Re: Expose 'array_length()' macro in or
On 2020-09-30 17:58, Joseph Myers wrote: For some reason http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2529.pdf doesn't seem to have reached the agenda of a WG14 meeting yet, but adding a language feature like that to the standard would be another approach. Hi Joseph, Yes, that would be great! I hope they add that to the language. When/if that happens, nitems() could be `#define nitems(arr) _Lengthof(arr)` for std >= c2x. In the meantime, I would add this macro to libc. Maybe gcc could add such a great feature as an extension even before the standard does... Too many wishes :) BTW, I sent a PATCH v4 that I should've sent --in-reply-to PATCH v3 in this thread (but I forgot to do so); I'll link to it here: https://sourceware.org/pipermail/libc-alpha/2020-September/117986.html Thanks, Alex
Re: Expose 'array_length()' macro in
Thanks again for your improvements.
I think this might be ready for a patch already.
Any more thoughts?
Thanks,
Alex
#if defined(__cplusplus)
# include
# if __cplusplus >= 201103L
template
constexpr inline std::size_t
__array_length(const _Tp(&)[_Len]) __THROW
{
return _Len;
}
template
constexpr inline std::ptrdiff_t
__array_slength(const _Tp(&)[_Len]) __THROW
{
return _Len;
}
# else /* __cplusplus < 201103L */
template
char (&__array_length(const _Tp(&)[_Len]))[_Len];
# define __array_length(_Arr) (sizeof(__array_length(_Arr)))
# define __array_slength(_Arr) \
(static_cast(__array_length(_Arr)))
# endif /* __cplusplus >= 201103L */
#else /* !defined(__cplusplus) */
#include
# 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(_Expr, _Msg)( \
0 * (int)sizeof(\
struct {\
_Static_assert((_Expr), _Msg); \
char _ISO_C_forbids_a_struct_with_no_members; \
} \
) \
)
# else
# define __must_be(_Expr, _Msg)( \
0 * (int)sizeof(\
struct {\
int : (-!(_Expr)); \
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_len(_Arr) (sizeof(_Arr) / sizeof((_Arr)[0]))
# define __array_length(_Arr) (__array_len(_Arr) + __must_be_array(_Arr))
# define __array_slength(_Arr) ((ptrdiff_t)__array_length(_Arr))
#endif /* defined(__cplusplus) */
static int a[5];
static int v[__array_slength(a)];
static int w[__array_length(v)];
static int *p;
int main(void)
{
int aa[5];
const int xx = 6;
int vv[xx];
int yy = 7;
int ww[yy];
int *pp;
(void)p;
(void)pp;
(void)ww;
return __array_slength(a)
+ __array_length(v)
+ __array_slength(w)
/* + __array_length(p) */ /* Always breaks :) */
+ __array_length(aa)
+ __array_slength(vv)
+ __array_length(ww) /* Not in C++ */
/* + __array_length(pp) */ /* Always breaks :) */
;
}
Re: Expose 'array_length()' macro in
* Jonathan Wakely:
> I don't see much point in using std::size here. If you're going to
> provide the alternative implementation for when std::size isn't
> defined, why not just use it always?
>
> template
> #if __cplusplus >= 201103L
> constexpr
> #endif
> inline std::size_t
> __array_length(const _Tp(&)[_Num]) __THROW
> {
> return _Num;
> }
>
> This only requires , not .
I agree that this is an advantage. But the version without constexpr is
not sufficient because __array_length does not produce a constant
expression.
I've seen something like this used instead:
template
char (&___array_length(const _Tp(&)[_Num]))[_Num];
#define __array_length(v) (sizeof(___array_length(v)))
If the function type is too cute, a helper struct could be used instead.
Thanks,
Florian
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Re: Expose 'array_length()' macro in
On 22/09/20 11:10 +0200, Alejandro Colomar via Libstdc++ wrote:
[[ CC += LKML ]]
Thanks for all your input. I learned some C++ :)
The following code works for all C and C++ standards:
g++ --std={c++98, c++03, c++11, c++14, c++17, c++20}
gcc --std={c89, c99, c11, c18, c2x}
With `-Wall -Wextra -Werror -pedantic -Wno-vla -Wno-sizeof-pointer-div`.
It doesn't compile when '+ __array_length(p)' is uncommented.
It compiles, and returns the correct value (18).
With some exceptions:
c++ doesn't accept the VLA (w[]):
array_length.c: In function 'int main()':
array_length.c:101:22: error: no matching function for call to
'__array_slength(int [y])'
101 | + __array_slength(w)
| ^
array_length.c:38:1: note: candidate: 'template
std::ptrdiff_t __array_slength(const T (&)[N])'
38 | __array_slength(const T(&)[N])
| ^~~
array_length.c:38:1: note: template argument deduction/substitution
failed:
array_length.c:101:22: note: variable-sized array type 'long int' is
not a valid template argument
101 | + __array_slength(w)
| ^
But we can live with limited support for VLAs in C++.
So I needed to comment '+ __array_slength(w)',
and then the program correctly returns 11.
