On Wed, 2 Oct 2013, Gabriel Redner wrote:
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 had been allowed in range-based for previously, but was removed before C++11 was released.
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?
I'm not sure. There might be a way to inherit from std::pair to be compatible while still allowing range-based for, but I am not sure. Another option would be a wrapper that converted an std::pair to a range. -- Jeremiah Willcock