c++ - 为什么 lock_guard 可以通过 unique_lock 得到一个已经锁定的互斥体？ - 还有问题

Question

我在学习this example .我找到了 this question并认为我会得到答案，但我仍然有一个问题。

为了方便起见，我把代码贴在这里:

std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;

void worker_thread()
{
    // Wait until main() sends data
    std::cout << "------------------------\n";
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{return ready;});

    // after the wait, we own the lock.
    std::cout << "Worker thread is processing data\n";
    data += " after processing";

    // Send data back to main()
    processed = true;
    std::cout << "Worker thread signals data processing completed\n";

    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    cv.notify_one();
}

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

    data = "Example data";
    // send data to the worker thread
    {
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        std::cout << "main() signals data ready for processing\n";
    }
    cv.notify_one();

    // wait for the worker
    {
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    std::cout << "Back in main(), data = " << data << '\n';

    worker.join();

    return 0;
}

不应该声明std::unique_lock<std::mutex> lk(m);阻塞主线程，因为 mutex m被 worker_thread 锁定？如果是，声明不是 cv.wait(lk, []{return processed;});在这个例子中不需要之后？当主线程可以锁定互斥锁时，processed将已经是真的。

Answer 1

调用 wait 会在等待期间解锁互斥量。参见 http://en.cppreference.com/w/cpp/thread/condition_variable/wait .

编辑:在您链接到的问题的答案中明确说明:https://stackoverflow.com/a/32030975/212870

编辑 2:“当主线程可以锁定互斥锁时，processed 已经为真”是不正确的。工作线程可能甚至还没有启动，或者即使已经启动它也可能没有看到 ready 已设置。