c++ - 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

现在，引用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.

注意 a.cpp 并不知道 B 和 C 的存在，B< 的唯一交互 & C 和 A 是 GetA() 和 A::Register() 的调用构建各自的实例。

据我所知，B 和 C 实例未使用 ODR，当然也不是来自 main()'翻译单位。他们的初始化显然有副作用，但在我看来，不能保证这个初始化会在进入 main() 之前发生，或者在 main() 打印注册的字符串 - 或者根本没有。

所以 - 最后 - 我的问题是:B 和 C 实例是在 main( ) 打印注册字符串不符合标准，而是 gcc 的实现定义行为？

如果是标准保证的，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?

标准不保证。报价中最相关的部分:

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

因为 b.cpp 和 c.cpp 都没有使用函数和变量，所以它们的静态变量可能未初始化(关于动态初始化)和因此它们初始化的副作用可能不可见。

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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。