单例模式

class QDH_MYSQL
{
public:
    static QDH_MYSQL* instance()
    {
        if (_instance == NULL)
            _instance = new QDH_MYSQL();
        return _instance;
    }
    static void destroy()
    {
        if (_instance)
        {
            delete _instance;
            _instance = NULL;
        }
    }
    
private:
    QDH_MYSQL();
    ~QDH_MYSQL();
    static QDH_MYSQL* _instance;
};