Hm.  The `CpGridData::global_id_set_` is (supposed to be) a `dune::cpgrid::GlobalIdSet*` and that class derives privately from the `dune::cpgrid::GlobalIdMapping` (ca line 223 of `dune/grid/cpgrid/Indexsets.hpp`).  The intentions are a little murky, I agree, but maybe the actual solution is to say
```C++
class GlobalIdSet : public GlobalIdMapping { /* ... */ };
```
instead?  @blattms: What do you think?