C语言：联合变量

#include <stdio.h>
union hold{
        int digit;
        double big;
        char letter;
};
int main(){
        union hold a;
        a.letter = 'R';

        union hold b = a;

        union hold c = {};

        union hold d = {
                .big = 99.9
        };

        union hold *ps = &a;
        printf("---%c---",ps->letter);
        printf("---%c---",a);
}