cpp如何让主线程等待，直到其他线程在周期内完成

#include <iostream>
#include <thread>
#include <future>

void worker(std::promise<int> result_promise) {
    // 模拟耗时操作
    std::this_thread::sleep_for(std::chrono::seconds(2));
    result_promise.set_value(42); // 设置结果值
}

int main() {
    std::promise<int> result_promise;
    std::future<int> result_future = result_promise.get_future();

    std::thread t(worker, std::move(result_promise));

    // 主线程等待子线程完成
    int result = result_future.get(); // 阻塞直到结果可用
    std::cout << "Result: " << result << std::endl;

    t.join(); // 确保线程资源被正确释放
    return 0;
}

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cv;
bool ready = false;

void worker() {
    // 模拟耗时操作
    std::this_thread::sleep_for(std::chrono::seconds(2));
    {
        std::lock_guard<std::mutex> lock(mtx);
        ready = true;
    }
    cv.notify_one(); // 通知等待的线程
}

int main() {
    std::thread t(worker);

    {
        std::unique_lock<std::mutex> lock(mtx);
        cv.wait(lock, []{ return ready; }); // 阻塞直到条件满足
    }

    std::cout << "Worker thread has finished!" << std::endl;
    t.join();
    return 0;
}

#include <iostream>
#include <thread>
#include <atomic>

std::atomic<bool> done(false);

void worker() {
    // 模拟耗时操作
    std::this_thread::sleep_for(std::chrono::seconds(2));
    done = true;
}

int main() {
    std::thread t(worker);

    while (!done) {
        // 忙等待，实际应用中可能不是最佳选择
    }

    std::cout << "Worker thread has finished!" << std::endl;
    t.join();
    return 0;
}