Hi Doug, On Feb 20, 2004, at 11:06 AM, Douglas Gregor wrote:

There are no semantics for null_vertex. I assume I can copy it, but can I compare it against other vertex descriptors via == and !=? Is it always different from "real" vertex descriptors?

By "null_vertex", do you mean the graph_traits<G>::null_vertex() function? I believe http://www.boost.org/libs/graph/doc/Graph.html does give the semantics. It says: Returns a special <tt>vertex_descriptor</tt> object which does not refer to any vertex of graph object which type is <tt>G</tt>. Also, in the section describing the associated types: <td><pre>boost::graph_traits<G>::vertex_descriptor</pre> A vertex descriptor corresponds to a unique vertex in an abstract graph instance. A vertex descriptor must be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html">Default Constructible</a>, <a href="http://www.sgi.com/tech/stl/Assignable.html">Assignable</a>, and <a href="http://www.sgi.com/tech/stl/EqualityComparable.html">Equality Comparable</a>. </td> </tr> Cheers, Jeremy _______________________________________________ Jeremy Siek http://www.osl.iu.edu/~jsiek Ph.D. Student, Indiana University Bloomington C++ Booster (http://www.boost.org) Office phone: (812) 856-1820 _______________________________________________