C++关于Condition Variable

#include <condition_variable>
#include <mutex>
#include <future>
#include <iostream>
#include <thread>
#include <queue>
using namespace std;

queue<int> q;
mutex queueMutex;
condition_variable queueCondVar;

void provider(int val)
{
    for (int i = 0; i < 6; ++i)
    {
        {
            this_thread::sleep_for(chrono::milliseconds(1000));
            cout << "\nWait\n";
            this_thread::sleep_for(chrono::milliseconds(1000));
            lock_guard<mutex> lg(queueMutex);
            q.push(val + i);
        }
        queueCondVar.notify_one();
        //this_thread::sleep_for(chrono::milliseconds(val));
    }
}

void consumer(int num)
{
    while (true)
    {
        int val;
        {
            unique_lock<mutex> ul(queueMutex);
            queueCondVar.wait(ul, []{return !q.empty(); });
            val = q.front();
            q.pop();
        }
        cout << "consumer" << num << ":" << val << endl;
    }
}

int main()
{
    auto p1 = async(launch::async, provider, 100);
    auto p2 = async(launch::async, provider, 300);
    auto p3 = async(launch::async, provider, 500);

    auto c1 = async(launch::async, consumer, 1);    //this_thread::sleep_for(chrono::milliseconds(5000));
    auto c2 = async(launch::async, consumer, 2);

    //system("pause");
    p1.get();
    p2.get();
    p3.get();
    c1.get();
    c2.get();
    return 0;
}

在consumer中：我们要使用

unique_lock<mutex> ul(queueMutex);
            queueCondVar.wait(ul, []{return !q.empty(); });
因为等待的函数中有可能锁定和解锁mutex.
另外，condition variable也许有所谓的假醒，所以我们使用

[]{return !q.empty(); }来防止假醒对程序的影响。
注意，两个consumer的并发等待被通知次序是不确定的。
还要condition variable也提供给你一个接口允许你等待某个最大时间量：wait_for()用来等待一个时间段，wait_until()用来等待直到某个时间点。