[Dune] dune-grid does not compile when implementations of EntityPointer and Iterator are not interoperable

[Steffen Müthing]

Hi Martin,

Am 27.04.2012 um 13:00 schrieb Martin Nolte:

> Hi Steffen,
> 
> you are right, the cast operator is indeed deprecated in the 2.1 release branch. This would mean it should be gone by now. Unfortunately, we are close to a release and I would ask the release managers whether such a change would still be in order. Is anybody still using the deprecated cast operator?

Well, regarding the cast operator: It is one of the major roadblocks towards strict aliasing support, and AFAIK
the only one that might be directly visible to users. Looking at the release, we have the following options:

1) Completely remove the cast (perhaps with an optional configure flag to keep it one more round).
2) Change the implementation to work correctly for non-interoperable types. This would make it possible for grid
  implementors to selectively disable the cast for their grids, which might simplify implementations (see e.g. your
  comment about the required memory layout of EntityPointer) and possibly save users from a bunch of ugly
  problems, while still staying backward compatible.
3) Leave everything as it is now.

If I had a vote, I would pick 1). Regarding 2) or 3), I simply don't know enough about the original intentions
behind the current code, so that's up to the developers to decide.

I noticed a missing const in my patch, an updated version is attached.

> 
> For the casting problem, you might want to have a look at GeometryGrid's entity pointer (see dune/grid/geometrygrid/entitypointer.hh). Here, the entity pointer stores the host iterator (which could, e.g., be a leaf iterator). You can then derive your iterator from the entity pointer (with the correct iterator passed as template argument) and, thus, satisfy the interoperability desired by Dune::EntityPointer. Of course, this trick has some consequences:
> (a) There is no guarantee that the code will work
>   (but that holds for the cast in Dune::EntityPointer as well).
> (b) The last member of your entity pointer must be the host iterator
>   (which is the one actually casted by the reinterpret_cast).
> Apart from that, the code seems to work wonderfully. I have to admit, though, that I never use the deprecated cast operator in my code.

That's what I do as well. The problem occured while I was trying to debug a problem where the compiler picked a wrong constructor
and wanted to make sure that the constructor argument would not be casted using the reference cast operator.

Steffen

> 
> Best,
> 
> Martin
> 
> 
> -- 
> Dr. Martin Nolte <nolte at mathematik.uni-freiburg.de>
> 
> Universität Freiburg                                   phone: +49-761-203-5630
> Abteilung für angewandte Mathematik                    fax:   +49-761-203-5632
> Hermann-Herder-Straße 10
> 79104 Freiburg, Germany

Steffen Müthing
Universität Stuttgart
Institut für Parallele und Verteilte Systeme
Universitätsstr. 38
70569 Stuttgart
Tel: +49 711 685 88429
Fax: +49 711 685 88340
Email: steffen.muething at ipvs.uni-stuttgart.de
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