Re: [Boost-users] Swap allocator problem in multi_array.hpp

From: Ronald Garcia (garcia_at_[hidden])
Date: 2004-10-19 17:49:45

On Oct 19, 2004, at 4:37 AM, Ewan Campbell wrote:

>
> I appear to have made a small error, the warning occurs 'whenever' 
> resize is called.
>
> So the problem now is:
>
> Every time the resize function of multi_array is called, a warning is 
> generated in the use of MSVC++ 6.0's implementation of std::swap.
>
> The problem is solved by commenting out the line
> swap(this->allocator_,new_array.allocator_);
> in multi_array.hpp
>
In general, I'd say this is a dangerous thing to do, particularly 
because I don't know how std::swap is implemented in your compiler.  It 
seems like the compiler is complaining about an unused variable in the 
implementation, which does not mean that the allocators are not being 
swapped.

> This hasn't incurred an execution problem under debug or release. 
> Could someone explain this? What is the purpose of this line, and why 
> wasn't it swapping the allocators?
>

The resize operation preserves as much of  the contents of the original 
array as possible.  To do so, it must allocate a new portion of memory 
and copy the salvageable elements to it.  The algorithm essentially 
builds a new array of the right size and swaps the guts of the original 
array out.

Hope that helps,

ron