[boost] [tuple] swap (also includes question about [sorting])

Subject: [boost] [tuple] swap (also includes question about [sorting])
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2009-05-08 15:12:38

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?

-- Jeremiah Willcock