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

17 Feb 2010

      Hello!

I've got somehow unexpected behaviour of std::swap with multi_array's 
subarray. For example, given a 2x2 multi_array:

	boost::multi_array<double, 2> a;

	a.resize( boost::extents[2][2] );

	for (int i = 0; i < 2; ++i)
		for (int j = 0; j < 2; ++j)
			a[i][j] = (i+1)*10 + j+1;

	std::swap( a[0], a[1] );

	for (int i = 0; i < 2; ++i)
	{
		for (int j = 0; j < 2; ++j)
			std::cout << a[i][j] << "   ";
		std::cout << std::endl;
	}

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

21   22
21   22

The fastest hacky solution i've came is this:

namespace std
{
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)
{
	boost::multi_array<TValue, K> tmp = lhs;
	lhs = rhs;
	rhs = tmp;
}
} // namespace std

Now, the rows are properly swapped and the output is:

21   22
11   12

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.

----------------
Sergey Mitsyn.