新手请教虚函数的定义的问题，谢谢哈
我想定义一个纯虚类，它的子类都要自己实现add_up函数，add_up函数操作的参数类型是子类型本身，如下：
class DAT_BASE{public: long sum_num; virtual int add_up(DAT_BASE & v_a, DAT_BASE & v_b) = 0; virtual ~DAT_BASE(){};};class DAT_ITEM: public DAT_BASE{public: vector < double > dat; int add_up(DAT_ITEM & v_a, DAT_ITEM & v_b) {.....};};class DAT_RCD: public DAT_BASE{public: double dat; int add_up(DAT_RCD & v_a, DAT_RCD & v_b) {.....};};class DAT_BASE{public: long sum_num; virtual int add_up(int & v_a, int & v_b) = 0; virtual ~DAT_BASE(){};};class DAT_ITEM: public DAT_BASE{public: vector < double > dat; int add_up(int & v_a, int & v_b) {.....};};class DAT_RCD: public DAT_BASE{public: double dat; int add_up(int & v_a, int & v_b) {.....};};