将std::jthread作为方法内局部变量时，线程回调为何不执行？

核心问题：start()函数里创建的std::jthread thr_A是局部变量，函数执行完毕后该对象会立即被销毁。std::jthread的析构逻辑是自动向关联线程发送停止请求，并等待线程退出，导致你的update函数还没来得及执行就被终止了，自然看不到预期输出。

修复方案：把thr_A改为类A的成员变量，让它的生命周期和类实例绑定，只要主程序里的a对象存在，线程就不会被提前销毁。

修改后的代码：

// includes and directives
#include <iostream>
#include <thread>
#include <functional>
#include <chrono>

// a class for testing stuff
class A {
private:
    int updates{ 0 }; // something to count on
    int tick{ 1000 }; // tick time set to 1000ms initially
    std::jthread thr_A; // 将线程改为类成员变量

public:
    void set_tick(int t) {
        tick = (t<1?1:t); // sanity check and set tick time
    }

    int get_updates() {
        return updates; // I did how many?
    }

    void update(std::stop_token st) {
        while (!st.stop_requested()) { // stop if they say stop
            updates++; // count this as an update
            std::cout << "Performing update " << updates << "\n"; // visually indicate something happened
            std::this_thread::sleep_for(std::chrono::milliseconds(tick)); // wait for tick time before doing it all again
        }
    }

    void start() { // start a ticking function
        std::cout << "Starting update thread\n"; // tell someone responsible that it's started
        // 给成员变量赋值，初始化线程
        thr_A = std::jthread(std::bind_front(&A::update, this));
    }
};

// main thing
int main()
{
    std::cout << "Start main thread\n"; // say that I'm starting up 
    A a; // make me a sandwich
    a.set_tick(10); // really fast(ish)
    a.start(); // go on, get on with it
    std::this_thread::sleep_for(std::chrono::milliseconds(10000)); // wait until sandwich ready
    std::cout << "Wake main thread and terminate\n"; // sandwich is ready, time to eat
}

内容的提问来源于stack exchange，提问作者Radiobiscuit