On Sat, Mar 30, 2024 at 5:05 AM Jonathan Wakely <jwakely.gcc@xxxxxxxxx> wrote: > > On Sat, 30 Mar 2024 at 06:12, NightStrike via Gcc-help > <gcc-help@xxxxxxxxxxx> 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++?
