Re: [Boost-users] [Graph] What can I do with null_vertex?

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2004-02-20 11:40:36

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&lt;G&gt;::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 <jsiek_at_[hidden]>
http://www.osl.iu.edu/~jsiek
Ph.D. Student, Indiana University Bloomington
C++ Booster (http://www.boost.org)
Office phone: [phone]
_______________________________________________