[[
CC += libc-co...@sourceware.org
CC += gcc@gcc.gnu.org
CC += libstd...@gcc.gnu.org
]]
Hi Florian,
On 2020-09-21 10:38, Florian Weimer wrote:
> * Alejandro Colomar via Libc-alpha:
>
>> I'd like to propose exposing the macro 'array_length()' as defined in
>> 'include/array_length.h' to the user.
>
> It would need a good C++ port, probably one for C++98 and another one
> for C++14 or later.
For C++, I use the following definition:
#include <cassert>
#include <sys/cdefs.h>
#include <type_traits>
#define is_array__(a) (std::is_array <__typeof__(a)>::value)
#define must_be_array(arr) \
static_assert(is_array__(arr), "Must be an array !")
#define array_length(arr) ( \
{ \
must_be_array(arr); \
__arraycount((arr)); \
} \
)
This solves the problem about G++ not having
__builtin_types_compatible_p().
However, there are a few problems:
1) This doesn't work for VLAs (GNU extension).
I couldn't find a way to do it. Maybe I should file a bug in GCC.
2) Also, this requires C++11; I don't know how to do it for older C++.
Again, support from the compiler would be great.
3) The macro can't be used in the same places as the C version,
because of the `({})`.
The `0 * sizeof(struct{...})` trick doesn't work in C++ due to:
error: types may not be defined in 'sizeof' expressions
>
>> Libbsd provides '__arraycount()' in <sys/cdefs.h> and some BSDs provide
>> 'nitems()' in <sys/param.h>, so any of those 2 headers may be a good
>> place to do it.
>
> In this case, I would prefer nitems in <sys/param.h>, given that there
> is precedent for it. <sys/cdefs.h> seems to be a bit drastic for a new
> macro with such a common name; it would create widespread build
> breakage.
Ok. I guess you would use 'nitems()' name, right?
>
> Maybe also ask on the libc-coord list.
Ok. Added CCs.
>
> Thanks,
> Florian
>
Thanks,
Alex
