C++11 中的延迟初始化顺序

Question

考虑以下代码，分为三个编译单元：

a.h:

struct A
{
    void Register(const char* s);

    const char* m_s[10];
    int m_i = 0;
};

A& GetA();

a.cpp:

#include "a.h"
#include <stdio.h>

void A::Register(const char* s)
{
    m_s[m_i++] = s;
}

A& GetA()
{
    static A instance;
    return instance;
}

int main(int argc, char* argv[])
{
    A& a = GetA();
    int n = a.m_i;
    for (int i = 0; i < n ; ++i)
        printf("%s\n", a.m_s[i]);
    return 0;
}

b.cpp:

#include "a.h"
struct B
{
    B() { GetA().Register("b"); }

    static B* instance;
};
B* B::instance = new B;

c.cpp:

#include "a.h"
struct C
{
    C() { GetA().Register("c"); }

    static C* instance;
};
C* C::instance = new C;

代码使用 gcc (-std=c++11) 构建并运行良好，产生输出：

c
b

现指cppreference.com:

Deferred dynamic initialization

It is implementation-defined whether dynamic initialization happens-before the first statement of the main function (for statics) or the initial function of the thread (for thread-locals), or deferred to happen after.

If the initialization of a non-inline variable is deferred to happen after the first statement of main/thread function, it happens before the first odr-use of any variable with static/thread storage duration defined in the same translation unit as the variable to be initialized. If no variable or function is odr-used from a given translation unit, the non-local variables defined in that translation unit may never be initialized (this models the behavior of an on-demand dynamic library). However, as long as anything from a TU is odr-used, all non-local variables whose initialization or destruction has side effects will be initialized even if they are not used in the program.

请注意 a.cpp 不知道 B 和 C 的存在，并且 B 和 C 与 [= 的唯一交互24=] 是 GetA() 和 A::Register() 在构造各自实例期间的调用。

据我所知，B 和 C 实例未使用 ODR，当然也不是来自 main() 的翻译单元。他们的初始化显然有副作用，但在我看来，不能保证这种初始化会在进入 main() 之前发生，或者在 main() 打印注册的字符串之前发生——或者根本不会发生。

所以 - 最后 - 我的问题是这个： B 和 C 实例是否在 main() 打印之前初始化注册的字符串不是符合标准，而是 gcc 的实现定义的行为？

如果按标准保证，怎么保证？

Answer 1

Is the fact that the B and C instances are initialised before main() prints the registered strings due not to the standard, but instead to gcc's implementation-defined behaviour?

它不是标准保证的。引用中最相关的部分：

If no variable or function is odr-used from a given translation unit, the non-local variables defined in that translation unit may never be initialized

由于 b.cpp 和 c.cpp 都没有使用函数或变量，它们的静态变量可能未初始化（关于动态初始化），因此它们的初始化会产生副作用可能不可见。

在实践中，我希望翻译单元静态链接时显示的初始化行为，以及动态加载（共享库）时可能出现的非初始化行为。但这两者都没有得到标准的保证，因为它没有指定共享库的行为方式。

C++11 中的延迟初始化顺序

Deferred initialisation order in C++11

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c++11

language-lawyer

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