Re: Large writable variables

On Tue, Oct 16, 2018 at 2:30 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> This just reminded me that a couple times I wanted a cast that casts
> away const, but otherwise makes sure the type stays the same. I don't
> think there's a way to do that in C, but we can write one that verifies
> the cast doesn't do something bad if gcc is used:
>
> #if defined(HAVE__BUILTIN_TYPES_COMPATIBLE_P)
> #define unconstify(cst, var) StaticAssertExpr(__builtin_types_compatible_p(__typeof(var), const cst), "wrong cast"), (cst) (var)
> #else
> #define unconstify(cst, var) ((cst) (var))
> #endif
>
> Does anybody besides me see value in adding a cleaned up version of
> that?

Under what circumstances would we consider this to be a legitimate thing to use?

I think if we add something this, we'd better accompany it with some
detailed and very clearly-written statements about when you're allowed
to use it. Otherwise, I predict that people will use it in cases
where it's not actually safe, and we'll end up with low-grade bugs.

--
Robert Haas
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