c++ - 在 C++ 中使用 `{}` 聚合初始化 union

Question

在下面的程序中 union U有两个字段 a和 b ，每个都有不同的默认值。如果创建了一个 U 类型的变量使用聚合初始化 {} union 的值(value)和活跃成员是什么？

#include <iostream>

struct A { int x = 1; };
struct B { int x = 0; };

union U {
    A a;
    B b;
};

int main() {
    U u{};
    std::cout << u.a.x;
}

令人惊讶的是，编译器在这里出现分歧:Clang 打印 1和 GCC 打印 0 , 演示:https://gcc.godbolt.org/z/8Tj4Y1Pv1
其中一个编译器是否存在错误，或者此处的行为不是由标准定义的？

Answer 1

Clang 是对的，GCC 是错的
根据 [dcl.init.aggr]/1 :

An aggregate is an array or a class ([class]) with

• (1.1) no user-declared or inherited constructors ([class.ctor]),
• (1.2) no private or protected direct non-static data members ([class.access]),
• (1.3) no virtual functions ([class.virtual]), and
• (1.4) no virtual, private, or protected base classes ([class.mi]).

A , B和 U都是聚合类，尽管前者是非 union 聚合类，前者不符合条件。
根据 [dcl.init.aggr]/5 [ 重点 矿]:

For a non-union aggregate, each element that is not an explicitly initialized element is initialized as follows:

• (5.1) If the element has a default member initializer ([class.mem]), the element is initialized from that initializer.
• (5.2) Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list ([dcl.init.list]).
• (5.3) Otherwise, the program is ill-formed.

If the aggregate is a union and the initializer list is empty, then

• (5.4) if any variant member has a default member initializer, that member is initialized from its default member initializer;
• (5.5) otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list.

因此

U u{};

是聚合初始化，结果是 union 类的第一个数据成员，即数据成员a类型 A (这是一个非 union 聚合类)，是从一个空的初始化列表中复制初始化的。作为单个数据成员 x类型 A有一个默认的成员初始值设定项，然后按照 [dcl.init.aggr]/5.1以上，数据成员x由其默认成员初始值设定项初始化。
因此，Clang 是正确的，而 GCC 是错误的。

GCC 错误报告

Bug 102013 - Incorrect aggregate initialization of union