[boost] Re: null vertex, edge descriptors

From: Tim Culver (tim.gmane_at_[hidden])
Date: 2002-09-17 13:04:28

David Abrahams wrote:
> From: "Tim Culver" <tim.gmane_at_[hidden]>
> 
>>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?
> 
> 
> Definitely not, and it shouldn't.
> For many graph types, vertex_descriptor is an integer type which is a
> zero-based index into the graph's vector of vertices. That's important for
> efficiency.

Now I understand!  We want to be able to have descriptors without 
constructors.

Still, this does not prevent writing a function that will generate a 
null-like value.  I think this would be a good addition to the library 
for its graph datatypes.

if (vdesc == graph_traits<My_graph>::null_vertex_descriptor()) {
   // vdesc is invalid
}

Also, the particular case of adjacency_list<listS,listS,...> appears to 
have actual default constructors that initialize something to zero.  Can 
I use these as my null generators in this particular case?

Tim Culver