dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/4/15, Dag Lindbo <dag@xxxxxxxxxx>:
> Thanks Martin! It is much clearer now. My main concern here is as a "user",
> since a release is forthcoming. I only wish to clarify things here for the
> benefit of (myself and) other users. Comments below.
>
>  Martin Sandve Alnæs wrote:
>
> > 2008/4/15, Dag Lindbo <dag@xxxxxxxxxx>:
> >
> > > Hello!
> > >
> > >  A lot has happened on the Vector/Function/<backend>Vector interface the
> > >  last weeks. For clarity, I would like to have a little "summing up"
> > >  thread. That is, I  think it would be beneficial to explicitly show how
> > >  the new interface is intended to be used. Sounds good? Here are some
> > >  situations, to which I propose solutions. Please correct if these are
> > >  not in line with the intended interface, or if there are better ones.
> > >
> > >  How to:
> > >  *) Get reference to Vector from a (discrete) Function u
> > >  Vector& v = dynamic_cast<Vector&>(u.vector());
> > >
> >
> > Why would you want that?
> > All dolfin library code will now use GenericVector references.
> > This way it will also work with f.ex. uBlasVector objects directly.
> >
>
>  I understand that Generic* is used for all internal DOLFIN code.
>
>  It is still not clear how "application code", i.e. an actual solver for a
> user, is supposed to do this. Is GenericVector from now on part of the
> public interface of DOLFIN? (sorry if I'm repeating this question)


Actually, I think you've pointed out a user interface problem here.
We can in general go from Vector to backend-type, but not the other way around.

You _can_ do this:
    Vector& v = dynamic_cast<Vector&>(u.vector());
_if_ your code has been using Vector consistently.
It will throw a std::bad_cast exception otherwise.

But that will limit that part of your code to built-in dolfin backends, which
is one of the things we're trying to avoid by using GenericVector& everywhere.

On the other hand, if you use
    GenericVector& v = u.vector();
this does, as you say, make GenericVector enter user code alongside Vector,
which was something Anders didn't want to begin with (if I remember correctly?).

>From Python, this is not an issue.

-- 
Martin