條件變量 condition_variable wait

wait(阻塞當前線程，直到條件變量被喚醒)

#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()
{
    // 等待直至 main() 發送數據
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{return ready;});
 
    // 等待后，我們占有鎖。
    std::cout << "Worker thread is processing data\n";
    data += " after processing";
 
    // 發送數據回 main()
    processed = true;
    std::cout << "Worker thread signals data processing completed\n";
 
    // 通知前完成手動解鎖，以避免等待線程才被喚醒就阻塞（細節見 notify_one ）
    lk.unlock();
    cv.notify_one();
}
 
int main()
{
    std::thread worker(worker_thread);
 
    data = "Example data";
    // 發送數據到 worker 線程
    {
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        std::cout << "main() signals data ready for processing\n";
    }
    cv.notify_one();
 
    // 等候 worker
    {
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    std::cout << "Back in main(), data = " << data << '\n';
 
    worker.join();
}