Re: [PATCH v3 3/5] stddef: Add const_cast()

[David Laight <david.laight.linux@gmail.com>]

On Tue, 10 Nov 2026 16:33:40 +0100
Alejandro Colomar <alx@kernel.org> wrote:

> This macro serves to cast const away without risking unwanted
> conversions due to mistakes in the cast.
> 
> It's similar to C++'s const_cast(), but it is more restrictive: it can
> only cast away 'const', and not arbitrary qualifiers.
> 
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
>  include/linux/stddef.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> index e1851c50c89b..d79a81406b78 100644
> --- a/include/linux/stddef.h
> +++ b/include/linux/stddef.h
> @@ -194,4 +194,16 @@ enum {
>  #define TRAILING_OVERLAP(TYPE, NAME, FAM, MEMBERS)				\
>  	__TRAILING_OVERLAP(TYPE, NAME, FAM, /* no attrs */, MEMBERS)
>  
> +/**
> + * const_cast() - Drop const in the left-most derivation of a pointer
> + *
> + * @T: Type for the cast.
> + * @p: Pointer to be casted.
> + *
> + * If @p is of type const @T, then it is casted to @T.
> + * Otherwise, @p is returned without any casts.
> + * This avoids mistakes that change anything other than 'const' in a cast.
> + */
> +#define const_cast(T, p)  _Generic(p, const T: (T) (p), default: (p))

Should it be de_const_cast(), or just de_const().
It is a shame you need to specify the type, but why not:
	_Generic(p, T: p, const T: (T)(p))
so that you get a compile time error from the _Generic() if the type
is wrong.

David


> +
>  #endif