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Message-ID: <20230526212951.148fc424@inria.fr>
Date: Fri, 26 May 2023 21:29:51 +0200
From: Jₑₙₛ Gustedt <jens.gustedt@...ia.fr>
To: NRK <nrk@...root.org>
Cc: musl@...ts.openwall.com
Subject: Re: [C23 const 1/2] C23: change bsearch to a macro that
respects the const contract
on Fri, 26 May 2023 23:20:37 +0600 you (NRK <nrk@...root.org>) wrote:
> On Fri, May 26, 2023 at 01:29:31PM +0200, Nat! wrote:
> > I think it's sort of obvious, that these macros increase code
> > brittleness due to now multiple execution of macro arguments vs. a
> > single execution in a function call.
>
> It would be heavily surprising if the controlling expression of
> _Generic was evaluated. Similar to `sizeof`, it only needs to know
> the type of the expression and thus doesn't require evaluation (only
> exception being VLAs in a sizeof).
Yes, exactly, that's the idea. And VLA (and similar) are not allowed
for the controling expression of a generic selection, so this problem
does not arise, there.
> | The controlling-expression and the expressions of the selections
> that | are not chosen are never evaluated.
>
> However, there is one thing that I don't quite understand about this
> patch:
>
> > + void const*: (void const*)bsearch((K), (void
> > const*)(B), (N), (S), (C)), \
>
> What's with the `(void const*)(B)` cast? It's already determined to be
> `void const *` via _Generic.
Even if not evaluated, all branches have to be syntactically and
semantically correct. If you don't use these kind of casts, you may get
a lot of warnings.
This "feature" makes the use of these beast a bit subtle, sometimes.
(Originally, generic selection had been designed to chose between
different function pointers similar as in <tgmath.h> to implement
something like overloading.)
Thanks
Jₑₙₛ
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