Re: [Boost-users] [BGL] Can't set non-const edge_descriptor = const edge_descriptor

On Fri, 3 Sep 2010, John Robertson wrote:

I have

struct edge_properties {     Traits::edge_descriptor ePredecessor; }

some_function ( const Graph g ) {

    e = edge(vertex_a, vertex_b, g).first;

    //fails here     g[e].ePredecessor = e;

}

compiler gives

./Includes/Utilities.cpp:451: error: passing ‘const boost::detail::edge_desc_impl’ as ‘this’ argument of ‘boost::detail::edge_desc_impl& boost::detail::edge_desc_impl::operator=(const boost::detail::edge_desc_impl&)’ discards qualifiers make: *** [pl] Error 1

My guess is that e is returned as a const but g[e].ePredecessor is declared a variable. How can I fix this?

The issue is actually (it appears) that since g is const, you cannot write to its properties. Note that the error message is about passing a const object as "this" to the operator=() call, not about the argument to the call. -- Jeremiah Willcock