线程局部全局作用域变量

Question

Consider the following code:

#include <iostream>
#include <thread>

using std::cout;
using std::thread;

thread_local int a;

void foo()
{
    a = a + 1;
    cout  << a << "\n";
}

void bar()
{
    cout << a << "\n";
}

void baz()
{
    cout << "Something\n";
}

int main()
{
    thread first(foo);
    thread second(bar);
    thread third(baz);
    second.join();
    first.join();
    third.join();
    cout << a;
}

Demo

Since a has thread storage duration, we have at least three different objects, denoted by a and used in first , second and main threads. We don't use a inside third . Is there zero-initialized a which can be used in third? I ask this question because I can't find anything about this in the Standard:

Non-local variables with thread storage duration are initialized as a consequence of thread execution.

Is it implementation defined?

Answer 1

N3337, 3.7.2.2

A variable with thread storage duration shall be initialized before its first odr-use (3.2) and, if constructed, shall be destroyed on thread exit.

3.2.2 is too long to copy it here, but essentially, it can´t be "odr-used" if it doesn´t appear in the executed code. So it is not (necessarily) initialized with something.