Hi List, as the Subject states. I would like to initialize a std::priority_queue with vertex_descriptors and a comparison object as a template parameter but the compiler tells me: ... /usr/include/c++/4.5/bits/stl_heap.h:303:4: error: invalid conversion from ‘unsigned int’ to ‘void*’ /usr/include/c++/4.5/bits/stl_heap.h:303:4: error: initializing argument 1 of ‘bool degree_compare::operator()(void* const&, void* const&)’ ... if i try to initialize the queue and i just don't know why 'bool degree_compare::operator()(void* const&, void* const&)' comes up. Could you please help me to initialize my priority_queue? best regards Christoph // begin code #include <iostream> #include <map> #include <queue> #include #include typedef boost::adjacency_list_traits link_adjacency_list_traits; struct vertex_properties; struct edge_properties; struct graph_properties; struct vertex_properties { std::map used_colors; // this one works fine, i can use a traits::edge_descriptor }; struct edge_properties { int color; }; struct degree_compare; typedef std::priority_queue < link_adjacency_list_traits::vertex_descriptor, std::vector , degree_compare> VERTEX_DEGREE_PQ; struct graph_properties { int delta; boost::shared_ptr pq; }; typedef boost::adjacency_list < boost::listS, boost::listS, boost::bidirectionalS, vertex_properties, edge_properties, graph_properties> link_graph; class degree_compare { private: link_graph g; public: degree_compare(const link_graph &g): g(g) {std::cout << "Num Vertices: " << num_vertices(g) << std::endl;} bool operator()(const boost::graph_traits::vertex_descriptor &a, const boost::graph_traits::vertex_descriptor &b) { return out_degree(a, g) < out_degree(b, g); } }; int main () { link_graph g; boost::graph_traits::vertex_descriptor a = add_vertex (g); degree_compare dc(g); // init the comparison object VERTEX_DEGREE_PQ pq(dc);// init the p_queue with comparison // object fails //pq.push(a); //pq.top(); } // end code