Hi! for some reason these two typdefs do not work in my code: typedef typename PriorityQueue::nth_index<0>::type DissimilarityIndex; typedef typename PriorityQueue::index<DissimilarityT>::type DissimilarityIndex; When I actually use them: const DissimilarityIndex& di =_priority_queue.get<DissimilarityT>(); I get error-messages like this: ../../../pocket_graph/src/clustering/NearestNeighbors.h:300: error: invalid initialization of reference of type 'typename boost::multi_index::multi_index_container<Value1, IndexSpecifierList1, Allocator1>::nth_index<0>::type&' from expression of type 'const boost::multi_index::detail::ordered_index<boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, float, &NearestNeighborPair<uint>::dissimilarity>, std::less<float>, boost::multi_index::detail::ordered_index<boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, uint, &NearestNeighborPair<uint>::object>, std::less<uint>, boost::multi_index::detail::ordered_index<boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, uint, &NearestNeighborPair<uint>::nearest_neighbor>, std::less<uint>, boost::multi_index::detail::index_base<NearestNeighborPair<uint>, boost::multi_index::indexed_by<boost::multi_index::ordered_non_unique<boost::multi_index::tag<DissimilarityT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, float, &NearestNeighborPair<uint>::dissimilarity>, boost::multi_index::detail::null_arg>, boost::multi_index::ordered_unique<boost::multi_index::tag<ObjectT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, uint, &NearestNeighborPair<uint>::object>, boost::multi_index::detail::null_arg>, boost::multi_index::ordered_non_unique<boost::multi_index::tag<NearestNeighborT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::const_mem_fun<NearestNeighborPair<uint>, uint, &NearestNeighborPair<uint>::nearest_neighbor>, boost::multi_index::detail::null_arg>, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, std::allocator<NearestNeighborPair<uint> > >, boost::multi_index::tag<NearestNeighborT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::detail::ordered_non_unique_tag>, boost::multi_index::tag<ObjectT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::detail::ordered_unique_tag>, boost::multi_index::tag<DissimilarityT, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, boost::multi_index::detail::ordered_non_unique_tag>' The following typedef works: typedef typename boost::multi_index::index<PriorityQueue,DissimilarityT>::type DissimilarityIndex; The first two typedefs are in some way the more "natural" way to use indices and I wonder if I have just missed to set some preprocessor symbol... ...I am using gcc 3.2.2 Thanks, Lars