Re: [boost] [BGL] Why source() needs a graph argument?

Subject: Re: [boost] [BGL] Why source() needs a graph argument?
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2009-10-02 10:41:27

On Fri, 2 Oct 2009, Cosimo Calabrese wrote:

> Hi to all,
>
> the question is in subject. I don't understand it. Either source() and 
> target() functions only return the m_source member and the m_target of 
> edge_base. They don't use the graph argument, that is not even used.
>
> I need to know it because I'm implementing an union_graph adaptor, that 
> provides the join's view of 2 graph. When I call the source() function on a 
> union_graph, I provide an edge_descriptor; but whereas the source() function 
> of an adjacency_list needs a graph argument, I need to know which graph 
> belongs the edge, which is an 2 * O|E| operation on 2 graphs.

Some edge descriptors need information from the graph and do not contain a 
pointer to it (which would use extra space).  For example, the compressed 
sparse row graph's edge descriptor contains a source vertex and edge 
index, and computes the target from that index and a table in the graph. 
I would suggest using the Boost.Variant technique I mentioned in a 
previous email to make it easy (and constant time) to distinguish which 
edge descriptors are in each graph.

-- Jeremiah Willcock