c++利用互斥锁实现读写锁

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  
};