c应该在锁定状态下通知条件变量

我在www.cppreference.com,http://en.cppreference.com/w/cpp/thread/condition_variable上找到了以下关于条件变量的示例.对cv.notify_one()的调用放在锁外.我的问题是,如果在保持锁定的同时进行调用以保证等待线程实际上处于等待状态并且将接收通知信号.

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

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);

    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();
}

应该在锁内移动notify_one()调用以保证等待线程接收通知信号,

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

解决方法

您无需在锁定下通知.但是,由于在实际值发生更改时逻辑上发生了通知(否则,为什么要通知？)并且该更改必须在锁定下进行,通常在锁定内完成.

没有实际可观察​​到的差异.