C++11 thread_local 用法

参考文献：
C++11 thread_local 用法 | 拾荒志
https://murphypei.github.io/blog/2020/02/thread-local

你可曾听过网络编程中应用线程本地存储？ - 知乎
https://zhuanlan.zhihu.com/p/103932878

C++11 thread_local用法 - 知乎
https://zhuanlan.zhihu.com/p/340201634

例子:
g++ testthread.cpp -lpthread

#include <iostream>
#include <thread>

void add(int n) {
    thread_local int count = 0;
    // static thread_local int count = 0; // 两种写法等价！
    count += n;
    // 休眠n秒，防止输出时数据交错（Mac会出现）
    std::this_thread::sleep_for(std::chrono::seconds(n));
    std::cout<<std::this_thread::get_id()<<":"<<count<<std::endl;
}

int main() {
    std::thread td[2];
    for (int i = 0; i < 2; i++) {
        td[i] = std::thread(add, i+1);
    }
    for (int i = 0; i < 2; i++) {
        td[i].join();
    }
    return 0;
}

#include <iostream>
#include <thread>
#include <mutex>
std::mutex cout_mutex;    //方便多线程打印

thread_local int x = 1;

void thread_func(const std::string& thread_name) {
    for (int i = 0; i < 3; ++i) {
        x++;
        std::lock_guard<std::mutex> lock(cout_mutex);
        std::cout << "thread[" << thread_name << "]: x = " << x << std::endl;
    }
    return;
}

int main() {
    std::thread t1(thread_func, "t1");
    std::thread t2(thread_func, "t2");
    t1.join();
    t2.join();
    return 0;
}

输出：

thread[t2]: x = 2
thread[t2]: x = 3
thread[t2]: x = 4
thread[t1]: x = 2
thread[t1]: x = 3
thread[t1]: x = 4
可以看到虽然是局部变量，但是在每个线程的每次 for 循环中，使用的都是线程中的同一个变量，也侧面印证了 thread_local 变量会自动 static。

如果我们不加 thread_local，输出如下：

thread[t2]: x = 2
thread[t2]: x = 2
thread[t2]: x = 2
thread[t1]: x = 2
thread[t1]: x = 2
thread[t1]: x = 2
体现了局部变量的特征。