From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2007-06-12 06:42:43
Andrej van der Zee <mavdzee_at_[hidden]> 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 <typename seq1, typename seq2>
> struct intersect
> {
> typedef typename copy<seq2
> , inserter< set<>, insert< _1, _2 > >
> >::type set_seq2;
>
> typedef typename copy_if<seq1
> , has_key <set_seq2, _1>
> , 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<S,S>::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