On 22/08/2012 22:38, 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?
As the name suggests, it converts a Fusion sequence type to a MPL sequence type. For example, it converts a fusion::vector to a mpl::vector, and a fusion::list to a mpl::list. Maybe pop_back shouldn't require that conversion though.