Re: [boost] [graph] ranges used by boost.graph not compatible with c++11 range-based-for

Subject: Re: [boost] [graph] ranges used by boost.graph not compatible with c++11 range-based-for
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2013-10-02 16:57:27

> I noticed today that code of this form does not compile:
>
> typedef boost::adjacency_list<boost::vecS, boost::vecS,
> boost::bidirectionalS, Node, Edge> Graph;
> typedef boost::graph_traits<Graph>::vertex_descriptor Vertex;
> typedef boost::graph_traits<Graph>::edge_descriptor Edge;
> Graph graph = ...;
> Vertex vertex = ...;
> for (Edge edge: boost::out_edges(vertex, graph)) {...}
>
> The reason is that out_edges returns a std::pair of iterators. This
> is a valid range in the sense of boost.range, and so is compatible
> with BOOST_FOREACH. However, this is not a valid range in the sense
> of c++11 range-based-for.
>
> It's not clear whether this can easily be changed. In a perfect world
> these functions should probably return something like
> boost::iterator_range which would be compatible with both boost- and
> language-level concepts of range-ness. However, the graph
> documentation explicitly names the return types of out_edges et. al.
> (see [1]) as std::pairs, which eliminates some wiggle room. Is there
> an alternate way to address this?

Sadly, I don't think there s a way of turn an std::pair of iterators into 
a range-based-for-compliant range (see [1]).

So, I think that if Boost.Range wants to interoperate with standard
ranges, it will need to deprecate std::pair of iterators as a range, and
dependent libraries will need to follow suit.

Regards,
Nate

[1] http://stackoverflow.com/questions/6167598/why-was-pair-range-access-removed-from-c11