Re: [Boost-users] [multi_array] std::swap and subarray

Hi Sergey, Ultimately, the problem is the compiler I think.

I've got somehow unexpected behaviour of std::swap with multi_array's subarray. For example, given a 2x2 multi_array: ...         std::swap( a[0], a[1] );

With msvc 2008 + boost 1.42, one of the rows overwrites the other and the output is:

21   22 21   22

The problem is that a[0] and a[1] are temporary objects, and they cannot be swapped in the std::swap sense (since it takes two reference to non-const). fortunately gcc 4 catches this problem and your original code doesn't compile. Then using your hack doesn't work either (i.e. doesn't compile in gcc 4)

template void swap( boost::detail::multi_array::sub_array &lhs,          boost::detail::multi_array::sub_array &rhs)

...because of the same reason.

Well, probably that's not a problem for now, but I dunno if it's a bug or std::swap is not supported - coundn't find anything about this.

it is a (conceptual) bug in your compiler that allows you to take a temporary as a non-const reference. (gcc wins for the conceptual superiority this time). Now, to solve your problem with the smallest change possible you can try with template void swap( boost::detail::multi_array::sub_array lhs, boost::detail::multi_array::sub_array rhs) { ... same code here } note two changes, 1) we must use boost::detail::multi_array::size_type instead of int, otherwise the overload template is not found in cases where the size_t is not int for example). 2) use plain variable type as argument, you may say: What a waste to pass by value!! well is not that bad because sub_arrays are really references already, elements are not copied. It works in gcc (actually this is the only variant that compiles with gcc), Let me know your results in VS2008. I think this solves the problem, (except for the fact that there it may be a more efficient way by swapping element by element). Good luck, Alfredo