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

Subject: Re: [Boost-users] [multi_array] std::swap and subarray
From: alfC (alfredo.correa_at_[hidden])
Date: 2010-02-20 00:26:03

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 <typename TValue, int K>
> void swap( boost::detail::multi_array::sub_array<TValue, K> &lhs,
>          boost::detail::multi_array::sub_array<TValue, K> &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 <typename TValue, boost::detail::multi_array::size_type /*was
int*/ K>
void swap(
  boost::detail::multi_array::sub_array<TValue, K> lhs,
  boost::detail::multi_array::sub_array<TValue, K> 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