Re: [eigen] Eigen Types as Parameters for Functions

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 26.01.2012 14:50, Benoit Jacob wrote:
> 2012/1/26 Christoph Hertzberg<chtz@xxxxxxxxxxxxxxxxxxxxxxxx>:
> > On 26.01.2012 14:33, Benoit Jacob wrote:
> > > 2012/1/26 Christian Seiler<christian@xxxxxxxx>:
> > >
> > > >   - subclass of MatrixBase, i.e. can be used inside the function like a
> > > >    Matrix object
> > > >   - has private members containing the above-mentioned bits of
> > > >    information (pointer to first element, dimension, ...)
> > > >   - can be implicitly converetd to from any compatible MatrixBase (i..e.
> > > >    foo(mat) and foo(mat.block(...)) can be used directly)
> > >
> > >
> > > Just note that this is only possible for those matrix expressions
> > > whose coefficients are stored in memory with a simple enough layout
> > > that can be described in terms of strides. That is what we call a
> > > "Direct Access" matrix expression; the test is ExpressionType::Flags&
> > > DirectAccessBit.
> >
> >
> > Yes, and I would say this would be the main application, if you have
> > complicated-enough functions (i.e. where you don't really bother about
> > evaluating expressions to a temporary if required).
> > And more importantly, when having parameters that are actually output
> > parameters, you are more or less stuck to direct access types anyway -- and
> > this is a very important point, where currently const-correctness is ignored
> > (almost) entirely.
> >
> >
> > > With that said, yes, we have been considering doing this, that's bug 58:
> > > http://eigen.tuxfamily.org/bz/show_bug.cgi?id=58
> > >
> > > If someone wants to do this and needs mentoring, I should find time to
> > > do at least that mentoring.
> >
> >
> > I'd be interested at least to join the "specification committee" ;)
>
> Really, I haven't thought about this in a long time so your thoughts
> on this subject are probably a lot fresher than mine. From the top of
> my head, the starting point is to look at MapBase (which is inherited
> both by Map and by Block<direct access type>) and ask: how much of the
> templated stuff there can we replace by runtime variables (data
> members), and try to make all direct-access types inherit the
> resulting base class (in particular, Matrix should inherit it).

Yes, my thoughts were about the same. Actually, the only difference 
between a Map/Block and a Matrix should be that the Matrix "owns" its 
memory, i.e. has to deallocate it on destruction (with a special case of 
fixed size matrices having static memory).

Furthermore, if one allows strides in the Matrix-class, some specialized 
types, such as AlignedVector3 would become just a typedef. Currently, if 
someone needs an aligned vector7, he'd have to to the same thing again 
-- also it might be interesting to store an aligned 3x3 matrix as the 
top 3 rows of a 4x3 matrix allowing a much more efficient matrix-vector 
product.

Christoph



--
----------------------------------------------
Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
Enrique-Schmidt-Straße 5
28359 Bremen

Tel: (+49) 421-218-64252
----------------------------------------------