As Ville suggested, I renamed the function/macro to __array_[s]length().
However, (some) BSDs already provide nitems() in ,
so it probably wouldn't be very drastic to
provide this function/macro with the name '[s]nitems()' there.
Would you like to add anything else before I write the patch?
BTW, I should note one more thing:
Linux has a macro named '__must_be_array()' with the same API,
but slightly different internal implementation,
so they should be aware of this change.
However, I don't think they include a lot,
so maybe it doesn't break anything; but they should be aware anyway.
I CC'd the LKML so they are aware and can give any suggestions.
Thanks,
Alex
#if defined(__cplusplus)
# include
# if __cplusplus >= 201703L
# include
# endif
template
You need to use reserved names here.
# if __cplusplus >= 201103L
constexpr
# endif
inline std::size_t
# if __cplusplus >= 201703L
__array_length(const T()[N])
# else
__array_length(const T(&)[N])
# endif
# if __cplusplus >= 201103L
noexcept
If this is going to be part of Glibc then you can use its __THROW
macro instead.
# endif
{
# if __cplusplus >= 201703L
return std::size(arr);
# else
return N;
# endif
}
I don't see much point in using std::size here. If you're going to
provide the alternative implementation for when std::size isn't
defined, why not just use it always?
template
#if __cplusplus >= 201103L
constexpr
#endif
inline std::size_t
__array_length(const _Tp(&)[_Num]) __THROW
{
return _Num;
}
This only requires , not .
template
This declaration is wrong, the array extent has type std::size_t. The
type you return from the function doesn't change that.
The __array_slength definition should be identical to __array_length
except for its name and return type.
template
#if __cplusplus >= 201103L
constexpr
#endif
inline std::ptrdiff_t
__array_slength(const _Tp(&)[_Num]) __THROW
{
return _Num;
}
#else /* !defined(__cplusplus) */
#include
# 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_len(arr) (sizeof(arr) / sizeof((arr)[0]))
# define __array_length(arr)(__array_len(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 v[x];
int y =
Re: Expose 'array_length()' macro in
[[ CC += LKML ]]
Thanks for all your input. I learned some C++ :)
The following code works for all C and C++ standards:
g++ --std={c++98, c++03, c++11, c++14, c++17, c++20}
gcc --std={c89, c99, c11, c18, c2x}
With `-Wall -Wextra -Werror -pedantic -Wno-vla -Wno-sizeof-pointer-div`.
It doesn't compile when '+ __array_length(p)' is uncommented.
It compiles, and returns the correct value (18).
With some exceptions:
c++ doesn't accept the VLA (w[]):
array_length.c: In function 'int main()':
array_length.c:101:22: error: no matching function for call to
'__array_slength(int [y])'
101 | + __array_slength(w)
| ^
array_length.c:38:1: note: candidate: 'template
std::ptrdiff_t __array_slength(const T (&)[N])'
38 | __array_slength(const T(&)[N])
| ^~~
array_length.c:38:1: note: template argument deduction/substitution
failed:
array_length.c:101:22: note: variable-sized array type 'long int' is
not a valid template argument
101 | + __array_slength(w)
| ^
But we can live with limited support for VLAs in C++.
So I needed to comment '+ __array_slength(w)',
and then the program correctly returns 11.
As Ville suggested, I renamed the function/macro to __array_[s]length().
However, (some) BSDs already provide nitems() in ,
so it probably wouldn't be very drastic to
provide this function/macro with the name '[s]nitems()' there.
Would you like to add anything else before I write the patch?
BTW, I should note one more thing:
Linux has a macro named '__must_be_array()' with the same API,
but slightly different internal implementation,
so they should be aware of this change.
However, I don't think they include a lot,
so maybe it doesn't break anything; but they should be aware anyway.
I CC'd the LKML so they are aware and can give any suggestions.
Thanks,
Alex
#if defined(__cplusplus)
# include
# if __cplusplus >= 201703L
# include
# endif
template
# if __cplusplus >= 201103L
constexpr
# endif
inline std::size_t
# if __cplusplus >= 201703L
__array_length(const T()[N])
# else
__array_length(const T(&)[N])
# endif
# if __cplusplus >= 201103L
noexcept
# endif
{
# if __cplusplus >= 201703L
return std::size(arr);
# else
return N;
# endif
}
template
# if __cplusplus >= 201103L
constexpr
# endif
inline std::ptrdiff_t
# if __cplusplus >= 202002L
__array_slength(const T()[N])
# else
__array_slength(const T(&)[N])
# endif
# if __cplusplus >= 201103L
noexcept
# endif
{
# if __cplusplus >= 202002L
return std::ssize(arr);
# else
return N;
# endif
}
#else /* !defined(__cplusplus) */
#include
# 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_len(arr) (sizeof(arr) / sizeof((arr)[0]))
# define __array_length(arr)(__array_len(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 v[x];
int y = 7;
int w[y];
int *p;
(void)p;
(void)v;
(void)w;
return __array_slength(a)
+ __array_slength(v)
+ __array_slength(w)
// + __array_length(p)
;
}
