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

From: Mark Ruzon (mark34_at_[hidden])
Date: 2007-01-24 12:11:50

Ronald Garcia <garcia <at> cs.indiana.edu> writes:

> 
> 
> 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.

To Ron's credit, the documentation does not say that multi_array models
Assignable.  If it did, it would have to obey the semantics listed above for any
two multi_arrays that have the same template parameters.  I am curious to know
the source of Ron's loathing, though.

But I agree with Markus' point that modeling Assignable is a good thing.  It
means multi_array would become Regular since it already has equality and a total
ordering (assuming the element type does).  This would make multi_array a more
useful class, in my opinion.

> 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.

I don't think creating more special cases is the way to proceed.  The question
seems to be: are two multi_arrays with the same template parameters the same
type or not?  Right now the answer is "sometimes" because of assignment.  I
think everyone would be better served if the answer was either "yes" or "no."
I will vote "yes" if given a chance, because it will simplify the class instead
of making it more complex.  If the answer is "no", the dimensions might as well
be set at compile time. 

Sincerely,
Mark