An union with a const and a nonconst member?

Question

This appears to be undefined behavior

union A {
  int const x;
  float y;
};

A a = { 0 };
a.y = 1;

The spec says

Cr

Answer 1

The latest C++0x draft standard is explicit about this:

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.

So your statement

a.y = 1;

is fine, because it changes the active member from x to y. If you subsequently referenced a.x as an rvalue, the behaviour would be undefined:

cout << a.x << endl ; // Undefined!

Your quote from the spec is not relevant here, because you are not creating any new object.