c/c++ 继承与多态 容器与继承1

问题：类b公有继承类a，类a有虚函数fun，类b覆盖了虚函数fun，有一个std::vector<a>，添加a的对象a，和b的对象b，到这个容器里，然后从vector里取出来，使用对象a.fun()，和对象b.fun()，会发生什么？

发现对象b.fun()的调用，实际调用的是父类a的fun()方法。也就是子类部分被切掉了。

如何解决呢，用std::vector<a*>或者std::vector<std::shared_ptr<a>> ，但推荐用智能指针。

quote3.h

#ifndef __quote3_h__
#define __quote3_h__

#include <iostream>
#include <memory>

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;
  }
}

main.cpp

#include "quote3.h"
#include <vector>

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(){

  std::vector<quote> qv;
  for(unsigned i = 0; i != 10; ++i){
    qv.push_back(bulk_quote("01", 100, 10, 0.1));
  }
  double total = 0;
  for(const auto s : qv){
    total += s.net_price(20);
  }
  std::cout << total << std::endl;

  std::cout << "-------------------------" << std::endl;

  std::vector<std::shared_ptr<quote>> sv;
  for(unsigned i = 0; i != 10; ++i){
    sv.push_back(std::make_shared<bulk_quote>("01", 100, 10, 0.1));
  }
  double total1 = 0;
  for(const auto s : sv){
    total1 += s->net_price(20);
  }
  std::cout << total1 << std::endl;
  
}

执行结果：

20000
-------------------------
18000

std::vecto<quote>的执行结果为：20000；std::vector<std::shared_ptr<quote>>部分的执行结果：18000。所以在容器里放智能指针，可以解决上面的问题。

c/c++ 学习互助qq群：877684253

本人微信：xiaoshitou5854