Re: [Boost-users] [multi_array] operator= requires resize - why?

From: Ronald Garcia (garcia_at_[hidden])
Date: 2007-01-23 17:13:04

On Jan 16, 2007, at 3:51 PM, Matthias Troyer wrote:

>
> On Jan 16, 2007, at 4:59 PM, Ronald Garcia wrote:
>
>> Hello Markus,
>>
>> On Jan 16, 2007, at 9:06 AM, Markus Werle wrote:
>>
>>> IMHO the behaviour of operator= for multi_array is a little bit
>>> broken.
>>> A lot of preconditions required:
>>
>> Indeed operator= for multi_array requires the sizes of the
>> multi_arrays to match, hence the assertions.  The multi_array type is
>> not meant to automatically resize when assigned to.  All resizing is
>> intentionally meant to be explicit.
>> For your application, you will need to resize your array prior to
>> assignment.
>
> This is awkward since it means that any class with a multi_array
> member cannot use the default operator= but needs to implement a
> special operator=. We just ran into a subtle bug with this because of
> the semantics of the valarray operator=, and I would strongly argue
> for an operator = that makes the semantics of
>
> A x = y;
>
> equivalent to
>
> A x;
> x = y;
>

Hi Matthias,

multi_array's operator=() was designed to behave in a manner  
consistent with the other array objects provided by the library  
(subarrays, views, multi_array_ref).  In particular, assignment  
between any of these other types requires that the dimensions of the  
arrays match, and it performs an assignment to the analogous  
locations in the array.  I am loath to alter these semantics.

One possible approach, which I'm not completely happy with, is to  
provide special behavior for default constructed multi_arrays. In  
particular, assignment to a default-constructed multi_array would  
cause it to resize during assignment.  This would make such default- 
constructed multi_arrays special.

Cheers,
ron