c++ - 具有 const 和 nonconst 成员的 union ？

Question

这似乎是未定义的行为

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

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

规范说

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.

但是没有编译器会警告我，因为它很容易诊断错误。我是否误解了措辞？

Answer 1

最新的 C++0x 草案标准明确说明了这一点:

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.

所以你的陈述

a.y = 1;

很好，因为它将事件成员从 x 更改为 y。如果您随后将 a.x 引用为右值，则行为将是未定义的:

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

您对规范的引用与此处无关，因为您没有创建任何新对象。