15-6.md

#include <iostream>
using std::cin; using std::cout; using std::cerr; using std::endl;
using std::ostream;
#include <string>
using std::string;

class Quote{
public:
    Quote() = default;
    Quote(const string &book, double sales_price) :
        bookNo(book), price(sales_price) {};
    string isbn() const { return bookNo;}
    //返回给定数量的书籍的销售总额
    //派生类负责改写并使用不同的折扣计算算法
    virtual double net_price(size_t n) const { return n * price;};
    virtual ~Quote() = default;     //对析构函数进行动态绑定
private:
    string bookNo;      //书籍的ISBN编号
protected:
    double price = 0.0;     //代表普通状态下不打折的价格
};

//计算并打印销售给定数量的某种书籍所得的费用
double print_total(ostream &os, const Quote &item, size_t n){
    //根据传入item形参的对象类型调用Quote::net_price
    //或者Bulk_quote::net_price
    double ret = item.net_price(n);
    os << "ISBN: " << item.isbn()   //调用Quote::isbn()
       << " # sold: " << n << " total due: " << ret << endl;
    return ret;
}

class Bulk_quote : public Quote{    //Bulk_quote继承自Quote
public:
    Bulk_quote() = default;
    Bulk_quote(const string &book, double p, size_t qty, double disc) :
        Quote(book, p), min_qty(qty), discount(disc) {};
    //覆盖基类的函数版本以实现基于大量购买的折扣政策
    double net_price(size_t) const override;
private:
    size_t min_qty = 0;     //适用折扣政策的最低购买量
    double discount = 0.0;  //以小数表示的折扣额
};

//如果达到了购买书籍的某个最低限量值，就可以享受折扣价格了
double Bulk_quote::net_price(size_t cnt) const {
    if (cnt > min_qty)
        return cnt * (1 - discount) * price;
    else
        return cnt * price;
}

int main(){
    Quote basic("978-7-121-15535-2", 128.00);
    Bulk_quote bulk("978-7-121-15535-2", 128.00, 100, 0.3);

    print_total(cout, basic, 20);
    print_total(cout, bulk, 120);
}