Re: [PATCH v2] _Generic.3: New page documenting _Generic()

[Florian Weimer <fweimer@redhat.com>]

* Alejandro Colomar:

> +.SH EXAMPLES
> +The following program demonstrates how to write
> +a replacement for the standard
> +.BR imaxabs (3)
> +function, which being a function can't really provide what it promises:
> +seamlessly upgrading to the widest available type.
> +.PP
> +.\" SRC BEGIN (_Generic.c)
> +.EX
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +
> +#define my_imaxabs(j)  _Generic((intmax_t) 0,  \e
> +    int:            abs(j),                    \e
> +    long:           labs(j),                   \e
> +    long long:      llabs(j)                   \e
> + /* long long long: lllabs(j) */               \e
> +)

The macro name does not really match what the function does.  It's a
type-generic abs function, not related to the max function or intmax_t.

Note that this approach does not really work that well in practice
because macros using _Generic expand all the alternatives (in current
implementations; doing this differently requires deviating from the
layered implementation strategy suggested in the C standard).  This
means that _Generic-using macros can only be nested maybe three or four
levels deep, depending on the number of _Generic alternatives on each
level.  For <tgmath.h>, this is really not enough, so a high-quality
implementation of <tgmath.h> using _Generic is not feasible.  GCC
provides __builtin_tgmath, which is designed in such a way that when
used in a macro, the macro argument is only expanded once.

Maybe mention this under BUGS?

C++ templates do not suffer from this particular problem.

Thanks,
Florian