[boost] [multi_array] Ticket#4874 "multi_array compile errors using Visual C++ 2010 in debug mode"

Subject: [boost] [multi_array] Ticket#4874 "multi_array compile errors using Visual C++ 2010 in debug mode"
From: Stephan T. Lavavej (stl_at_[hidden])
Date: 2011-09-14 22:14:40

Hi Boost devs,

VC10 users are occasionally encountering https://svn.boost.org/trac/boost/ticket/4874 and reporting it to MS. For example, see http://connect.microsoft.com/VisualStudio/feedback/details/687655/compilation-issue-in-debug-mode-with-boost-library where I've explained what's going on (I wrote this analysis back in Feb 2010 and I believe it's still correct):

The problem is that multi_array iterators report themselves as input iterators, which can't be written to (e.g. as the destination of a copy()).  Under IDL=0, copy() doesn't actually care how its destination iterator is marked with an iterator category tag, it just tries to write to the destination iterator, and that works.  Under IDL > 0, copy() wants to know whether its source and destination iterators are both random-access, in which case it can perform a range check and unwrap the destination iterator for increased performance.  It does this by overloading foo(input_iterator_tag, output_iterator_tag) and foo(random_access_iterator_tag, random_access_iterator_tag) and calling foo(category(source), category(destination)).  If both source and destination are random-access, both overloads are viable (i.e. callable) and the second overload is preferred, so it's actually called.  Otherwise, source is required to be at least an input iterator (maybe stronger) and destination is required to be at least an output iterator (maybe stronger) so the first overload must be viable and the second overload will be non-viable, so the first overload wins by default.

However, if the destination is not marked as an output iterator or stronger, then the first overload is non-viable as well as the second, so overload resolution fails and boom.

Thanks,
Stephan T. Lavavej
Visual C++ Libraries Developer