使用 `{}` 在 C++ 中聚合初始化联合

Question

在下面的程序中，联合 U 有两个字段 a 和 b，每个字段都有不同的默认值。如果使用聚合初始化创建类型 U 的变量 {} 联合的值和活动成员是什么？

#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 而 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此处的行为未由标准定义？

Answer 1

Clang 正确，GCC 错误

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 classes，虽然之前是 non-union聚合 classes，前者不符合条件。

根据[dcl.init.aggr]/5[强调我的]：

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union的第一个数据成员class，即A类型的数据成员a（这是一个非联合聚合class), 是从一个空的初始化列表复制初始化的。由于类型 A 的单个数据成员 x 具有默认成员初始值设定项，因此根据上面的 [dcl.init.aggr]/5.1，数据成员 x 由其默认成员初始值设定项初始化。

因此，Clang是对的，GCC是错的。

GCC 错误报告

Bug 102013 - Incorrect aggregate initialization of union

使用 `{}` 在 C++ 中聚合初始化联合

Aggregate initialization of a union in C++ with `{}`

c++

unions

language-lawyer

aggregate-initialization

Clang 正确，GCC 错误

GCC 错误报告