Type information-encoded flags in C

metamap.c

#include <stdio.h>

enum {
	FOO = 1 << 0,
	BAR = 1 << 1,
	BYX = 1 << 2,
	QUE = 1 << 3
};

#define maybe_signed(T, cond) __typeof__(  \
        _Generic(&(char[2 - !(cond)]){0},  \
                char(*)[1]: (unsigned T)0, \
                char(*)[2]: (signed T)0))

#define if_signed(T, x, then)                              \
        _Generic((__typeof__(x) *){0},                     \
                signed T const *: (then), unsigned T *: 0, \
                signed T *: (then), unsigned T const *: 0)

#define define_metaflag(flag, flags) \
        maybe_signed(char, flag & (flags)) meta_##flag

#define decode_metaflag(T, flag) \
        if_signed(char, (T){0}.meta_##flag, flag)

#define define_metamap(T, flags)     \
typedef union {                      \
        define_metaflag(FOO, flags); \
        define_metaflag(BAR, flags); \
        define_metaflag(BYX, flags); \
        define_metaflag(QUE, flags); \
} T

#define decode_metamap(T) (          \
        0u | decode_metaflag(T, FOO) \
           | decode_metaflag(T, BAR) \
           | decode_metaflag(T, BYX) \
           | decode_metaflag(T, QUE) \
)

int main (void)
{
	define_metamap(foo_and_byx, FOO | BYX);
	define_metamap(bar_and_que, BAR | QUE);
	define_metamap(without_foo, BAR | BYX | QUE);

	printf("0x%04x\n0x%04x\n0x%04x\n",
	       decode_metamap(foo_and_byx),
	       decode_metamap(bar_and_que),
	       decode_metamap(without_foo));
}