On Apr 1, 2009, at 6:19 AM, Florian.PONROY@fr.thalesgroup.com wrote:
Hi,
I want to check if a vertex_descriptor is valid before using it. As
I saw on
this list, I perform the following test :
Graph_t graph(10);
vertexDescriptor srcVertex = boost::vertex(424242, graph);
if (Graph_t::null_vertex() == boost::vertex(424242, graph))
{
return;
}
Obviously, vertex indice 424242 does not exist, but test fails, and
function
does not return, leading the a crash of the following algorithm
(BFS or
Dijkstra).
What did I do wrong?
Thanks for your help.
Hi, Florian.
The call:
boost::vertex(424242, graph);
attempts to return the vertex_descriptor for the 424242 vertex in the
graph, without validating that there in fact are 424242 in the graph.
Here is the implementation for adjacency_list using a random access
container for vertices (boost 1.37):
template
inline typename Config::vertex_descriptor
vertex(typename Config::vertices_size_type n,
const vec_adj_list_impl&)
{
return n;
}
static vertex_descriptor null_vertex()
{
return (std::numeric_limits::max)();
}
Here is the implementation for adjacency_list using a non-random
access container for vertices:
template
inline typename Config::vertex_descriptor
vertex(typename Config::vertices_size_type n,
const adj_list_impl& g_)
{
const Derived& g = static_cast(g_);
typename Config::vertex_iterator i = vertices(g).first;
while (n--) ++i; // std::advance(i, n); (not VC++ portable)
return *i;
}
static vertex_descriptor null_vertex()
{
return 0;
}
So in neither case are you likely to get back something equal to
null_vertex() when you ask for the vertex_descriptor of the 424242th
vertex in graph with 10 vertices.
You could validate the index before calling boost::vertex() by
checking that it falls in [0, num_vertices(g)).
-- Michael
