On 2018-10-17 21:06:13 +0200, Peter Eisentraut wrote: > On 16/10/2018 08:30, Andres Freund 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? > > I've had the attached patch lying around.
Heh. > As you can see there, there are a few places where it could be useful, > but not too many. Yea. > The name CONST_CAST() is adapted from C++. > > Your version with __builtin_types_compatible_p() doesn't work for > casting between char * and const char *, so it wouldn't be very useful. Huh, why wouldn't it work for char *? Seems to work fine for me? > +/* > + * Safer casting -- use this for casting away const or volatile. It ensures > + * that the source and target types are otherwise compatible. > + */ > +#define CONST_CAST(t, x) ((void)((t)0 == (x)), (t)(x)) > + Has the advantage that it probably works for globals, but OTOH, it only works correctly for pointers, and it doesn't reliably trigger warnigns / errors. Greetings, Andres Freund
