Is it possible to un-const typeof in gcc pure C?

Question

I have a macro that uses GCC's typeof to create a variable of the same type of a macro argument. The problem is: if that argument has const type, the variable created inside the macro is const and I can't use it. For instance:

#include <stdio.h>

#define DECR(x) ({typeof(x) y; y = x; y--; y;})

int main(void)
{
    const int v = 5;
    printf("%d\n", DECR(v));
    return 0;
}

Compilation gives:

$ cc    -c -o t.o t.c
t.c: In function 'main':
t.c:9:2: error: assignment of read-only variable 'y'
t.c:9:2: error: decrement of read-only variable 'y'
make: *** [t.o] Error 1

Is there a way to copy the typeof a value and un-const it?

Answer 1

If you don't mind the possible arithmetic promotion you can do this:

#define DECR(x) ({typeof(x + 0) y; y = x; y--; y;})

The trick is that the expression for typeof is x + 0, which is a r-value, and so the l-value-constness (which is what you want to avoid) is lost.

The same trick can be done with 1 * x, but curiously enough, +x and -x don't work.

Answer 2

This is a rather late answer, but if you don't mind using more GCC extensions you can do this like this (building upon a previous answer somewhat).

#define UNCONST_HAX_(TYPE) ({TYPE _tmp_macro_var_; _tmp_macro_var_;})
#define UNCONST(x)                                                      \
    __typeof__(_Generic((x),                                            \
            signed char:              UNCONST_HAX_(signed char),        \
            const signed char:        UNCONST_HAX_(signed char),        \
            unsigned char:            UNCONST_HAX_(unsigned char),      \
            const unsigned char:      UNCONST_HAX_(unsigned char),      \
            short:                    UNCONST_HAX_(short),              \
            const short:              UNCONST_HAX_(short),              \
            unsigned short:           UNCONST_HAX_(unsigned short),     \
            const unsigned short:     UNCONST_HAX_(unsigned short),     \
            int:                      UNCONST_HAX_(int),                \
            const int:                UNCONST_HAX_(int),                \
            unsigned:                 UNCONST_HAX_(unsigned),           \
            const unsigned:           UNCONST_HAX_(unsigned),           \
            long:                     UNCONST_HAX_(long),               \
            const long:               UNCONST_HAX_(long),               \
            unsigned long:            UNCONST_HAX_(unsigned long),      \
            const unsigned long:      UNCONST_HAX_(unsigned long),      \
            long long:                UNCONST_HAX_(long long),          \
            const long long:          UNCONST_HAX_(long long),          \
            unsigned long long:       UNCONST_HAX_(unsigned long long), \
            const unsigned long long: UNCONST_HAX_(unsigned long long), \
            float:                    UNCONST_HAX_(float),              \
            const float:              UNCONST_HAX_(float),              \
            double:                   UNCONST_HAX_(double),             \
            const double:             UNCONST_HAX_(double),             \
            long double:              UNCONST_HAX_(long double),        \
            const long double:        UNCONST_HAX_(long double)         \
    ))

And it could be used as follows:

#define DECR(x) ({UNCONST(x) y; y = x; y--; y;})

Yes, it is pretty ugly.