Re: [eigen] Operations involving constants

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 22.10.2014 18:30, oz wrote:
> So I'm working on my own little test class to learn expression
> templates, etc. and have run into a bit of a snag.
> I'd appreciate any help in understanding. I'm uncertain how to implement
> the following kinds expressions:
>
> 2*array
> array+1
>
> It looks like Eigen somehow implements this in the Unary Operators,
> which makes sense, but I can't figure it out.

ArrayBase<Derived>::operator*(Scalar) is defined in
  Eigen/src/plugins/CommonCwiseUnaryOps.h
it returns a
CwiseUnaryOp<internal::scalar_mult_op<Scalar>, Derived> >.

operator+(Scalar) is defined in ArrayCwiseBinaryOp (but still returns a 
CwiseUnaryOp object.

> I tried encapsulating the constant into a Constant<T> class, and using
> the BinaryOps like normal, but that gave me segfaults for some reason.
> How would y'all suggest I approach this issue?

See ArrayBase::max(Scalar) (also in ArrayCwiseBinaryOp.h) for an example 
how to implement a unary operation as a BinaryOp with a Constant.



Developer-question:
Is there a reason why we implement operator+(Scalar) as CwiseUnaryOp, 
but max(Scalar) as CwiseBinaryOp?

Christoph


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