Make filtered_graph conditionally model the underlying graph

[tsnorri]

According to filtered_graph's documentation, the refinements of the model of the underlying graph are optional.

[jeremy-murphy]

Thanks for this, but I'm afraid I don't really understand what it is doing or why it is a good thing. :)
Could you quote relevant paragraphs from the documentation and maybe even provide an example of how it changes behaviour? Thanks.

[tsnorri]

Right, I was quite terse, sorry about that. The issue is with adapting an existing graph type G for use with BGL and especially filtered_graph. If G does not model all the graph concepts filtered_graph supports, filtered_graph cannot be specialised for G since its graph_traits may lack some of the types required by the graph concepts. For instance, if G does not model BidirectionalGraph, the graph_traits specialisation for G may lack in_edge_iterator but filtered_graph requires it anyway. I attempted to fix this by defining the types conditionally in filtered_graph.

I think that this is something that should be fixed in filtered_graph since its documentation (§ Model of) states that “if the underlying Graph type models fewer or smaller concepts than these, then so does the filtered graph.” I think the unspecialised version of graph_traits handles the issue by defining the missing types as void, but the LEDA graph example in the documentation suggests that specialising graph_traits is the preferred approach.

Defining the types required by each of the graph concepts in BGL in the graph_traits specialisation as void or otherwise would also fix the issue but I do not think it is preferable. (On the other hand, defining the required types in G would make it possible to use the unspecialised graph_traits but may not be possible.) By looking at the example code for using the LEDA graph, I do not think the problem occurs there, since the existing graph type seems to model at least all of the concepts filtered_graph supports and hence the associated graph_traits specialisation seems to define all the types used by filtered_graph.