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 23:27:25

On Fri, 18 Feb 2011, Cedric Laczny wrote:

> On Friday, 18. February 2011 07:50:09 Jeremiah Willcock wrote:
>> 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.
>
> I just saw, that reverse_graph.hpp does not provide add_edge()... Which
> function is then called? The one of the underlying graph, that is _not_
> reversed?

Yes -- he's calling add_edge on g.m_g, the underlying graph.

> Because I was assuming that add_ege would simply return an edge_descriptor of
> the same type as the input graph, which in this case actually is
> reverse_graph<>. Therefore I didn't quite see the implicit conversion...

It's implicitly converting from edge_descriptors of the underlying graph 
to those of the reverse_graph.

-- Jeremiah Willcock