零初始化忽略构造函数

Question

以下this and discussion about zero-initialization, I would like to clarify in which situation these two paragraphs zero-initialization CPP reference同时发生：

As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of elements of aggregates for which no initializers are provided.

从这一段我了解到class成员必须没有构造函数。

If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.

但是从这个我了解到 constructor of class members, if any (他们不应该有），被忽略。

那么，在什么情况下我可以为 class 成员定义一个构造函数并且仍然 忽略它的构造函数 ？我对这两个 classes A 和 B 进行了实验，并构建了 A{} 调用 B()（显然）因为 c=1。只有 B() = default; 会使 c=0.

class B
{
public:
    B() : c{1} {}
    int c;
};

class A
{
public:
    A() = default;
    B b;
};

可能吗？提前谢谢你。

Answer 1

Then, in which case can I define a constructor for a class member and still ignore its constructor?

静态存储。具有静态存储持续时间的 class 实例将在静态初始化阶段初始化为零，这将忽略构造函数（如果有）。稍后在动态初始化阶段调用构造函数。

Could you give an example?

这是一个 class 具有静态存储持续时间的实例示例：

int main()
{
    static A a;
}

零初始化忽略构造函数

Zero initialization ignoring constructor

c++

constructor

initialization