15.7节练习

练习15.24 哪种类需要虚析构函数？虚析构函数必须执行什么样的操作？

有基类和派生类的继承体就要虚析构函数。

当指向基类的指针绑定到派生类上，要确保删除基类指针时将运行正确的析构函数版本，及使用被绑定的派生类中的析构函数。

练习15.25 我们为什么为Disc_quote定义了一个默认构造函数？如果去除掉该构造函数的话会对Bulk_quote的行为产生什么影响？

合成的Bulk_quote默认构造函数运行Disc_quote的默认构造函数，后者又运行Quote的默认构造函数。如果去除DIsc_quote的构造函数，那么程序将不能初始化qyt和discount成员。

练习15.26 定义Quote和Bulk_Quote的拷贝控制成员，令其与合成的版本行为一致。为这些成员以及其他的构造函数添加打印状态的语句，使得我们能够知道正在运行哪个程序。使用这些类编写程序，预测程序将创建和销毁哪些对象。重复实验，不断比较你的预测和实际输出结果是否相同。

#include <iostream>#include <string>using namespace std;class Quote {friend double print_total(ostream&, const Quote&, size_t);public:Quote() = default;Quote(const std::string &book, double sales_price) :bookNo(book), price(sales_price) {}std::string isbn() const { return bookNo; }virtual double net_price(std::size_t n) const { return n*price; }virtual void debug() const;Quote(const Quote&);Quote(Quote&&);Quote& operator=(const Quote&);Quote& operator=(Quote&&);virtual ~Quote();private:std::string bookNo;protected:double price = 0.0;};double print_total(ostream &os, const Quote &item, size_t n){double ret = item.net_price(n);os << "ISBN: " << item.isbn() << " # sold: " << n << " total due: " << ret << endl;return ret;}void Quote::debug()const{cout << "bookNo: " << bookNo << " price: " << price;}Quote::Quote(const Quote &rhs){bookNo = rhs.isbn();price = rhs.price;cout << "used Quote&" << endl;}Quote::Quote(Quote &&rhs){bookNo = rhs.isbn();price = rhs.price;cout << "used Quote&&" << endl;}Quote& Quote::operator=(const Quote &rhs){bookNo = rhs.isbn();price = rhs.price;cout << "used Quote operator= &" << endl;return *this;}Quote& Quote::operator=(Quote &&rhs){if (this != &rhs){bookNo = rhs.isbn();price = rhs.price;}cout << "used Quote operator &&" << endl;return *this;}Quote::~Quote(){bookNo.clear();cout << "~used Quote" << endl;}class Bulk_Quote :public Quote {public:Bulk_Quote() = default;Bulk_Quote(const std::string &book, double p, std::size_t qty, double disc) :Quote(book, p), min_qty(qty), discount(disc) {}double net_price(std::size_t)const override;void debug()const override;Bulk_Quote(const Bulk_Quote&);Bulk_Quote(Bulk_Quote &&);Bulk_Quote& operator=(const Bulk_Quote &);Bulk_Quote& operator=(Bulk_Quote &&);~Bulk_Quote();private:std::size_t min_qty = 0;double discount = 0.0;};double Bulk_Quote::net_price(std::size_t cnt) const{if (cnt >= min_qty) {return cnt*(1 - discount)*price;}else{return cnt*price;}}void Bulk_Quote::debug()const{cout << "min_pty: " << min_qty << " discount: " << discount;}Bulk_Quote::Bulk_Quote(const Bulk_Quote &rhs){min_qty = rhs.min_qty;discount = rhs.discount;cout << "used Bulk_Quote &" << endl;}Bulk_Quote::Bulk_Quote(Bulk_Quote &&rhs){min_qty = rhs.min_qty;discount = rhs.discount;cout << "used Bulk_Quote &&" << endl;}Bulk_Quote& Bulk_Quote::operator=(const Bulk_Quote &rhs){min_qty = rhs.min_qty;discount = rhs.discount;cout << "used Bulk_Quote operator= &" << endl;return *this;}Bulk_Quote& Bulk_Quote::operator=(Bulk_Quote &&rhs){if (this != &rhs) {min_qty = rhs.min_qty;discount = rhs.discount;}cout << "used Bulk_Quote operator= &&" << endl;return *this;}Bulk_Quote::~Bulk_Quote(){cout << "used ~Bulk_Quote" << endl;}int main(){//Quote q1("qqq", 3.5);//Quote q2 = q1;//Quote q3 = std::move(q1);Bulk_Quote b1("bbb", 3.5, 10, 2.4);Bulk_Quote b2 = b1;Bulk_Quote b3 = std::move(b1);}

练习15.27 重新定义你的Bulk_quote类，令其继承构造函数

Bulk_Quote(const std::string &book, double price, std::size_t qty, double disc) :Disc_quote(book, price, qty, disc) {}

或者

using Disc_quote::Disc_quote;