使用默认默认构造函数的 C++11 值初始化

Question

在下面的例子中:

#include <iostream>

struct A {
    int z;
    A(std::string) {}
    A() = default;
};

int main() {
    char buf[1000];
    std::fill(buf, buf + 1000, 'x');
    auto a = new(buf) A{};
    std::cerr << "A.z: " << a->z << std::endl;
}

使用 GCC 4.8 编译 outputs zero (与 Clang 3.4 的行为相同)。这似乎表明 a 在调用默认构造函数之前被零初始化。

但根据value-initialization rules on cppreference.com ，对象不应在默认构造函数调用之前初始化。 A 类符合 C++11 下的要点 #1:

1) If T is a class type with at least one user-provided constructor of any kind, the default constructor is called.

另一个可能有用的数据点是，如果我们在上面的示例中将 A() = default 替换为 A() {}，no zero-initialization happens正如预期的那样。这似乎表明在初始示例中遵循了值初始化的要点#2，这是错误的:

2) If T is an non-union class type without any user-provided constructors, then the object is zero-initialized and then the implicitly-declared default constructor is called (unless it's trivial)

Answer 1

您在 C++11 中为值初始化引用的措辞被认为是有缺陷的，请参阅 Core Working Group DR 1368和 Core Working Group DR 1301 中的决议改变了措辞(用粗体显示添加，用删除线显示删除):

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 9 [class]) with either no default constructor (12.1 [class.ctor]) or a default constructor that is user-provided or deleted constructor (12.1 [class.ctor]), then the object is default-initialized default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if T's implicitly-declared default constructor is T has a non-trivial default constructor, that constructor is called. default-initialized;

一些编译器——我相信 GCC 4.8 和自 3.3ish 以来的 clang——已经实现了 DR 1301 的决议。