Re: [eigen] Matrix assignment

[Cristóvão Sousa <crisjss@xxxxxxxxx>]

> Now what about the automatic resizing of operator= ? Honestly 
I would
> not be against removing this feature and enforcing that the 
left-hand
> side has the correct size. This is also more consistent with
> fixed-size matrices which cannot be resized anyway. In that 
case we
> could add the folfowing exception: the lhs will still be 
automatically
> resized if its size is 0. Well, currently we cannot create 
such an
> empty matrix, but I think that would be nice to allow that 
regardless
> of the automatic resizing issue.

Well, if the auto re-size was removed from operator= then a 
couple of checks would be removed, right? But, introducing auto 
re-size for zero sized matrices would introduce a check...

I think that the need of an auto/non-auto re-size feature will 
always be dependent on the application:

- In a Matlab like style the matrices should be automatically
  re-sized.
- Also, in a more traditional C++ approach, coping an object to
  an other should make the destination object "equal" to the
  original, as is proposed to happen with zero sized matrices.
- But in many cases, as in my specific application needs, it is
  desirable to maintain the destination matrix size unaltered,
  raising an failure if matrices have different sizes and debug
  mode is enabled. This can eliminate the overhead (if any) of
  checking sizes in non debug mode, and is more consistent with
  fixed-size matrices, as you said.


I did had not mentioned but when I realise that the operator= 
re-sizes the destination matrix, the first solution, which did 
not worked, that comes to my mind was to use the .cwise() in the 
lhs matrix:

   m0.cwise() = m1;   

as in "m0.cwise() += scalar".

This can be a solution too, although it could turn to be not 
very clear if the rhs expression should be a matrix with equal 
size or a scalar.

Another solution that may be nice is to deny any resizing unless 
you call some function, like:

    m0.copy(m1);
or
    m0.assign() = m1; 



Cheers,
Cristóvão Sousa



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