C++多线程基础-condition_variable

condition_variable是同步原语，被使用在std::mutex去阻塞块在不同线程，直到线程修改共享变量并且唤醒条件变量；

线程尝试修改共享变量必须：

1、获得mutex；例如std::lock_guard

2、获得锁后修改共享变量；（即使共享变量是原子量，也要获得锁才能修改）

3、接着调用notify_one或者notify_all；

线程等等待条件变量必须：

1、获得std::unique_lock

2、做其中一种：检查条件；调用wait，wait_for，wait_until；检查条件和如果没有唤醒则重新等待，

实现简单的条件变量：

#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();
}

参考：

std::condition_variable - cppreference.com