（原创）C++ 同步队列

同步队列作为一个线程安全的数据共享区，经常用于线程之间数据读取，比如半同步半异步线程池的同步队列。

其实做起来比较简单，要用到list、锁和条件变量，条件变量的作用是在队列满了或者空了的时候等待通知。先看一个简单的同步队列：

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

template<typename T>
class SimpleSyncQueue
{
public:
    void Put(const T& x)
    {
        std::lock_guard<std::mutex> locker(m_mutex);
        m_queue.push_back(x);
        m_notEmpty.notify_one();
    }

    void Take(T& x)
    {
        std::unique_lock<std::mutex> locker(m_mutex);
        m_notEmpty.wait(locker, [this]{return !m_queue.empty();});
        x = m_queue.front();
        m_queue.pop_front();
    }

private:
    std::list<T> m_queue;
    std::mutex m_mutex;
    std::condition_variable_any m_notEmpty;
};

再看一个带上限的同步队列：

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

template<typename T>
class SyncQueue
{
public:
    SyncQueue(int maxSize) :m_maxSize(maxSize), m_needStop(false)
    {
    }

    void Put(const T&x)
    {
        Add(x);
    }

    void Put(T&&x)
    {
        Add(std::forward<T>(x));
    }

    void Take(std::list<T>& list)
    {
        std::unique_lock<std::mutex> locker(m_mutex);
        m_notEmpty.wait(locker, [this]{return m_needStop || NotEmpty(); });
        
        if (m_needStop)
            return;
        list = std::move(m_queue);
        m_notFull.notify_one();
    }

    void Take(T& t)
    {
        std::unique_lock<std::mutex> locker(m_mutex);
        m_notEmpty.wait(locker, [this]{return m_needStop || NotEmpty(); });
        
        if (m_needStop)
            return;
        t = m_queue.front();
        m_queue.pop_front();
        m_notFull.notify_one();
    }

    void Stop()
    {
        {
            std::lock_guard<std::mutex> locker(m_mutex);
            m_needStop = true;
        }
        m_notFull.notify_all();
        m_notEmpty.notify_all();
    }

    bool Empty()
    {
        std::lock_guard<std::mutex> locker(m_mutex);
        return m_queue.empty();
    }

    bool Full()
    {
        std::lock_guard<std::mutex> locker(m_mutex);
        return m_queue.size() == m_maxSize;
    }

    size_t Size()
    {
        std::lock_guard<std::mutex> locker(m_mutex);
        return m_queue.size();
    }

    int Count()
    {
        return m_queue.size();
    }
private:
    bool NotFull() const
    {
        bool full = m_queue.size() >= m_maxSize;
        if (full)
            cout << "full, waiting，thread id: " << this_thread::get_id() << endl;
        return !full;
    }

    bool NotEmpty() const
    {
        bool empty = m_queue.empty();
        if (empty)
            cout << "empty,waiting，thread id: " << this_thread::get_id() << endl;
        return !empty;
    }

    template<typename F>
    void Add(F&&x)
    {
        std::unique_lock< std::mutex> locker(m_mutex);
        m_notFull.wait(locker, [this]{return m_needStop || NotFull(); });
        if (m_needStop)
            return;

        m_queue.push_back(std::forward<F>(x));
        m_notEmpty.notify_one();
    }

private:
    std::list<T> m_queue; //缓冲区
    std::mutex m_mutex; //互斥量和条件变量结合起来使用
    std::condition_variable m_notEmpty;//不为空的条件变量
    std::condition_variable m_notFull; //没有满的条件变量
    int m_maxSize; //同步队列最大的size

    bool m_needStop; //停止的标志
};

测试代码比较简单，就不写了。实际的运用在后面的线程池中会用到。