Union 的默认构造函数被隐式删除

Question

以下代码：

struct non_trivially {
    non_trivially() {};
};

union U {
    bool dummy{false};
    non_trivially value;
};

int main() {
    U tmp;
}

https://godbolt.org/z/1cMsqq9ee

在 clang (13.0.0) 上产生下一个编译器错误：

source>:11:7: error: call to implicitly-deleted default constructor of 'U'
    U tmp;
      ^ <source>:7:19: note: default constructor of 'U' is implicitly deleted because variant field 'value' has a non-trivial default constructor
    non_trivially value;

但使用 MSVC (19.30) 成功编译。

根据 cppreference 它应该是一个有效的代码：https://en.cppreference.com/w/cpp/language/union

If a union contains a non-static data member with a non-trivial default constructor, the default constructor of the union is deleted by default unless a variant member of the union has a default member initializer .

在我的示例中，U 中有一个带有默认成员初始值设定项的替代方案，因此不应删除默认构造函数，但确实如此。我错过了什么？

Answer 1

这一切都有些神秘。 gcc 与 clang 的行为相同。

standard 有话要说（强调我的）：

Absent default member initializers, if any non-static data member of a union has a non-trivial default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, or destructor, the corresponding member function of the union must be user-provided or it will be implicitly deleted for the union.

但我觉得这里的措辞有点模糊。我认为他们的实际意思是您必须为具有（在您的示例中）非平凡构造函数的成员提供初始化程序，如下所示：

struct non_trivially { non_trivially() {}; }; union U { bool dummy; non_trivially value { }; // <-- note that I have added an initialiser }; int main() { U tmp; }

然后，it compiles.

Answer 2

这是CWG2084。如果您提供默认成员初始值设定项，Clang 和 gcc 在删除默认构造函数方面是错误的。

采纳后加入标准的相关引用（在[class.default.ctor]）：

A defaulted default constructor for class X is defined as deleted if:

X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,

[...]

X is a union and all of its variant members are of const-qualified type (or array thereof),

(仅有的两点引用联合类型，两者都不适用，所以不应该隐式删除)

解决方法是用户提供构造函数：

union U {
    constexpr U() : dummy{false} {}
    bool dummy;
    non_trivially value;
};

// Or this also seems to work
union U {
    constexpr U() {}
    bool dummy{false};
    non_trivially value;
};

Union 的默认构造函数被隐式删除

Union's default constructor is implicitly deleted

c++

language-lawyer