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;
};