[Dune] [#651] Declaration of hasHierarchicIndexSet for GeometryGrid

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FS#651 - Declaration of hasHierarchicIndexSet for GeometryGrid
User who did this - Martin Nolte (nolte)

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Let me remind you of a discussion on the mailing list started by commit 5686:

Oliver:
I am getting old and my memory is weak: when did we decide
that hierarchic index sets become part of the interface?

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Martin:
we never did. And that's why they are _not_ part of the interface (i.e.
dune/grid/common/capabilities.hh). Yet, Geogrid tries to take advantage of
them and therefore needs this capability to work on that grid. So, if you want
GeometryGrid< YaspGrid >, you need this capability, otherwise not.

Of course, we can decide not to add this capability and leave it up the user
to declare it. I would leave that decision up to the grid implementor...

I hope this course of action was not an offense against good style - I just
needed this capability to make it work

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Christian:
I do understand that there is a need for additional coapabilities, if
you want to implement new features. Still I have some questions/remarks:

1) If you need a new capabiity you can do the following:

   - add the complete capability to the new module

     geogrid.hh contains

     template<class T>
     struct HasHierarchicIndexSet
     {
 	     enum { value = false };
	 };

     #if HAVE_ALUGRID
     #include <dune/grid/alugrid.hh>
     template<...>
     struct HasHierarchicIndexSet<Alugrid>
     {
         enum { value = true };
     };
     #endif

   This way it will work for all grids in dune-grid, without the need
   to change all implementations. The only inconvience is that the
   user has alugrid.hh is automatically included.

2) The HierarchicIndexSets are something _very_ far from being part
   of the interface, thus I must agree with Oliver that it is more
   than ugly to have these capabilities in all headers.

I think that the solution proposed in point 1 is a good compromise..

Martin:
I understand. In this case I will not declare the capability at all. A default 
implementation is great, but only useful if I know where to include it from. 
And I don't think that geogrid.hh would be a good place for this one (I 
wouldn't want ALUGrid or AlbertaGrid to depend on geogrid.hh).

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This is where everybody lost interest in the problem. It works for me (dune-fem always declares this capability) and the test is an example how to make it work.

If you want me to add the code snippet to the documentation, I can do that.

As for a real solution, all my attempts to be able to add extensions to dune-grid were blocked somewhere. For example, I suggested a directory "extensions", "experimenal" or something like that for interface extensions. One could then add a namespace Extensions (like Capabilities, but stating definitely that these features are non-standard and have never been agreed upon).

Whatever the solution will be, just give me a vote on what to do and, if it does not involve a lot of work, it shall be done.
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More information can be found at the following URL:
http://www.dune-project.org/flyspray/index.php?do=details&task_id=651#comment1307

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