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From: "andrew2085 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/95349] Using std::launder(p) produces unexpected behavior where (p) produces expected behavior
Date: Mon, 14 Nov 2022 04:53:49 +0000 [thread overview]
Message-ID: <bug-95349-4-iYokLxuGPD@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-95349-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95349
--- Comment #44 from Andrew Downing <andrew2085 at gmail dot com> ---
(In reply to Richard Biener from comment #43)
> (In reply to Andrew Downing from comment #41)
> > > Thus for types without a non-trivial ctor/dtor you do not need to use
> > > placement new. So take your example and remove the placement new.
> > > Does that change its semantics?
> >
> > These are C++17 rules.
> >
> > 4.5/1) An object is created by a definition, by a new-expression, when
> > implicitly changing the active member of a union, or when a temporary object
> > is created.
> >
> > 6.8/1) The lifetime of an object of type T begins when: storage with the
> > proper alignment and size for type T is obtained, and if the object has
> > non-vacuous initialization, its initialization is complete.
> >
> > double d;
> >
> > My interpretation of the above rules would be that only a double object is
> > created in the storage for d because T in 6.8/1 is set to double by the
> > definition of d. According to these rules the only way to change the dynamic
> > type of the object in d's storage would be with placement new (pre C++20).
> > memcpy only overwrites the object representation. It doesn't affect it's
> > type or lifetime.
>
> What would
>
> *(long *)&d = 1;
>
> do? My reading of earlier standards say it starts lifetime of a new object
> of type long (the storage of 'd' gets reused). Following that stmt a read
> like
>
> foo (d);
>
> invokes undefined behavior (it accesses the storage of effective type long
> via an effective type of double). The same example with placement new
> would be
>
> *(new (&d) long) = 1;
>
> and I'm arguing the placement new is not required to start the lifetime
> of an object of type long in the storage of 'd'.
It's been a while since I've though about this stuff.
double d;
*(long *)&d = 1;
That would be lead to undefined behavior because it's breaking the strict
aliasing rules.
*(new (&d) long) = 1;
That would be ok because new creates a long object in the storage of d before
dereferencing.
next prev parent reply other threads:[~2022-11-14 4:53 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-26 20:45 [Bug c++/95349] New: " andrew2085 at gmail dot com
2020-05-27 8:04 ` [Bug c++/95349] " rguenth at gcc dot gnu.org
2020-05-27 9:14 ` redi at gcc dot gnu.org
2020-05-27 9:40 ` rguenther at suse dot de
2020-05-27 11:05 ` redi at gcc dot gnu.org
2020-05-27 14:45 ` andrew2085 at gmail dot com
2020-05-27 15:07 ` redi at gcc dot gnu.org
2020-05-27 15:19 ` andrew2085 at gmail dot com
2020-05-27 16:01 ` andrew2085 at gmail dot com
2020-05-29 10:59 ` ed at catmur dot uk
2020-05-29 11:23 ` rguenth at gcc dot gnu.org
2020-05-29 11:32 ` rguenth at gcc dot gnu.org
2020-05-29 13:53 ` ed at catmur dot uk
2020-05-29 14:15 ` redi at gcc dot gnu.org
2020-05-29 14:24 ` rguenther at suse dot de
2020-05-29 15:05 ` andrew2085 at gmail dot com
2020-05-29 18:07 ` richard-gccbugzilla at metafoo dot co.uk
2020-05-29 21:00 ` andrew2085 at gmail dot com
2020-05-29 21:50 ` richard-gccbugzilla at metafoo dot co.uk
2020-05-29 23:13 ` andrew2085 at gmail dot com
2020-05-29 23:25 ` richard-gccbugzilla at metafoo dot co.uk
2020-06-02 12:09 ` rguenth at gcc dot gnu.org
2020-06-02 12:20 ` rguenth at gcc dot gnu.org
2020-06-02 16:00 ` andrew2085 at gmail dot com
2020-06-02 16:23 ` rguenther at suse dot de
2020-06-02 16:34 ` andrew2085 at gmail dot com
2020-06-02 16:37 ` andrew2085 at gmail dot com
2020-06-02 17:54 ` rguenther at suse dot de
2020-06-02 18:43 ` andrew2085 at gmail dot com
2020-06-02 20:53 ` andrew2085 at gmail dot com
2020-06-03 6:52 ` rguenth at gcc dot gnu.org
2020-06-04 0:27 ` andrew2085 at gmail dot com
2020-06-04 6:14 ` rguenther at suse dot de
2020-06-04 16:05 ` andrew2085 at gmail dot com
2020-06-05 6:52 ` rguenth at gcc dot gnu.org
2020-06-05 14:30 ` andrew2085 at gmail dot com
2020-06-15 9:29 ` rguenth at gcc dot gnu.org
2020-06-15 21:45 ` richard-gccbugzilla at metafoo dot co.uk
2020-06-16 3:27 ` andrew2085 at gmail dot com
2020-06-16 6:50 ` rguenther at suse dot de
2020-06-16 6:57 ` rguenther at suse dot de
2020-06-16 13:56 ` andrew2085 at gmail dot com
2022-01-11 12:43 ` rguenth at gcc dot gnu.org
2022-01-11 12:48 ` rguenth at gcc dot gnu.org
2022-11-14 4:53 ` andrew2085 at gmail dot com [this message]
2024-06-03 8:02 ` Christopher.Nerz at de dot bosch.com
2024-06-03 8:51 ` rguenth at gcc dot gnu.org
2024-06-03 9:13 ` Christopher.Nerz at de dot bosch.com
2024-06-03 9:23 ` rguenth at gcc dot gnu.org
2024-06-03 10:26 ` Christopher.Nerz at de dot bosch.com
2024-06-03 11:19 ` rguenther at suse dot de
2024-06-03 15:53 ` redi at gcc dot gnu.org
2024-06-03 16:00 ` redi at gcc dot gnu.org
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