c++ - thread_local 对象的销毁

Question

在问题Using QSqlQuery from multiple threads结果是线程存储解决了这个问题。

我制作了一个简单的演示代码，以绝对清楚 C++11 thread_local 说明符。下面的代码创建了两个线程，它们将 ThreadLocal 对象作为本地唯一对象。 Storage::get 函数是线程特定的单例。标准是否保证在加入或退出线程函数时调用 ThreadLocal 析构函数？

使用 GCC 5.4.0 编译 (g++ -o main main.cpp --std=c++11 -lpthread)

#include <thread>
#include <mutex>
#include <string>
#include <chrono>
#include <iostream>
#include <atomic>

static std::mutex mtx;

struct ThreadLocal {
    std::string name;

    ~ThreadLocal() {
        mtx.lock();
        std::cout << "destroy " << name << std::endl;
        mtx.unlock();
    }
};

struct Storage {
    static ThreadLocal &get() {
        /* Thread local singleton */
        static thread_local ThreadLocal l;
        static std::atomic<int> cnt(0);
        l.name = std::to_string(cnt);
        cnt++;
        return l;
    }
};

void thread() {
    mtx.lock();
    std::cout << Storage::get().name << std::endl;
    mtx.unlock();
    std::this_thread::sleep_for(std::chrono::seconds(1));
}

int main(int argc, const char **argv) {
    std::thread t1(&thread);
    std::thread t2(&thread);
    t1.join();
    t2.join();
}

Answer 1

如果对象被构建，它将在线程函数退出时销毁。几乎在 [basic.stc.thread]/2 上的那些确切的话:

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