[Boost-users] Boost Graph: why can't get() take a const argument?

Hi, I have a question about line 54 of the canonical_ordering example: graph g(6); ... property_map::type e_index = get(edge_index, g); If I'm reading the Boost source code correctly, the get function's graph argument is declared const. This makes sense intuitively because, well, why would the graph need to be modified in order to retrieve a property? So the graph parameter passed to get() could be made const, correct? Like this: graph g(6); ... const graph g2 = g; property_map::type e_index = get(edge_index, g2); However, the above change gives a compiler error: conversion from ‘boost::adj_list_edge_property_map, boost::edge_index_t>’ to non-scalar type ‘boost::adj_list_edge_property_map, boost::edge_index_t>’ requested Can anyone explain why? Thank you, Trevor