[BGL] Property map question

What determines when boost::get(Graph, vertex_myproperty_t) returns a boost::property_map<graph_t, vertex_myproperty>::type vs. a ::const_type? Suppose: enum vertex_myproperty_t { vertex_myproperty = 12345}; BOOST_INSTALL_PROPERTY(vertex,myproperty); typedef property<vertex_myproperty_t, MyPropertyObject, ....> vertex_properties_t; typedef adjacency_list<vecS,vecS,bidirectionalS, vertex_properties_t> graph_t; typedef property_map<graph_t, vertex_myproperty_t>::type vertex_myproperty_map_t; graph_t Graph;