[Boost-users] [graph] Modifying properties in a visitor?

Subject: [Boost-users] [graph] Modifying properties in a visitor?
From: James C. Sutherland (James.Sutherland_at_[hidden])
Date: 2009-01-15 16:52:50

Can a visitor modify properties in a graph?  I would like to use the  
following visitor with the breadth_first_search algorithm:

   struct ExecPriorityVisitor : public boost::default_dfs_visitor
   {
     ExecPriorityVisitor();
     void start_vertex( Vertex v, Graph& g ){ g[v].priority = 0; }
     void examine_edge( Edge e, Graph& g ){
       const Vertex src  = boost::source(e,g);
       Vertex dest = boost::target(e,g);
       int& priority = g[dest].priority;
       priority = std::max( priority, g[src].priority + 1 );
     }
   };

However, it fails to compile because apparently the graph argument  
must be const.  That implies that I cannot modify graph properties  
from within the visitor?  Why not?

Is the suggested approach to record the modifications in yet another  
object and later impose that on the graph, or is there a way to  
accomplish this through a visitor?

James