Re: g++ with VLA

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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.


>
> 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++?