From: NightStrike <nightstrike@gmail.com>
To: Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: gcc-help <gcc-help@gcc.gnu.org>
Subject: Re: g++ with VLA
Date: Sat, 30 Mar 2024 14:39:09 -0400 [thread overview]
Message-ID: <CAF1jjLtCqgNUfeFjJB+QTykWu_i3-9D=OToxX6WMkHugTcX-4g@mail.gmail.com> (raw)
In-Reply-To: <CAH6eHdTtjRM6iT0yzyZd5-aazGjh=s7+7z93wmiD79e-M1k3fA@mail.gmail.com>
On Sat, Mar 30, 2024 at 5:05 AM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Sat, 30 Mar 2024 at 06:12, NightStrike via Gcc-help
> <gcc-help@gcc.gnu.org> wrote:
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
> >
> > That doc states that VLAs are supported in C++ as an extension.
> > Compiling the following:
> >
> > void f(int a, int b[a]);
> > void f() {
> > int c[2];
> > f(2, c);
> > }
> >
> > with g++ -std=gnu++20 results in the error: "use of parameter outside
> > function body before ']' token". The docs say nothing about partial
> > support of VLA, just that they can be used.
>
> I don't think GNU C++ has any changes to the function declarator
> syntax to support variably modified types. You can define a VLA
> object, but you can't use earlier function parameters later in the
> function-parameter-list, and you can't use int b[*] in a parameter
> either.
It looks like clang added support for this in v12. Maybe it would be
a good candidate to add support? I guess I can file the PR and see
where it lands.
> > Where this is useful as an extension is in the somewhat reasonable
> > case of including a C header in a C++ program. If that C header
> > declares a function using a C99 VLA, it would be awesome if g++ were
> > to accept it in -std=gnu++ mode (and it would be fine if it were
> > rejected in -std=c++ mode). Consider a situation where you cannot
> > modify the header, and so have to use it as-is.
> >
> > Is there perhaps another option to achieve the intended behavior?
> >
> > If it's not possible, can this be added?
> >
> > If the consensus is that it shouldn't be added, could the docs at
> > least clarify what is supported and what is not in C++?
next prev parent reply other threads:[~2024-03-30 18:39 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-30 6:10 NightStrike
2024-03-30 9:05 ` Jonathan Wakely
2024-03-30 18:39 ` NightStrike [this message]
2024-03-30 18:46 ` NightStrike
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