[Boost-users] Boost Graph: edge indices

Subject: [Boost-users] Boost Graph: edge indices
From: Trevor Harmon (Trevor.W.Harmon_at_[hidden])
Date: 2010-03-31 21:19:12

Hi,

In the Boost Graph library, vertices have an index (vertex_index_t),  
and I was under the impression that edges have one, too. At least,  
there's a predefined type called edge_index_t. However, I can't figure  
out how to access an edge's index. For example, consider a graph  
defined like:

typedef adjacency_list<vecS, vecS, directedS, Foo*> MyGraph;
typedef graph_traits<MyGraph>::vertex_descriptor Vertex;
typedef graph_traits<MyGraph>::edge_descriptor Edge;

And let's say I've got a vertex descriptor called "u". I can iterate  
over u's outgoing edges like this:

typedef graph_traits<MyGraph>::out_edge_iterator out_iter;
std::pair<out_iter, out_iter> outp;
for (outp = out_edges(u, g); outp.first != outp.second; ++outp.first) {
     Edge e = *outp.first;
     Vertex v = target(e, g);
     cout << index[v] << std::endl;  // Works fine
     cout << index[e] << std::endl;  // Compiler error
}

In this example, index[v] works as expected, but index[e] gives me a  
compiler error: "no match for ‘operator[]’ in ‘index[e]".

Perhaps my graph typedef needs to be changed so that its edges have a  
property map for edge_index_t, although I'm not sure how to do that.  
(And anyway I wouldn't expect vertices to have indices by default but  
edges not...)

Thanks for any help,

Trevor