使用條件變數

條件變數是與互斥體結合使用的原語，用於協調執行緒之間的通訊。雖然它既不是完成此任務的唯一或最有效的方式，但對於熟悉該模式的人來說，它可能是最簡單的方法之一。

一個用 std::unique_lock<std::mutex> 等待 std::condition_variable。這允許程式碼在決定是否繼續獲取之前安全地檢查共享狀態。

下面是一個生產者 - 消費者草圖，它使用 std::thread，std::condition_variable，std::mutex 和其他一些來使事情變得有趣。

#include <condition_variable>
#include <cstddef>
#include <iostream>
#include <mutex>
#include <queue>
#include <random>
#include <thread>

int main()
{
    std::condition_variable cond;
    std::mutex mtx;
    std::queue<int> intq;
    bool stopped = false;

    std::thread producer{[&]()
    {
        // Prepare a random number generator.
        // Our producer will simply push random numbers to intq.
        //
        std::default_random_engine gen{};
        std::uniform_int_distribution<int> dist{};

        std::size_t count = 4006;    
        while(count--)
        {    
            // Always lock before changing
            // state guarded by a mutex and
            // condition_variable (a.k.a. "condvar").
            std::lock_guard<std::mutex> L{mtx};

            // Push a random int into the queue
            intq.push(dist(gen));

            // Tell the consumer it has an int
            cond.notify_one();
        }

        // All done.
        // Acquire the lock, set the stopped flag,
        // then inform the consumer.
        std::lock_guard<std::mutex> L{mtx};

        std::cout << "Producer is done!" << std::endl;

        stopped = true;
        cond.notify_one();
    }};

    std::thread consumer{[&]()
    {
        do{
            std::unique_lock<std::mutex> L{mtx};
            cond.wait(L,[&]()
            {
                // Acquire the lock only if
                // we've stopped or the queue
                // isn't empty
                return stopped || ! intq.empty();
            });

            // We own the mutex here; pop the queue
            // until it empties out.

            while( ! intq.empty())
            {
                const auto val = intq.front();
                intq.pop();

                std::cout << "Consumer popped: " << val << std::endl;
            }

            if(stopped){
                // producer has signaled a stop
                std::cout << "Consumer is done!" << std::endl;
                break;
            }

        }while(true);
    }};

    consumer.join();
    producer.join();
    
    std::cout << "Example Completed!" << std::endl;

    return 0;
}