C ++中引用成员默认初始化程序的生命周期是多少？

Question

请考虑这个简短的代码示例：

#include <iostream>

struct A
{
    A() { std::cout << "A() "; }
    ~A() { std::cout << "~A() "; }
};

struct B { const A &a = A(); };

int main()
{
    B x;
    std::cout << ". ";
    auto y = B();
    std::cout << ". ";
    auto z = B{};
    std::cout << ". ";
}

GCC 在此处打印 (https://gcc.godbolt.org/z/a83bn54qT):

A() ~A() . A() ~A() . A() . ~A()

表示A-对象初始化引用在x和y中的生命周期很短，但在z中生命周期延长到结束范围。

能否从 C++ 标准的角度解释一下为什么会这样？

Answer 1

无法保证生命周期延长，因为代码格式错误。

[class.base.init]/11:

A temporary expression bound to a reference member from a default member initializer is ill-formed. [Example 8:
struct A {
  A() = default;        // OK
  A(int v) : v(v) { }   // OK
  const int& v = 42;    // OK
};
A a1;                   // error: ill-formed binding of temporary to reference
A a2(1);                // OK, unfortunately
— end example]

C ++中引用成员默认初始化程序的生命周期是多少？

What is the lifetime of reference member default initializer in C++?

c++

default-value