Re: [eigen] To initialize or not to initialize

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

2010/4/16 Adolfo Rodríguez Tsouroukdissian <adolfo.rodriguez@xxxxxxxxxxxxxxxxx>:
> Looking into the code of various decompositions I've seen that the
> constructors for input matrix are sometimes implemented like this (dummy
> example):
>
> fooDecomp(const MatrixType& matrix) : m_mat(matrix.rows(), matrix.rows())
> {
>   compute(matrix);
> }
>
> and sometimes like this:
>
> fooDecomp(const MatrixType& matrix) // <- Notice no initialization
> {
>   compute(matrix);
> }
>
> First of all, I'd like to propose uniforming this (I could do it), but I
> also wanted to know how big a difference is there between the two
> alternatives.

The better version is the one with the initialization list. The other,
without initialization list, means that in case of a dynamic matrix,
it is initialized with size 0 and then resized by the compute()
method. This amounts almost exactly to the same, there is just a tiny
bit of inefficiency here.

OK to uniformize all decompositions with initialization lists.

> I know that in general it's best to follow the first approach and initialize
> class members, but is it still so in the case of Eigen matrices?. Are the
> differences greater when certain macros are defines, such as
> EIGEN_INITIALIZE_BY_ZERO_IF_THAT_OPTION_IS_ENABLED?

It doesn't make a difference evec if EIGEN_INITIALIZE_BY_ZERO is
defined, because the matrices get filled by the compute() method
anyway.

Don't bother about EIGEN_INITIALIZE_BY_ZERO. The only reason why I was
OK for it was that it was a small  local change that the rest of Eigen
doesn't need to care about. If that changed, we'd probably prefer to
remove that option, than have one more thing to constantly worry
about.

Benoit

>
> TIA,
>
> Adolfo
>
> --
> Adolfo Rodríguez Tsouroukdissian, Ph. D.
>
> Robotics engineer
> PAL ROBOTICS S.L
> http://www.pal-robotics.com
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