AMDG Jeremiah Willcock wrote:

Thanks to Marcin Zalewski, the problem in my last email has been tracked down to the inability to swap boost::tuples that contain references. There have been a couple of proposals to add that:

http://lists.boost.org/Archives/boost/2006/03/102603.php http://lists.boost.org/Archives/boost/2003/05/48488.php

but it appears that none of them has made it into Boost. This feature is important because of sorting data structures using zip_iterators as in my previous message:

http://www.nabble.com/-iterator--std::sort-on-zip_iterator-td23348384.html

and the older posts:

http://lists.boost.org/Archives/boost/2004/07/68758.php http://lists.boost.org/boost-users/2009/03/46623.php

In particular, std::swap (from GCC 4.0.1) does not work correctly on the result of dereferencing a zip_iterator, which is a Boost tuple of references. The std::swap function does not work at all (fails at compile time) because the result of operator* is not a non-const reference, and std::iter_swap tries to use assignment between the tuples, producing incorrect answers at run time. Is it true that there is no standards-compliant way to solve this problem? If so, would that be a good justification for Boost.Sorting to include an algorithm similar to std::sort but that handles more generalized swappable types?

Even overloading swap is insufficient, since the standard does not specify that std::sort calls swap using ADL or even that it calls any swap at all. In particular overloading swap will not work on msvc, because std::sort calls std::iter_swap (without ADL), which in turn calls std::swap. In Christ, Steven Watanabe