非局部变量的动态初始化是线程安全的吗

Question

I have following code in one of the source file of my application:

// file1.cpp
#include <memory>

static auto global_variable = std::make_unique<int>(123);

int get_global_variable() { return *global_variable; }

Let assume that my application has some threads which call the get_global_variable . Is the initialization of the global_variable thread-safe?

As far as I know, the global_variable is dynamically initialized . I also know that the initialization of static local variables is thread-safe since C++11. So, I wonder to know if that exception proves the rule that the other types of variables are not thread-safe initialized or it is also thread-safe and does not produce data races .

I've found this answer , but after reading, I'm more confused because the answerer suggested using such pattern:

const T& f()
{
    static T t(a,b,c);
    return t;
}

which supposedly guarantees the thread-safe initialization.

I also found this answer . It states that all globals are initialized before main, so there is only one thread ( Peter rightly pointed out that it is not true - or not every time). However, what if my piece of code is a shared library loaded by dlopen function to a program where there is more than one thread?

Answer 1

[basic.start.dynamic]/4 It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized.

I believe this requires the implementation to perform initialization of global variables in a thread-safe manner (if it chooses to defer such initialization).