是什么让工会会员活跃起来？

Question

是什么让工会成员活跃起来？

我已经阅读了 C++14 标准的第 9.5 章（关于联合的那一章），但是对于什么使联合成员活跃，我还没有找到明确的答案。

有备注：

In general, one must use explicit destructor calls and placement new operators to change the active member of a union.

例如，

union U {
  int i;
  short s;
} u;

new(&u.i) int(42);

好的，placement new 改变活跃成员，很清楚了。但是在处理具有普通构造函数的类型时，我们通常不使用 placement new。

operator=是否更换没有UB的活跃会员？

u.i = 42;

这里，operator= 调用了一个未构造的对象。定义好了吗？

这个呢？

struct A {
  int i0;
  int i1;
};
union U {
  A a;
  short s;
} u;

是什么让 a 成为 u 的活跃成员？同时设置 i0 和 i1 是否足够？

u.a.i0 = 42;
u.a.i1 = 99;

如果我写：

u.a.i0 = 42;     // supposedly this doesn't change the active member to a, as i1 isn't set
int x = u.a.i0;  // is it fine to read from a.i0? a is not an active member supposedly

u.a.i0 = 42;之后，active成员没有改成a（我觉得），所以做int x = u.a.i0;是不是UB？

C++17 是否改进了活跃成员的描述？

Answer 1

在 C++17 中，添加了一段明确讨论 u.i = 42:

等情况

[class.union]/5 When the left operand of an assignment operator involves a member access expression (8.2.5) that nominates a union member, it may begin the lifetime of that union member, as described below. For an expression E, define the set S(E) of subexpressions of E as follows:

(5.1) — If E is of the form A.B, S(E) contains the elements of S(A), and also contains A.B if B names a union member of a non-class, non-array type, or of a class type with a trivial default constructor that is not deleted, or an array of such types.

(5.2) — If E is of the form A[B] and is interpreted as a built-in array subscripting operator, S(E) is S(A) if A is of array type, S(B) if B is of array type, and empty otherwise.

(5.3) — Otherwise, S(E) is empty.

In an assignment expression of the form E1 = E2 that uses either the built-in assignment operator (8.18) or a trivial assignment operator (15.8), for each element X of S(E1), if modification of X would have undefined behavior under 6.8, an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment. [ Note: This ends the lifetime of the previously-active member of the union, if any (6.8). —end note ]

（后面是一个较长的示例，我懒得正确格式化，但您可以看到 here。）

是什么让工会会员活跃起来？

What makes a union member active?

c++

unions

language-lawyer

c++14

c++17