220 36329 <36d5cdb5-0213-4efd-9d36-8179177f604b@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Trivial copyability and `optiona/variant`. 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As I see now the Microsoft >> and >> > libc++ implementation maintain trivial copyability, but libstdc++ >> doesn't: >> > https://godbolt.org/g/tL29Hd >> >> The proposal for that hasn't apparently been adopted yet, so this >> hasn't popped up as a high-priority >> item on the libstdc++ todo list. I suppose I should implement it >> anyway, but I can't make any promises >> that it'll make it into gcc 8. >> > > Coincidentally, "trivial copyability of std::optional" just popped up over > on the SG14 list as well: > > https://groups.google.com/a/isocpp.org/forum/#!msg/sg14/ZqrAbZ2Vq9s/iNVCL3naDAAJ > > The fundamental problem (IMO) is that if you make optional trivially > copyable, then you have > > optional a = 42; // correctly constructs an `int` and begins its > lifetime > optional b = a; // trivially copies some bits into the memory > address of `b` > > In the `b` case, there is no `int` object actually constructed (assuming > the common implementation of `optional` as containing a union of a dummy > char and a `T` object, where the `T` object is not constructed by default). > So the first access to `b.value()` invokes undefined behavior, as far as > I'm aware. > That would basically mean that any trivially copyable `union` type cannot be copied at all. I don't buy that the standard says that. Indeed, it very much does not [basic.types]/3: > For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes (4.4) making up obj1 are copied into obj2 , 44 obj2 shall subsequently hold the same value as obj1. And that's just for doing a memcpy-based copy assignment. For a `union`, to "hold the same value" as another instance of that union also means to have active the same member subobject as the other `union`. Do you have some specific statement in the standard that says otherwise? -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/36d5cdb5-0213-4efd-9d36-8179177f604b%40isocpp.org. ------=_Part_14502_392247051.1514175339817 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Sunday, December 24, 2017 at 8:40:14 PM UTC-5, = Arthur O'Dwyer wrote:
On Saturday, December 23, 2017 at 4:47:05 AM UTC-8, Ville Vouti= lainen wrote:
On 23 December 2017 at= 14:17, Andrey Davydov <andrey.a...@gmail.com>= ; wrote:
> Is there any progress being made on this? As I see now the Microso= ft and
> libc++ implementation maintain trivial copyability, but libstdc++ = doesn't:
> https://godbolt.org/g/tL29Hd

The proposal for that hasn't apparently been adopted yet, so this
hasn't popped up as a high-priority
item on the libstdc++ todo list. I suppose I should implement it
anyway, but I can't make any promises
that it'll make it into gcc 8.

= Coincidentally, "trivial copyability of std::optional" just poppe= d up over on the SG14 list as well:

The fundamental problem (IMO) is that if= you make optional<T> trivially copyable, then you have
=C2=A0 =C2=A0 optional<int> a =3D 42; =C2=A0// correctly = constructs an `int` and begins its lifetime
=C2=A0 =C2=A0 optiona= l<int> b =3D a; =C2=A0// trivially copies some bits into the memory a= ddress of `b`

In the `b` case, there is no `int` o= bject actually constructed (assuming the common implementation of `optional= <T>` as containing a union of a dummy char and a `T` object, where th= e `T` object is not constructed by default). So the first access to `b.valu= e()` invokes undefined behavior, as far as I'm aware.

That would basically mean that any trivially cop= yable `union` type cannot be copied at all. I don't buy that the standa= rd says that. Indeed, it very much does not [basic.types]/3:

=
>=C2=A0For any trivially copyable type T, if two pointers to = T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is= a base-class subobject, if the underlying bytes (4.4) making up obj1 are c= opied into obj2 , 44 obj2 shall subsequently hold the same value as obj1.

And that's just for doing a memcpy-based copy a= ssignment.

For a `union`, to "hold the same v= alue" as another instance of that union also means to have active the = same member subobject as the other `union`. Do you have some specific state= ment in the standard that says otherwise?

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/36d5cdb5-0213-4efd-9d36-8179177f604b= %40isocpp.org.
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