class Item_base{public:Item_base(const std::string &book = " ",double sales_price =0.0):isbn(book),price(sales_price) {}std::string book() const{return isbn;}virtual double net_price(size_t n) const{return n*price;}virtual Item_base* clone() const{return new Item_base(*this);}virtual ~Item_base() {}private:std::string isbn;protected:double price;};//保存折扣率和可實(shí)行折扣策略的數(shù)量，派生類將使用這些數(shù)據(jù)實(shí)現(xiàn)定價(jià)策略class Disc_item : public Item_base{public:Disc_item(const std::string& book=" ",double sales_price =0.0,size_t qty = 0,double disc_rate = 0.0):Item_base(book,sales_price),quantity(qty),discount(disc_rate) {}double net_price(size_t) const = 0;std::pair<size_t,double> discount_policy() const{return std::make_pair(quantity,discount);}protected:size_t quantity; //可實(shí)行折扣策略的購買量double discount; //折扣率};//批量購買折扣類class Bulk_item : public Disc_item{public:Bulk_item(const std::string& book=" ",double sales_price = 0.0,size_t qty = 0 ,double disc_rate = 0.0):Disc_item(book,sales_price,qty,disc_rate) {}double net_price(size_t cnt) const{if (cnt >= quantity)return cnt*(1-discount)*price;else return cnt*price;}Bulk_item* clone() const{return new Bulk_item(*this);}};//有限折扣率class Lds_item : public Disc_item{public:Lds_item(const std::string& book = " ",double sales_price = 0.0,size_t qty = 0,double disc_rate = 0.0) : Disc_item(book,sales_price,qty,disc_rate) {}double net_price(size_t cnt) const{if (cnt <= quantity)return cnt*(1-discount)*price;else return cnt*price - quantity*discount*price;}Lds_item* clone() const{return new Lds_item(*this);}};然而在VC++ 6.0平臺上編譯時(shí)，提示如下錯(cuò)誤：（1）error C2555: 'Bulk_item::clone' : overriding virtual function differs from 'Item_base::clone' only by return type or calling convention；（2）error C2555: 'Lds_item::clone' : overriding virtual function differs from 'Item_base::clone' only by return type or calling convention