tie like functionality for ranges

[philipp.moeller@…]

The Graph library uses std::pair to pass ranges and this allows a nice pattern to use old-fashioned for-loops:

Graph g; boost::graph_traits<Graph>::vertex_iterator b, e; for(boost::tie(b, e) = vertices(g); b != e; ++b) {

}

This works without any modifications with pair.

The functionality would be great for all ranges as it provides users that don't want to use Boost.Range algorithms with a simple way to use their preferred style or algorithms from std.