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Message-ID: <e734429a-d543-7e75-48e9-a8297a94b035@gmail.com>
Date: Mon, 21 Sep 2020 23:52:13 +0200
From: Alejandro Colomar <colomar.6.4.3@...il.com>
To: Jonathan Wakely <jwakely@...hat.com>, Florian Weimer <fweimer@...hat.com>
Cc: gcc@....gnu.org, libstdc++@....gnu.org,
 Libc-alpha <libc-alpha@...rceware.org>, libc-coord@...ts.openwall.com
Subject: Re: Expose 'array_length()' macro in <sys/param.h>

I have developed this draft code, the C++ part being based on what you 
wrote.

I am a C programmer, and my C++ is very basic, and I tend to write 
C-compatible code when I need C++, so I can't really write the C++ part.

I tested the code with all C versions (--std= {c89, c99, c11, c18, 
c2x}), and it worked for all of them (correctly returning 18 in all of 
them), and if I uncomment the part of the pointer, it has a nice error 
message.  I used `-Wall -Wextra -Werror -pedantic -Wno-vla 
-Wno-sizeof-pointer-div`.

However, the C++ part needs some work to be able to compile.

Would you mind finishing it?


Thanks,

Alex
------------------------------------------------------------------

#if defined(__cplusplus)
# include <cstddef>

# if __cplusplus >= 201703L
#  include <array>
#  define array_length(arr)	(std:size(arr))
# else

#  if __cplusplus >= 201103L
constexpr
#  endif
inline std::size_t
array_length(const T(&array)[N])
#  if __cplusplus >= 201103L
noexcept
#  endif
{
	return	N;
}
# endif

# if __cplusplus >= 202002L
#  define array_slength(arr)	(std:ssize(arr))
# else

#  if __cplusplus >= 201103L
constexpr
#  endif
inline std::ptrdiff_t
array_slength(const T(&array)[N])
#  if __cplusplus >= 201103L
noexcept
#  endif
{
	return	N;
}
# endif


#else /* !defined(__cplusplus) */
#include <stddef.h>

# define __is_same_type(a, b)						\
	__builtin_types_compatible_p(__typeof__(a), __typeof__(b))
# define __is_array(arr)	(!__is_same_type((arr), &(arr)[0]))

# if __STDC_VERSION__ >= 201112L
#  define __must_be(e, msg)	(					\
	0 * (int)sizeof(						\
		struct {						\
			_Static_assert((e), msg);			\
			char ISO_C_forbids_a_struct_with_no_members__;	\
		}							\
	)								\
)
# else
#  define __must_be(e, msg)	(					\
	0 * (int)sizeof(						\
		struct {						\
			int	: (-!(e));				\
			char ISO_C_forbids_a_struct_with_no_members__;	\
		}							\
	)								\
)
# endif

# define __must_be_array(arr)	__must_be(__is_array(arr), "Must be an 
array!")

# define __array_length(arr)	(sizeof(arr) / sizeof((arr)[0]))
# define array_length(arr)	(__array_length(arr) + __must_be_array(arr))
# define array_slength(arr)	((ptrdiff_t)array_length(arr))
#endif


int main(void)
{
	int a[5];
	const int x = 6;
	int b[x];
#if __cplusplus >= 201103L
	constexpr
#endif
int y = 7;
	int c[y];
	int *p;
	(void)p;

	return	array_slength(a) + array_slength(b) +
		array_length(c) /*+
		array_length(p)*/;
}

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