Re: [Boost-users] [Boost.Graph] Accessing underlying graph of a boost::reverse_graph

Subject: Re: [Boost-users] [Boost.Graph] Accessing underlying graph of a boost::reverse_graph
From: Björn Pollex (bjoern.pollex_at_[hidden])
Date: 2016-09-13 04:49:26

Hi Daniel,

the use case is as follows. I have defined a custom graph concept, in which each edge has certain properties, such as a length and a sequence (basically a string). I’ve defined custom property tags for them as well. Now users can make their graphs implement this concept by providing property getters for these properties. Now I’d like to provide a generic mechanism for exposing these properties on reversed graphs. So when a user provides a property getter for the sequence of edges of their graph type, then the reverse_graph of their type should automatically expose the same property, only with reversed sequences. The problem is, in order to generically define the property getter for the reversed graph, I need to be able to access the underlying graph. Here is a more concrete example:

struct edge_sequence_tag {
    using kind = boost::edge_property_tag;
};

Let G be some user-defined graph type, then they could implement a property getter like this (skipping all the boilerplate trait specialisations for brevity):

boost::property_map<G, edge_sequence_tag>::const_type
get(edge_sequence_tag, const G& graph) {
    // return appropriate property map
};

Similarly, I could define a generic version of this function to support reversed graphs (trait specialisations can be written generically as well):

class reverse_sequence_property_map {
    // class that wraps a property map and reverses the sequences it produces
};

template<typename U>
boost::property_map<boost::reverse_graph<U>, edge_sequence_tag>::const_type
get(edge_sequence_tag, const boost::reverse_graph<U>& graph) {
    const auto & underlying_graph = ???;
    return reverse_sequence_property_map{get(edge_sequence_tag{}, underlying_graph())};
};

As you can see, this fails because there is no mechanism to access the underlying graph. Do you have any advice on how to achieve this?

Cheers,

        Björn

> On 12 Sep 2016, at 16:58, Daniel Hofmann <daniel_at_[hidden]> wrote:
> 
> On 09/12/2016 04:35 PM, Björn Pollex wrote:
>> Hello,
>> 
>> I am currently trying to find out if there is a way to access the underlying graph of a boost::reverse_graph [1]. There does not seem to be support for this. I want to use this to generically define interior edge-properties for reversed graphs, if the underlying graph has them as well. Currently this does not work, because I cannot access the underlying graph object.
> 
> For a reverse_graph's bundled properties operator[] simply dispatches to
> the underlying graph's operator[].
> 
> reverse_graph has a public GraphRef m_g member attribute.
> It should be private and a implementation detail. But it isn't.
> 
> But can you elaborate a bit on your use-case?
> 
> Cheers,
> Daniel J H
> 
>> 
>> Am I overlooking something? If not, where could I request to add this?
>> 
>> Cheers,
>> 
>> 	Björn Pollex
>> 
>> [1]: http://www.boost.org/doc/libs/release/libs/graph/doc/reverse_graph.html
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