get_driver_instance()
不管从何处调用都是同一个实例
在构造函数里最终调用mysql_library_init(argc, argv, groups);析构函数里最终调用:mysql_library_end()
sql::Connection
在初始化sql::Connection对象时调用MySQL_Connection(this, proxy->conn_init(),hostName, userName, password)->new MySQL_NativeConnectionWrapper(api)->api->init()->::mysql_init(mysql);