentation for iterator_facade, there's a bit of a booboo where the function is first written:
> iterator-category(CategoryOrTraversal, value_type, reference)
>
> Then defined as:
>
> iterator-category(C,R,V)
>
> So I think the argument order is flipped.
>

Please open Trac tickets for any of these that don't already have them.

Thanks!

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com