I've created a branch to develop dual V2 + V3 support via namespace hoisting, as suggested by Scott McMurray, Andrey Semashev, and others. See https://svn.boost.org/svn/boost/branches/filesystem3 Seems to work well, and is lots easier to use that the switching approach currently in trunk. I've tested only on Windows, but will give Linux and Mac OS X a try later today or tomorrow. Namespace hoisting will break some existing V2 code: * User specializations in namespace filesystem. This is presumably very unusual, breaks noisily, and the fix is trivial, so I don't see it as a show stopper. * Possible ADL problems. I don't fully understand when this might be a problem, but neither V2 or V3 hit it in any of the tests, so I doubt it is very serious. --Beman