std::unique_lock和std::condition_variable如何工作

我需要弄清楚锁和条件变量是如何工作的。

在此处稍微修改的代码中 cplusplusreference

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::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);
    std::this_thread::sleep_for(std::chrono::seconds(1));

    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();
}

我感到困惑的是,如果worker_thread已经锁定了互斥锁,那么主线程如何锁定互斥锁。

从…起 this answer 我看到这是因为 cv.wait 解锁 互斥锁。

但现在我对此感到困惑:如果 cv.wait公司 会解锁吗?

例如,我可以这样做吗?

std::unique_lock<std::mutex> lk(m, std::defer_lock);

因此,我创建锁定对象是因为cv需要它,但我在创建时不会锁定它。

现在有什么不同吗?

我不明白为什么会出现“运行时错误” here 那样的话