I think I've found an error in the MPL docs (In trunk and 1.35, and presumably everything in between). The documentation for mpl::map state that "count<m,k>::type" yields "The number of elements with the key k in m", where in fact what it returns is the number of elements *equal* to k in m. This is consistent with the documentation of mpl::count. I suspect this is an error in the docs rather than a bug, since there is also mpl::has_key, which serves the purpose count claims to be serving (but isn't). Consider this code: #include <boost/mpl/has_key.hpp> #include <boost/mpl/count.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/set.hpp> #include <boost/mpl/map.hpp> #include <boost/mpl/insert.hpp> using namespace boost::mpl; typedef set< int
::type my_set;
typedef map< boost::mpl::pair<int, int>
::type my_map;
BOOST_MPL_ASSERT((has_key<my_set, int >::type)); BOOST_MPL_ASSERT((count<my_set, int >::type)); BOOST_MPL_ASSERT((has_key<my_map, int >::type)); BOOST_MPL_ASSERT((count<my_map, int >::type)); BOOST_MPL_ASSERT((count<my_map, pair<int,int> >::type)); The fourth assertion fails. If the map docs were accurate then it would be the fifth assertion that failed. The same error is in the Associative Sequence docs, and perhaps other places. John Bytheway