class readwrite_lock

{

public:

    readwrite_lock()

        : read_cnt(0)

    {

    }  

    void readLock()

    {

        read_mtx.lock();

        if (++read_cnt == 1)

            write_mtx.lock();  

        read_mtx.unlock();

    }  

    void readUnlock()

    {

        read_mtx.lock();

        if (--read_cnt == 0)

            write_mtx.unlock();  

        read_mtx.unlock();

    }  

    void writeLock()

    {

        write_mtx.lock();

    }  

    void writeUnlock()

    {

        write_mtx.unlock();

    }  

private:

    mutex read_mtx;

    mutex write_mtx;

    int read_cnt; // 已加读锁个数

};