[Dune] [Dune-Commit] dune-localfunctions r943 - trunk/dune/localfunctions/test

[Jö Fahlke]

Am Mon,  3. Jan 2011, 10:07:17 +0100 schrieb Martin Nolte:
> let me get this straight. Your code example shows that the facade
> does not guarantee a method to be present on the implementation just
> because the facade implements it -- granted. But there is a subtle
> difference: Let's assume we would call bar instead of foo in the
> main program of your example. With the facade, the error occurs in
> Facade::bar, while without the facade the error occurs in the main
> program (read: user code). So, with the facade, the user tell the
> difference between an error in his code and an error in the grid
> implementation.

On the other hand, it makes error messages longer, and missing interfaces
should be detected by unit-tests anyway.

> Finally, the incomplete type problem is as follows. When I wrote my
> first DUNE grid, I tried to extract the types from the const Grid
> provided to, e.g., the geometry. This failed due to the grid being
> an incomplete type. Today I know that I have to use remove_const to
> get around this. But the lesson I learned is: Think whether it is
> necessary to extract stuff from a template argument which is not a
> traits class.

I still don't understand. For instance, why is the geometry provided with a
const Grid template parameter?

> Just by the way, I wanted to specialize as follows:
> 
> template< int mydim, int cdim, class GridImp, template< int, int, class > Impl >
> struct MyObjectWrapper< Geometry< mydim, cdim, GridImp, Impl > > { ... };
> 
> template< int codim, int dim, class GridImp, template< int, int, class > Impl >
> struct MyObjectWrapper< Entity< codim, dim, GridImp, Impl > > { ... };
> 
> You can't do this trick without the Facade. But your example of
> specializing over the codimension is a really good one, too.

Ah, OK, I misunderstood you there.  Yes, you are right, you need some
provision from the Geometry/Entity to be able to do that, and I guess
Facade classes are one of the better ways to do this.  This is the best
argument in favour of them I have heard so far, and it explains why there is a
problem with MockGeometry.

In summary, I am convinced that we need some way to check whether a type is a
geometry, and the Dune::Geometry template is probably as good as any.  I am
not yet convinced about the mydim, cdim, and ctype template parameters, but I
regard that as a minor issue.  IMO it is most important that we get rid of
Dune::Geometry's GridImp template parameters.

Bye,
Jö.

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