The Unicode implementations of the file system classes should support long path names on Windows, by prepending "\\?\" to Unicode path names before passing them to WIN32 API calls.

See https://msdn.microsoft.com/en-us/library/windows/desktop/aa363851(v=vs.85).aspx

Note: simply prepending "\\?\" to every path will not work, see https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx#maxpath.

Update: also applies to Process::launch() using CreateProcessW().
See https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx.