Re: [eigen] is returning Eigen::Ref objects legit?

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 20.05.2013 19:06, Sébastien Barthélémy wrote:
> ok, got it, I should have returned the Ref object by value. It seems I

Yes, that works as intended, but it has a small drawback that Ref<const 
FixedSizeMatrix> objects optionally contain a copy of the passed matrix 
(which is only used if the passed matrix does not have direct access).
However, even if it's not used, passing that object by value might cause 
some significant copy-overhead (very compiler-dependent, look for Return 
Value Optimization and Copy Elision if you are interested in details).

I do see the usability advantage of that trick, but maybe we should 
allow this only when explicitly requested so that no silent overhead is 
introduced.
Another situation where silent overhead is introduced is passing objects 
with non-trivial inner stride, e.g. passing a row of a (column major) 
matrix. This causes copying all elements, although it could be accessed 
element-wise. I know there is the possibility to have Ref objects have 
dynamic stride, but I think it might be not obvious to the user (i.e. 
the lazy one, who stopped reading the docu after the first paragraph) 
when that does make a difference.


> expect Eigen to be a bit too magical.

Returning references to temporaries is more a C++ issue. As this issue 
is well known, most compilers are able to warn about that (as you said 
in your original mail).

Christoph


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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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