the "paired, sliding arg list" macro trick

paired_sliding_arg_list_macro_trick_1

// Accept any number of args >= N, but expand to just the Nth one. In this case,
// we have settled on 5 as N. We could pick a different number by adjusting
// the count of throwaway args before N. Note that this macro is preceded by
// an underscore--it's an implementation detail, not something we expect people
// to call directly.
#define _GET_NTH_ARG(_1, _2, _3, _4, N, ...) N

// Count how many args are in a variadic macro. Only works for up to N-1 args.
#define COUNT_VARARGS(...) _GET_NTH_ARG(__VA_ARGS__, 4, 3, 2, 1)

int main() {
    printf("one arg: %d\n", COUNT_VARARGS(1));
    printf("three args: %d\n", COUNT_VARARGS(1, 2, 3));
}
 
// ------ output --------
one arg: 1
three args: 3

Based on a technique described here: http://stackoverflow.com/a/11763277