Re: [boost] [repost][multiarray] Bug with boost::detail::multi_array::array_iterator on MSVC 2010RC?

Subject: Re: [boost] [repost][multiarray] Bug with boost::detail::multi_array::array_iterator on MSVC 2010RC?
From: Thomas Klimpel (Thomas.Klimpel_at_[hidden])
Date: 2010-03-13 10:27:39

Jesse Perla wrote:
> I have attached my test files.  Any ideas or patches would be appreciated

I could reproduce the problem with MSVC 2010beta2. I noticed that only the debug version fails to build. So I added the line
#define _HAS_ITERATOR_DEBUGGING 0
and everything compiled fine.

So the simplest solution is to add
ADD_DEFINITIONS(-D_HAS_ITERATOR_DEBUGGING=0)
to your CMakeLists.txt, which many people recommend in any case, because they claim that the debug version under MSVC would otherwise violate the "runtime complexity specifications" of the corresponding containers and iterators. (I don't know whether this is really true, but I really think that this workaround is not unreasonable.)

But of course, you want to know whether this is a bug in boost::multi_array or a bug in MSVC 2010beta2. Before we come to that, another "workaround" would be to change multi_array/iterator.hpp:56 for 
        , boost::random_access_traversal_tag
to
        , std::random_access_iterator_tag

This will most probably work, but would lying to the standard library, because the reference reference type of the corresponding iterator is not value_type& (but a proxy):

http://lists.boost.org/boost-users/2006/11/23565.php

What effectively happens it that the iterator gets the category "input_iterator", but std::copy wants to have an "output_iterator". I think I understand why the iterator doesn't satisfies the requirement of a "forward_iterator", but it's unclear to me why it didn't qualify as an "output_iterator". There is an "input_output_iterator_tag" in "boost/iterator/detail/facade_iterator_category.hpp", so it would have been possible for the iterator to be both "input_iterator" and "output_iterator". However, it seems to me that "input_output_iterator_tag" is never used in the Boost.Iterator library. I would have to read more about "output_iterator" to understand whether Boost.Iterator is correct here. With my current understanding of "output_iterator", I would say it's a bug in "boost/iterator/detail/facade_iterator_category.hpp".

Perhaps somebody with more knowledge about "output_iterator" and Boost.Iterator can explain to me why an iterator using the "boost::random_access_traversal_tag" for the "CategoryOrTraversal" template parameter of "iterator_facade" will never get the category "output_iterator" (or "input_output_iterator_tag").

Regards,
Thomas