Can an anonymous union be instantiated in c++?

Question

I have this piece of code:

class test {
  private:
  union {
    double x;
    std::vector<double> y;
  } amIValid;
};

I wonder if the union instance amIValid is valid?

Possible duplicate of [Why does C++ disallow anonymous structs and unions?](https://stackoverflow.com/questions/2253878/why-does-c-disallow-anonymous-structs-and-unions) — underscore_d, Dec 07 '17 at 12:15
`gcc` gives me this: `note: ‘test::::()’ is implicitly deleted because the default definition would be ill-formed:|` — Pekov, Dec 07 '17 at 12:17
So you answered your own question before even posting it? Yes, you need to provide a constructor in such cases, to ensure a single member is active after the object is initialised. — underscore_d, Dec 07 '17 at 12:19
Strictly speaking, you don't have an anonymous union. You have a member of an unnamed union type. Anonymous unions inject their members into the enclosing class/union. — StoryTeller - Unslander Monica, Dec 07 '17 at 12:28

score 2 · Accepted Answer · answered Dec 07 '17 at 14:43

An unnamed union can be instantiated in C++:

union { int i; double d; } my_thing;
my_thing.i = 3;
// etc.

An anonymous union is an unnamed union that is not instantiated (scroll down). You can access its members directly:

union { int i; double d; };
i = 3;
// etc.

So the answer to the question in the title is that an anonymous union cannot be instantiated because instantiating it means that it is not an anonymous union.

Can an anonymous union be instantiated in c++?

1 Answers1