https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108846
--- Comment #24 from Jonathan Wakely <redi at gcc dot gnu.org> --- (In reply to Arthur O'Dwyer from comment #23) > - There may be padding in the middle of an object, and I'm not confident > that the Standard actually forbids it from being used. Of course your > approach works fine on the Itanium ABI, and probably works everywhere that > actually exists. If you've got chapter+verse proving that it must work > *everywhere*, I'd appreciate seeing it, just for my own information. I don't care about everywhere, only GCC-like compilers on the targets they support. > - If GCC were ever to add a builtin for this notion, IMO the proper name > would be `__datasizeof(T)`. See > https://danakj.github.io/2023/01/15/trivially-relocatable.html#data-size I think it should have a __builtin_ prefix unless it directly implements a standard trait like __is_object. > - You can implement your library trait like this; see > https://quuxplusone.github.io/blog/2023/03/04/trivial-swap-prize-update/ > #step-1.-std-swap-can-t-assume-it > > template <class _Tp> > struct __libcpp_datasizeof { > struct _Up { > [[__no_unique_address__]] _Tp __t_; > char __c_; > }; > static const size_t value = __builtin_offsetof(_Up, __c_); > }; > > Unfortunately it looks like GCC doesn't support > `__attribute__((__no_unique_address__))` in C++03 mode. (Neither does Clang. > What is up with that!) Which is OK, because you can't have final types in C++03 either, so testing it using inheritance is good enough for C++03. > Your suggested trait implementation is slightly wrong for `is_final` types: > you return 0 but really a final type _can_ have usable tail padding. See > https://godbolt.org/z/P6x459MEq Ah, I didn't think that padding would actually be used by an adjacent member subobject. But of course it can be for an empty class, where it consists of a single byte of padding which can be reused by another member of a different type.
