如何在运行时区分 c 中的联合成员

Question

I have a doubt regarding unions. Suppose there is a union defined as

union emp{
    float f;
    int i;
};

Is there a way to determine which field of union is used. I have come across a situation in which I have to print float if float field is used and print int if int field is used.The function may be like

void print(union u){

    // if float field is initialized
    printf("float field = %f\n",u.f);

    // if int field is initialized
    printf("int field = %d\n",u.i);


}

Thanks in advance.

Answer 1

You can't tell with just the union ; typically you wrap it in a struct which includes a tag value of some kind.

struct val {
  enum { EMP_INT, EMP_FLOAT } tag;
  union emp {
    int i;
    float f;
  } emp;
};

Answer 2

Not really, depending on which field you access the memory will be interpreted as either a float or an int .

You best choice would be to make the union part of a struct and also keep a flag indicating which field is used.

Answer 3

You can't, the information isn't stored anywhere at runtime.

You could do something like:

struct emp {
        enum storedValue type;
        union {
                float f;
                int   i;
        }
};

to manually store the type. The enum then has values like floatval and intval or similiar.

Instead of an enum, you could use a boolean like isFloat .