我在www.example.com上找到了条件变量http://en.cppreference.com/w/cpp/thread/condition_variable的以下示例www.cppreference.com。对cv.notify_one（）的调用被放置在锁之外。我的问题是，是否应该在持有锁的同时进行调用，以确保等待线程实际上处于等待状态，并将接收通知信号。

#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();
}

如果notify_one（）调用被移动到锁内部以保证等待线程接收到通知信号，

// 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";
}