on Tue Sep 09 2008, "Kris Rousey" <krousey-AT-gmail.com> wrote:
2008/9/9 David Abrahams <dave@boostpro.com>:
Try this patch to boost/iterator/iterator_facade.hpp; does it fix the problem for you?
Yes. Well... It fixed THAT problem. After I took it a step further and tried to use the iterator like this:
*iter = make_tuple(i, i*2.0);
I then get this error:
boost/tuple/detail/tuple_basic.hpp:377: error: no match for 'operator=' in '((boost::tuples::cons<void, boost::tuples::cons<void, boost::tuples::null_type> >*)this)->boost::tuples::cons<void, boost::tuples::cons<void, boost::tuples::null_type> >::head = u->boost::tuples::cons<int, boost::tuples::cons<double, boost::tuples::null_type> >::head'
After looking into it further, it seems that std::back_insert_iterator inherits from std::iterator<std::output_iterator_tag, void, void, void, void>. Since it obviously has no interest in populating the right type information, would it make sense add partial specializations to boost/detail/iterator.hpp to grab the appropriate types from the container? e.g.
template <class CONTAINER> struct iterator_traits<std::back_insert_iterator<CONTAINER> > { typedef typename CONTAINER::value_type value_type; typedef typename CONTAINER::reference reference; typedef typename CONTAINER::pointer pointer; typedef typename CONTAINER::difference_type difference_type; typedef std::output_iterator_tag iterator_category; };
Well, that's a very clever idea. It subverts the purpose of boost/detail/iterator.hpp which was intended to be not much more than a wrapper for std::iterator_traits to work around compiler/library implementation deficiencies, but something like that just might work. That said, the reference type is definitely wrong in your specialization; it should be the same as the type returned by dereferencing the back_insert_iterator (I think that might be the back_insert_iterator type itself, IIRC).
I'll try this out and see if it works, but what is this the right way to go about it?
I'm fairly new to C++ and boost development, so I'm looking to learn the correct ways.
Well, the *really* right way to go about it is to write a fix that's not specific to back_insert_iterator, but that works for all kinds of output iterators that are not also forward iterators. That would take a little thought to get right, but it would be a good project for you to take on. -- Dave Abrahams BoostPro Computing http://www.boostpro.com