Union tested for current member in use

Question

Do unions have a control structure to test which member is currently in use (or if it has any at all)? I'm asking this because undefined behavior is never a good thing to have in your program.

Anisometric

Answer 1

No, no such mechanism exists off-the-shelf. You'll have to take care of that yourself. The usual approach is wrapping the union in a struct:

struct MyUnion
{
   int whichMember;
   union {
      //whatever
   } actualUnion;
};

So you have MyUnion x; and x.whichMember tells you which field of x.actualUnion is in use (you have to implement the functionality though).

Notarize

+1: This type of structure is often called either a "discriminated union" or a "tagged union". – Stochastic

Yes, I thought of a similar approach, but because I prefer language constructs to self-made ones I just had to know if one existed. Thanks for the quick answer. – Anisometric

@AlexD. Pascal had a tagged union, but it amounted to the same thing as this. You still had to test the tag to determine which element was in use. – Sergius