From: Jonathan Wakely <jwakely.gcc@gmail.com>
To: Xi Ruoyao <xry111@xry111.site>
Cc: Jonny Grant <jg@jguk.org>, gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: std::string add nullptr attribute
Date: Thu, 9 Feb 2023 17:52:40 +0000 [thread overview]
Message-ID: <CAH6eHdR23=FG+a72JKdvFEd6ynR8XaR9kaNS5GouX5d=2J+s6Q@mail.gmail.com> (raw)
In-Reply-To: <6e30ed8e6c6f08407a5b8259e73fd18a492376b5.camel@xry111.site>
On Thu, 9 Feb 2023 at 16:30, Xi Ruoyao wrote:
>
> On Thu, 2023-02-09 at 14:56 +0000, Jonathan Wakely via Gcc-help wrote:
> > > Note, my code isn't like this, it is just an example to suggest
> > > adding the nullptr attribute, as its clearly already rejected at
> > > runtime.
> >
> > I assume you mean the nonnull attribute. That was added in 2020 and
> > then reverted because it broke some things:
>
> I remember I'd once made the same mistake when I suggested to add
> nonnull for ostream::operator<<(const string &) and I was lectured:
> nonnull is not only a diagnostic attribute, it also allows the compiler
> to assume the parameter is never null and rendering std::string(nullptr)
> an undefined behavior.
Yes, I think that's what might have happened with the std::string change.
> Then the example may just silently continue to run, instead of throwing
> an exception. It would be an ironic example: an attempt to improve
> diagnostic finally made diagnostic more difficult.
Indeed.
Maybe we can add __attribute__((access(read, 1))) instead, which says
that we will read from the pointer, which also implies it must be
non-null.
N.B. in C++23 string(nullptr) produces an error, although
string((const char*)nullptr) doesn't, so in practice it only prevents
the dumbest calls with a literal 'nullptr' token, and not the more
realistic problems where you have a pointer that happens to be null.
next prev parent reply other threads:[~2023-02-09 17:52 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-09 13:26 Jonny Grant
2023-02-09 14:56 ` Jonathan Wakely
2023-02-09 16:30 ` Xi Ruoyao
2023-02-09 17:52 ` Jonathan Wakely [this message]
2023-02-10 21:30 ` Jonny Grant
2023-02-10 22:03 ` Jonathan Wakely
2023-02-10 22:38 ` Jonny Grant
2023-02-11 0:32 ` Jonathan Wakely
2023-02-13 22:02 ` Jonny Grant
2023-02-19 20:43 ` Jonny Grant
2023-02-19 21:33 ` Jonny Grant
2023-02-20 10:26 ` Xi Ruoyao
2023-02-20 10:37 ` Jonathan Wakely
2023-02-20 10:54 ` Xi Ruoyao
2023-02-20 11:10 ` Gabriel Ravier
2023-02-20 11:18 ` Marc Glisse
2023-02-20 11:28 ` Segher Boessenkool
2023-02-20 12:00 ` Jonny Grant
2023-02-20 14:50 ` Gabriel Ravier
2023-02-20 11:44 ` Jonny Grant
2023-02-21 15:02 ` Jonny Grant
2023-02-20 11:38 ` Jonny Grant
2023-02-20 11:30 ` Jonny Grant
2023-02-20 12:59 ` Xi Ruoyao
2023-02-20 13:44 ` Jonathan Wakely
2023-02-20 19:21 ` Jonny Grant
2023-02-20 19:35 ` Jonathan Wakely
2023-02-20 19:39 ` Jonny Grant
2023-02-22 20:27 ` Jonny Grant
2023-02-21 15:04 ` Jonny Grant
2023-02-21 22:48 ` Jonny Grant
2023-03-04 15:00 ` Jonny Grant
2023-02-20 11:25 ` Jonny Grant
2023-03-12 22:10 ` Jonny Grant
2023-03-13 10:10 ` Jonathan Wakely
2023-03-13 19:55 ` Jonny Grant
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