https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101061
--- Comment #14 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 16 Jun 2021, alexander.gr...@tu-dresden.de wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101061 > > --- Comment #13 from Alexander Grund <alexander.gr...@tu-dresden.de> --- > > But what you can see is that the resulting pointer is used for the > > initialization and not the placement address as literally written in source. > > So I assume it was supposed to be "Y::Y (D_6557, 1);" ? > > > I'm not sure how one can solve this issue with using placement new > > but are unions not sufficiently restricted so that copy assignment > > should work (and activate the appropriate union member)? Thus > > > > slot->mutable_value = pair<const K, V>(k, v); > > The problem is not the copy, the problem is that the value may contain any > kind > of data, think e.g. a pair of strings. And at the initial point (i.e. first > emplace) the slot is a casted pointer into uninitialized data. I.e. the above > would be an assignment into an object which does not exist. And (especially) > for such non-trivial types this would break. > > I think it will work for trivial types though, although it is UB due to > lifetime rules: You can't use an object (here: assign to) which has not > started > its lifetime yet. I see. I would need to read up what kind of restrictions recent C++ standards place on union members, but in C a store to a non-active union member makes that active and IIRC for tradidional POD data types the same should hold true for C++, even w/o requiring an explicit placement new. > However e.g. pair has custom copy and regular constructors so I think it will > run into the issue you mentioned: The ctor will access the object via the > this-pointer and not via the full union-thing and hence might misoptimise > later > > This would mean that in conclusion the use case of putting std::pairs in an > union and accessing them via aliasing is unsupported by (at least) GCC. Is > that > correct? Without restricting the set of C++ features used, yes. Even accessing the data via union.memb.getX (); would involve a 'this' pointer and thus break things. std::pair is probably a special-case that might work since you use pair.first rather than a method though ;) You can see this type-punning via unions exception was invented for C ;)
