[boost] Re: null vertex, edge descriptors

From: Tim Culver (tim.gmane_at_[hidden])
Date: 2002-09-17 08:16:26

Oops, that's what I get for posting made-up code snippets.  What I meant 
was:

typedef adjacency_list<listS, listS, ...> Graph;
typedef graph_traits<Graph>::vertex_descriptor Vdesc;
typedef graph_traits<Graph>::edge_descriptor Edesc;
typedef graph_traits<Graph>::vertex_iterator Viter;
typedef graph_traits<Graph>::edge_iterator Eiter;

Vdesc find_vertex(Graph& g) {
   Viter v, v_end;
   for (tie(v, v_end)=g.vertices(); v!=v_end; v++)
     if (my criterion)
       return *v;
   return Vdesc();
}

I could just return a Viter instead of a Vdesc.  But I have lots of code 
that finds it convenient to hold on to vertex descriptors instead of 
iterators.  Mixing them would be a lot of trouble, since you can't 
efficiently convert a descriptor to an iterator.  I'd have to switch to 
use iterators throughout.

Also, I think the convention above might be a good alternative.  I 
notice that the vertex descriptor does have a default constructor.  My 
question is, does a default-constructed vertex descriptor act like a 
null value, in that it never corresponds to a valid vertex, and two 
nulls always compare equal?  The documentation specifies that the 
default constructor and operator== must be defined, but the semantics 
are not specified.

http://www.boost.org/libs/graph/doc/Graph.html

Thanks for your response, David!

Tim