How to know that which variable from Union is Used?

Question

If I declare a Union as:

union TestUnion
{
    struct 
    {
      unsigned int Num;
      unsigned char Name[5];
    }TestStruct;
    unsigned char Total[7];
};         


        

Answer 1

Short answer: there is no way except by adding an enum somewhere in your struct outside the union.

enum TestUnionPart
{
  TUP_STRUCT,
  TUP_TOTAL
};

struct TestUnionStruct
{
  enum TestUnionPart Part;
  union
  {
    struct
    {
      unsigned int Num;
      unsigned char Name[5];
    } TestStruct;
    unsigned char Total[7];
  } TestUnion;
};

Now you'll need to control creation of your union to make sure the enum is correctly set, for example with functions similar to:

void init_with_struct(struct TestUnionStruct* tus, struct TestStruct const * ts)
{
  tus->Part = TUP_STRUCT;
  memcpy(&tus->TestUnion.TestStruct, ts, sizeof(*ts));
}

Dispatch on the correct values is now a single switch:

void print(struct TestUnionStruct const * tus)
{
  switch (tus->Part)
  {
    case TUP_STRUCT:
      printf("Num = %u, Name = %s\n",
             tus->TestUnion.TestStruct.Num,
             tus->TestUnion.TestStruct.Name);
      break;
    case TUP_TOTAL:
      printf("Total = %s\n", tus->TestUnion.Total);
      break;
    default:
      /* Compiler can't make sure you'll never reach this case */
      assert(0);
  }
}

---

As a side note, I'd like to mention that these constructs are best handled in languages of the ML family.

type test_struct = { num: int; name: string }
type test_union = Struct of test_struct | Total of string