On 8/23/2012 4:38 AM, Eric Niebler wrote:
Say I have a valid fusion sequence. I want to use an mpl algorithm on it. I include boost/fusion/mpl.hpp and put a nested tag type in my sequence that is an alias for fusion::fusion_sequence_tag. So far, so good. Then I try to call mpl::pop_back on this sequence, and I find that, despite fulfilling all the documented requirements for Fusion ForwardSequence, the code fails to compile. That's because of this in boost/fusion/mpl/pop_back.hpp:
template <> struct pop_back_impl<fusion::fusion_sequence_tag> { template <typename Sequence> struct apply { typedef typename fusion::result_of::pop_back<Sequence>::type result;
typedef typename fusion::result_of::convert< typename fusion::detail::tag_of<Sequence>::type, result>::type type; }; };
My code doesn't compile because I haven't implemented the fusion::convert customization point. Where is that documented and what does it do?
The basic problem is that MPL has this Extensible Sequence concept that Fusion does not have. Keep in mind that mpl::pop_back requires a Front Extensible Sequence and Fusion only guarantees adaptation of an MPL Forward, Bidirectional or Random Access Sequence (http://tinyurl.com/c7tmyfj). In this regard, we are actually going beyond what's guaranteed by Fusion. That being said, I am really considering adding extensible sequences into Fusion. The Deque, for example is a good example. Regards, -- Joel de Guzman http://www.boostpro.com http://boost-spirit.com