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

Hello alfC, Thanks for explanation! Yes, the code you provided works perfectly with my compiler :). The actual problem I've faced is that std::random_shuffle makes a mess with multi_array in msvc 2008. The interesting fact is that std::iter_swap, which is used by std::random_shuffle, works in gcc without one more overload in std: //std::swap( a[0], a[1] ); std::iter_swap( a.begin(), a.begin()+1 ); Looks like msvc's std::iter_swap always uses std::swap, while gcc's one uses it only when iterator_traits<...>::reference is the same as value_type&. Ok, now I see I don't really understand how stl handles types with reference semantics. Looks like std::swap should be overloaded. Maybe stl algorithms shouldn't be used with such types at all? Cheers, Sergey Mitsyn. On 20.02.2010 8:26, alfC wrote:

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