Re: [eigen] Interesting C++ standard proposal

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi,

(browsing through unanswered mails ...)

On 11.09.2013 09:00, Hauke Heibel wrote:
> I just thought some of you might be interested in this proposal:
> http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3748.pdf

Yes, that looks interesting, but as you said, even if this gets accepted 
it will take a long time until we can profit from it.
Until then, we must rely on Gael to implement the expression-evaluator 
and educate our users that storing expression trees is often not the 
most efficient solution (but that depends on the expression and how 
often it is used).

E.g.
MatrixXd A, B, C, X, Y, Z;

auto A_plus_B = A+B;  // usually ok (not always ...)
auto A_times_B= A*B;  // in many cases (A*B).eval() is better

X = A_plus_B + C; // more efficient than evaluating A+B.
Y.noAlias() = C + A_times_B; // currently fails, but efficient later.

// fails at the moment and inefficient (product gets evaluated twice):
Z.noAlias() = A_times_B + A_times_B.transpose();


Christoph

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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
Enrique-Schmidt-Straße 5
28359 Bremen

Tel: +49 (421) 218-64252
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