说明了容器里使用继承关系的方法，这里再弄一个练习，巩固一下。

做一个类basket，它有个multiset成员，key是智能指针std::shared_ptr<quote>，由于key是自定义对象，所有必须给一个比较key的函数decltype(compare)*，关于自定义key的set，参考 。有个公有的additem成员方法，

quote3.h

#ifndef __quote3_h__ #define __quote3_h__  #include <iostream>  class quote{  public:   quote() = default;   quote(const std::string& book, double pri)     :bookno(book), price(pri){}   std::string isbn() const{return bookno;}   virtual double net_price(std::size_t n)const{     return n * price;   }   virtual void debug()const{     std::cout << bookno << " " << price << std::endl;   }   virtual ~quote() = default;  private:   std::string bookno;  protected:   double price = 0.0; };  class disc_quote : public quote{  public:   disc_quote() = default;   disc_quote(const std::string& book, double price,          std::size_t qyn, double disc):quote(book, price),     quantity(qyn), discount(disc){}      double net_price(std::size_t) const override = 0;  protected:   std::size_t quantity = 0;//折扣适用的数量   double discount = 0.0;   //折扣率 };  class bulk_quote : public disc_quote{  public:      bulk_quote() = default;      bulk_quote(const std::string& book, double price,   std::size_t qyn, double disc)   :disc_quote(book, price, qyn, disc){}      double net_price(std::size_t) const override; };  class min_quote : public disc_quote{  public:      min_quote() = default;   min_quote(const std::string& book, double price,        std::size_t qyn, double disc)    :disc_quote(book, price, qyn, disc){}      double net_price(std::size_t) const override; };  #endif 

quote3.cpp

#include "quote3.h"  double bulk_quote::net_price(std::size_t cnt) const{   if(cnt >= quantity){     return cnt * (1 - discount) * price;   }   else{     return cnt * price;   } }   double min_quote::net_price(std::size_t cnt) const{   if(cnt < quantity){     return cnt * (1 - discount) * price;   }   else{     return cnt * price;   } }  

basket.h

#ifndef __basket_h__ #define __basket_h__ #include "quote3.h" #include <set> #include <memory>   class basket{  public:   void add_item(const std::shared_ptr<quote>& sale){     items.insert(sale);   }   double total_receipt(std::ostream&) const;  private:   static bool compare(const std::shared_ptr<quote>& lhs,               const std::shared_ptr<quote>& rhs){     return lhs->isbn() < rhs->isbn();   }   std::multiset<std::shared_ptr<quote>, decltype(compare)*>     items{compare}; };  #endif 

basket.cpp

#include "basket.h"  double print_total(std::ostream& os,            const quote& item, size_t n); double basket::total_receipt(std::ostream& os) const{   double sum = 0.0;   for(auto iter = items.cbegin();       iter != items.cend();       iter = items.upper_bound(*iter)){     sum += print_total(os, **iter, items.count(*iter));   }   os << "total sale: " << sum << std::endl;   return sum; } 

main.cpp

#include "quote3.h" #include "basket.h" #include <vector> #include <iostream>  double print_total(std::ostream& os,            const quote& item, size_t n){   double ret = item.net_price(n);   os << "isbn: " << item.isbn()      << " # sold: " << n << " total due: " << ret << std::endl;   return ret;  }  int main(){   basket bsk;   //bsk.add_item(std::make_shared<quote>("01", 100));   bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.total_receipt(std::cout); } 

执行结果1：

isbn: 01 # sold: 3 total due: 270 total sale: 270

把main函数改成下面：

int main(){   basket bsk;   bsk.add_item(std::make_shared<quote>("01", 100));   //bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.add_item(std::make_shared<bulk_quote>("01", 100, 2, 0.1));   bsk.total_receipt(std::cout); }

执行结果2：

isbn: 01 # sold: 3 total due: 300 total sale: 300

代码有点长，主要的关注点：