Re: [Boost-users] MPL intersection of type sequences and compile-time performance

Andrej van der Zee writes:

Than you for the quick reply :) Now I reimplemented intersect by converting the type sequences to mpl::set and do a mpl::has_key instead of mpl::contains:

template struct intersect { typedef typename copy, insert< _1, _2 > > >::type set_seq2;

typedef typename copy_if , back_inserter < vector<> > >::type type; };

Now I would like to optimize this and only convert to a set if the type sequence does not support mpl::has_key. How should I do this?

Ideally, we should have a metafunction that would allow you to query whether the sequence (or any other library entity) supports a particular concept (Associative Sequence in your case). In absence of that, something like the following should work: template< typename Sequence, typename Dummy = boost::mpl::false_ > struct is_Associative_Sequence : boost::mpl::false_ { }; template< typename S > struct is_Associative_Sequence< S , typename boost::mpl::has_key_impl< typename boost::mpl::sequence_tag<S>::type >::template apply::type > : boost::mpl::true_ { }; int main(int argc, char *argv[]) { BOOST_MPL_ASSERT_NOT(( is_Associative_Sequence< boost::mpl::vector0<> > )); BOOST_MPL_ASSERT(( is_Associative_Sequence< boost::mpl::map0<> > )); } HTH, -- Aleksey Gurtovoy MetaCommunications Engineering