I'm not understanding something about the interaction of map with the algorithms. I've got a map whose value_type is another map. I'd like to iterate through the map entries and add all the "sub-map" entries to the "primary" map. I wrote a little testcase, attached below. When I compile it with g++ 3.3.3, I get this: boost-1_32/boost/mpl/pair.hpp: In instantiation of `boost::mpl::second<mpl_::_2>': fold2.cc:23: instantiated from `boost::mpl::value_type_impl<boost::mpl::aux::map_tag>::apply<map_types, mpl_::_2>' fold2.cc:23: instantiated from `boost::mpl::value_type<map_types, mpl_::_2>' fold2.cc:23: instantiated from here boost-1_32/boost/mpl/pair.hpp:56: error: no type named `second' in `struct mpl_::_2' fold2.cc:23: error: `types' is not a member of type ` boost::mpl::value_type<map_types, mpl_::_2>' fold2.cc:24: error: template argument 1 is invalid fold2.cc:24: error: ISO C++ forbids declaration of `type name' with no type boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp: In instantiation of `boost::mpl::apply_wrap2<int, map_types, boost::mpl::pair<key1, entry> >': boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: instantiated from `boost::mpl::apply2<int, map_types, boost::mpl::pair<key1, entry> >' boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: instantiated from `boost::mpl::aux::fold_impl<1, boost::mpl::m_iter<map_types, 2, 3>, boost::mpl::m_iter<map_types, 3, 3>, map_types, int>' boost-1_32/boost/mpl/fold.hpp:39: instantiated from `boost::mpl::fold<map_types, map_types, int>' fold2.cc:24: instantiated from here boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: error: ` int' is not a class, struct, or union type boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp: In instantiation of `boost::mpl::aux::fold_impl<1, boost::mpl::m_iter<map_types, 2, 3>, boost::mpl::m_iter<map_types, 3, 3>, map_types, int>': boost-1_32/boost/mpl/fold.hpp:39: instantiated from `boost::mpl::fold<map_types, map_types, int>' fold2.cc:24: instantiated from here boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:49: error: no type named `type' in `struct boost::mpl::apply2<int, map_types, boost::mpl::pair<key1, entry> >' boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp: In instantiation of `boost::mpl::aux::fold_impl<1, boost::mpl::m_iter<map_types, 2, 3>, boost::mpl::m_iter<map_types, 3, 3>, map_types, int>': boost-1_32/boost/mpl/fold.hpp:39: instantiated from `boost::mpl::fold<map_types, map_types, int>' fold2.cc:24: instantiated from here boost-1_32/boost/mpl/aux_/preprocessed/gcc/fold_impl.hpp:53: error: no type named `type' in `struct boost::mpl::apply2<int, map_types, boost::mpl::pair<key1, entry> >' boost-1_32/boost/mpl/fold.hpp: In instantiation of `boost::mpl::fold<map_types, map_types, int>': fold2.cc:24: instantiated from here boost-1_32/boost/mpl/fold.hpp:39: error: no type named `state' in `struct boost::mpl::aux::fold_impl<1, boost::mpl::m_iter<map_types, 2, 3>, boost::mpl::m_iter<map_types, 3, 3>, map_types, int>' fold2.cc:24: error: syntax error before `;' token fold2.cc:26: error: syntax error before `::' token An EDG-based compiler gives much the same error. What am I doing wrong? Much thanks for the help! -Dave #include <boost/mpl/fold.hpp> #include <boost/mpl/placeholders.hpp> #include <boost/mpl/copy.hpp> #include <boost/mpl/map.hpp> #include <boost/mpl/value_type.hpp> #include <boost/mpl/inserter.hpp> #include <boost/mpl/insert.hpp> #include <boost/mpl/pair.hpp> using namespace boost; using namespace boost::mpl; struct key1 {}; struct key2 {}; struct entry { typedef map<pair<key2, int> > types; }; typedef map<pair<key1, entry> > map_types; typedef fold<map_types, map_types, copy<value_type<map_types, _2>::types, inserter<_1, insert<_1, _2> > > >::type result; result::blah; // Get a printout