第十五章 15.4节练习

练习15.15

定义你自己的Disc_quote和Bulk_quote。

解答：

这里，就参考书中的实现吧。可以在这两个类中再添一些自己喜欢的内容。

后面的习题用书中的实现来做。

练习15.16

改写你在15.2.2节（第533页）练习中编写的数量受限的折扣策略，令其继承Disc_quote.

解答：

这里其实就是实现Bulk_quote类中的net_price函数。

#include <iostream>
#include <string>

using namespace std;

class Quote {
public:
  Quote() = default;
  Quote(const std::string &book, double sales_price):
        bookNo(book), price(sales_price) { }

  virtual ~Quote() = default; // dynamic binding for the destructor

  std::string isbn() const { return bookNo; }

  virtual double net_price(std::size_t n) const { return n * price; }
  virtual void debug();
private:
  std::string bookNo; // ISBN number of this item
protected:
  double price = 0.0; // normal, undiscounted price
};

void Quote::debug(){
  std::cout << "bookNo : " << bookNo << std::endl;
  std::cout << "price : " << price << std::endl;
}

class Disc_quote : public Quote{
public:
  Disc_quote() = default;
  Disc_quote(const string& book, double price, size_t qty, double disc):
    Quote(book, price), quantity(qty), discount(disc){}
  double net_price(size_t) const = 0;
protected:
  size_t quantity = 0;
  double discount = 0.0;
};

class Bulk_quote : public Disc_quote{
public:
  Bulk_quote() = default;
  Bulk_quote(const string& book, double price, size_t qty, double disc):
    Disc_quote(book, price, qty, disc){}
  double net_price(size_t) const override;
};

double Bulk_quote::net_price(size_t cnt) const{
  if (cnt > quantity){
    return quantity * discount * price + (cnt - quantity) * price;
  } else {
    return cnt * discount * price;
  }
}

double print_total(ostream &os, const Disc_quote &item, size_t n){
  double ret = item.net_price(n);
  os << "ISBN: " << item.isbn() << " # sold: " << n << " total due: " << ret << endl;
  return ret;
}

int main(){
  Bulk_quote bulk("book", 20, 10, 0.5);
  print_total(std::cout, bulk, 12);
  print_total(std::cout, bulk, 10);
  print_total(std::cout, bulk, 2);
}

练习15.17

尝试顶一个Dis_quote的对象，看看编译器给出的错误信息是什么？

解答：

这是g++4.9.1给出的错误提示

test.cpp: In function ‘int main()’:
test.cpp:70:14: error: cannot declare variable ‘test’ to be of abstract type ‘Disc_quote’
   Disc_quote test;
              ^
test.cpp:30:7: note:   because the following virtual functions are pure within ‘Disc_quote’:
 class Disc_quote : public Quote{
       ^
test.cpp:35:10: note: 	virtual double Disc_quote::net_price(size_t) const
   double net_price(size_t) const = 0;
          ^

VS2013

1>          “double Disc_quote::net_price(size_t) const”: 是抽象的

clang 3.4

test.cpp:65:16: error: variable type 'Disc_quote' is an abstract class
    Disc_quote test;
               ^
test.cpp:35:16: note: unimplemented pure virtual method 'net_price' in
      'Disc_quote'
        double net_price(size_t) const = 0;
               ^
1 error generated.