[Dune] [#674] strict-aliasing warnings with gcc-4.4

[Dune]

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The following task has a new comment added:

FS#674 - strict-aliasing warnings with gcc-4.4
User who did this - Carsten Gräser (Carsten)

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As Christian said we have discussed the options. Regarding Martins options:
(a) is obviously not an option and (b) as well. It will break at lot of code if EntityPointers& ep=it is not supported. And allowing the temporary object needed if iterators are passed as function arguments is neither clean nor intuitive. I don't really understand what you mean with (c).

Our main idea is to derive ItImp from EP<EPImp> instead of EPImp. Then we could savely return the reference to the base class of ItImp.

The clean solution would then by to abolish the strange EP<ItImp> and to make ItImp a member of It. Notice that this will not change the memory layout of any class.

I also have a small patch to EntityPointer with a quick solution for a transition. It returns

(1) EP<Imp> if Imp=EPImp
(2) EP<Imp>::realIterator if Imp is derived from EP<EpImp>
(3) reinterpret_cast<EP<EpImp>&>(EP<Imp>) else

For the current grids this is the same as before: (1) or (3). For all adapted grids with ItImp deriving from EP<EPImp> this selects (1) or (2) and thus is a clean solution. This allowes for a smooth transition iterator per iterator. I have tested this with a quick for the UGLevelIterator.

We also suggests to have additional new interface classes which could be selected (by the GridTraits) for all grids that completely implement the new behaviour.

Since this only adjusts the inheritance hierarchy and does not introduce any additional members, copies or creation of temporaries there is no performance problem with this.

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