Re: [Boost-users] [graph] Is reverse_graph<> read-only?

Subject: Re: [Boost-users] [graph] Is reverse_graph<> read-only?
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2011-02-18 01:50:09

On Fri, 18 Feb 2011, Cedric Laczny wrote:

> Hi Jeremiah,
>
> On Friday, 18. February 2011 00:14:08 Jeremiah Willcock wrote:
>> On Fri, 18 Feb 2011, al.zatv wrote:
>>> Jeremiah Willcock <jewillco_at_[hidden]> писал(а) в своём письме Fri, 18
>>> Feb
>>>
>>> 2011 01:49:27 +0300:
>>>>> Is reverse_graph<> in Boost::Graph Library, read-only?
>>>>
>>>> It appears to be read-only.  It should be possible to add mutation; it
>>>> just isn't there yet.  Do you need that feature?
>>>
>>> Yes. My program build two trees in the same graph. First tree is
>>> "forward". Second tree is "backward": build by the same algorithm, but
>>> in reverse order. So I need add_vertex and add_edge for reverse trees. I
>>> wrote them this way (please look if I'm correct - because I'm a newbie
>>> and can make stupid errors).
>>
>> Do those versions work?  I forgot whether edge_descriptors in the original
>> graph are implicitly convertible to edge_descriptors in the reverse_graph;
>> your implementations require that.
>
> Just of pure interest, where do you see this requirement in the code? It's not
> that I doubt the fact that it actually is like this but I am interested in
> knowing how this can be seen in these short, two functions?

He's directly returning the result of add_edge() on the underlying graph 
as a pair<reverse_graph::edge_descriptor, bool> without explicitly 
converting the underlying graph's edge_descriptor to the reverse_graph's 
version.

Anyway, it turns out that the descriptors are exactly the same between the 
underlying and reverse graphs; that is not documented, however.

>> Your code would be easy to fix for the
>> other case, though.
>
> Again, I would really like to know how.

See above about the types being the same.  If not, there is no documented 
way to do the conversion.

-- Jeremiah Willcock