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

Creating a new object at the storage location that a const object with static, thread, or automatic storage duration occupies or, at the storage location that such a const object used to occupy before its lifetime ended results in undefined behavior.

But no compiler warns me while it's an easy to diagnose mistake. Am I misinterpreting the wording?

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.