From gregod@cs.rpi.edu Fri Feb 20 11:06:04 2004
From: Douglas Gregor <gregod@cs.rpi.edu>
To: boost-users@lists.preview.boost.org
Subject: [Boost-users] [Graph] What can I do with null_vertex?
Date: Fri, 20 Feb 2004 11:06:11 -0500
Message-ID: <200402201100.25580.gregod@cs.rpi.edu>
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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?

Doug

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From jsiek@osl.iu.edu Fri Feb 20 11:40:42 2004
From: Jeremy Siek <jsiek@osl.iu.edu>
To: boost-users@lists.preview.boost.org
Subject: Re: [Boost-users] [Graph] What can I do with null_vertex?
Date: Fri, 20 Feb 2004 11:40:36 -0500
Message-ID: <829410A8-63C3-11D8-A891-000A95C45228@osl.iu.edu>
In-Reply-To: <200402201100.25580.gregod@cs.rpi.edu>
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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@osl.iu.edu>
http://www.osl.iu.edu/~jsiek
Ph.D. Student, Indiana University Bloomington
C++ Booster (http://www.boost.org)
Office phone: (812) 856-1820
_______________________________________________

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