Re: _Generic.3: EXAMPLES: C++'s static_cast() in C

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 11/11/22 18:08, Alejandro Colomar wrote:>>>> Dear readers,
>>>>
>>>> I've been developing a cast macro for C that is similar to C++'s 
>>>> static_cast(), which allows to:
>>>>
>>>> - Cast between a limited set of types (actually, pointers to those types).
>>>> - Can add const, but not remove it.
>>>> - Easily grep for all casts.
>>>>
>>>> But forbids:
>>>>
>>>> - Cast between unrelated types.
>>>> - Removing const.
>>>>
>>>> This improves type safety in C, where casts allow programmers to commit all 
>>>> kinds of crimes by disabling most compiler warnings.
>>>>
>>>> The macro I originally wrote, allows to convert between u_char and char, for 
>>>> a code base that uses u_char internally but has to interface libc and 
>>>> syscalls. That one is unnecessary for most code bases which just use char 
>>>> (and that's how it should be for strings; if you want unsigned 'char's, you 
>>>> should use '-funsigned-char').
>>>>
>>>> But there are cases where the kernel or libc forces us to use casts, like 
>>>> for example in bind(2).  That's for what the following macro is, and I plan 
>>>> to add it to the EXAMPLES section in _Generic(3), replacing the previous 
>>>> program.
>>>>
>>>> The macro itself is quite huge, which might discourage some, but it is not 
>>>> so complex.  I'll release it to the public domain.  Could you please review it?
>>>>
>>>> Cheers,
>>>>
>>>> Alex
>>>>
>>>
>>> With C2x's typeof_unqual(), it can be made a bit smaller, and better (it now 
>>> supports volatile).
>>
>>
>> And since it makes no sense to convert between some of the derivative types 
>> (e.g., sa_in to sa_un), it can be further simplified:
>>
> v4:
> 
> - Reintroduce some typeof_unqual() that were accidentally removed in v3.
> - Align casts for readability.
> 

v5:

- Casting to sockaddr_storage is useless.  Only casting from it makes sense.

I'll send shortly after an v6, if I manage to simplify after this change, but 
I'll post this v5 to not have a sudden jump with an unreadably interdiff.

#define sockaddr_cast(t, p)                            \
     _Generic(&*(p),                                    \
     struct sockaddr *:                                 \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr_in *:             (t) (p),     \
         struct sockaddr_in6 *:            (t) (p),     \
         struct sockaddr_un *:             (t) (p),     \
         default: (p)),                                 \
     struct sockaddr **:                                \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr_in **:            (t) (p),     \
         struct sockaddr_in6 **:           (t) (p),     \
         struct sockaddr_un **:            (t) (p),     \
         default: (p)),                                 \
     const struct sockaddr *:                           \
         _Generic((t) NULL,                             \
         const struct sockaddr_in *:       (t) (p),     \
         const struct sockaddr_in6 *:      (t) (p),     \
         const struct sockaddr_un *:       (t) (p),     \
         default: (p)),                                 \
                                                        \
     struct sockaddr_in *:                              \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *:                (t) (p),     \
         default: (p)),                                 \
     struct sockaddr_in **:                             \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr **:               (t) (p),     \
         default: (p)),                                 \
     const struct sockaddr_in *:                        \
         _Generic((t) NULL,                             \
         const struct sockaddr *:          (t) (p),     \
         default: (p)),                                 \
                                                        \
     struct sockaddr_in6 *:                             \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *:                (t) (p),     \
         default: (p)),                                 \
     struct sockaddr_in6 **:                            \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr **:               (t) (p),     \
         default: (p)),                                 \
     const struct sockaddr_in6 *:                       \
         _Generic((t) NULL,                             \
         const struct sockaddr *:          (t) (p),     \
         default: (p)),                                 \
                                                        \
     struct sockaddr_un *:                              \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *:                (t) (p),     \
         default: (p)),                                 \
     struct sockaddr_un **:                             \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr **:               (t) (p),     \
         default: (p)),                                 \
     const struct sockaddr_un *:                        \
         _Generic((t) NULL,                             \
         const struct sockaddr *:          (t) (p),     \
         default: (p)),                                 \
                                                        \
     struct sockaddr_storage *:                         \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr *:                (t) (p),     \
         struct sockaddr_in *:             (t) (p),     \
         struct sockaddr_in6 *:            (t) (p),     \
         struct sockaddr_un *:             (t) (p),     \
         default: (p)),                                 \
     struct sockaddr_storage **:                        \
         _Generic((typeof_unqual(t)) NULL,              \
         struct sockaddr **:               (t) (p),     \
         struct sockaddr_in **:            (t) (p),     \
         struct sockaddr_in6 **:           (t) (p),     \
         struct sockaddr_un **:            (t) (p),     \
         default: (p)),                                 \
     const struct sockaddr_storage *:                   \
         _Generic((t) NULL,                             \
         const struct sockaddr *:          (t) (p),     \
         const struct sockaddr_in *:       (t) (p),     \
         const struct sockaddr_in6 *:      (t) (p),     \
         const struct sockaddr_un *:       (t) (p),     \
         default: (p)),                                 \
                                                        \
     default:                                           \
         (p)                                            \
     )


-- 
<http://www.alejandro-colomar.es/>

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