实验5 多态

实验任务一：

源码：

#include <iostream>
#include <string>
#include "publisher.hpp"

// Publisher类：实现
Publisher::Publisher(const std::string &name_): name {name_} {
}


// Book类: 实现
Book::Book(const std::string &name_ , const std::string &author_ ): Publisher{name_}, author{author_} {
}

void Book::publish() const {
    std::cout << "Publishing book《" << name << "》 by " << author << '\n';
}

void Book::use() const {
    std::cout << "Reading book 《" << name << "》 by " << author << '\n';
}


// Film类：实现
Film::Film(const std::string &name_, const std::string &director_):Publisher{name_},director{director_} {
}

void Film::publish() const {
    std::cout << "Publishing film <" << name << "> directed by " << director << '\n';
}

void Film::use() const {
    std::cout << "Watching film <" << name << "> directed by " << director << '\n';
}


// Music类：实现
Music::Music(const std::string &name_, const std::string &artist_): Publisher{name_}, artist{artist_} {
}

void Music::publish() const {
    std::cout << "Publishing music <" << name << "> by " << artist << '\n';
}

void Music::use() const {
    std::cout << "Listening to music <" << name << "> by " << artist << '\n';
}

#pragma once

#include <string>

// 发行/出版物类：Publisher （抽象类）
class Publisher {
public:
    Publisher(const std::string &name_ = "");            // 构造函数
    virtual ~Publisher() = default;

public:
    virtual void publish() const = 0;                 // 纯虚函数，作为接口继承
    virtual void use() const = 0;                     // 纯虚函数，作为接口继承

protected:
    std::string name;    // 发行/出版物名称
};

// 图书类: Book
class Book: public Publisher {
public:
    Book(const std::string &name_ = "", const std::string &author_ = "");  // 构造函数

public:
    void publish() const override;        // 接口
    void use() const override;            // 接口

private:
    std::string author;          // 作者
};

// 电影类: Film
class Film: public Publisher {
public:
    Film(const std::string &name_ = "", const std::string &director_ = "");   // 构造函数

public:
    void publish() const override;    // 接口
    void use() const override;        // 接口            

private:
    std::string director;        // 导演
};


// 音乐类：Music
class Music: public Publisher {
public:
    Music(const std::string &name_ = "", const std::string &artist_ = "");

public:
    void publish() const override;        // 接口
    void use() const override;            // 接口

private:
    std::string artist;      // 音乐艺术家名称
};

#include <memory>
#include <iostream>
#include <vector>
#include "publisher.hpp"

void test1() {
   std::vector<Publisher *> v;

   v.push_back(new Book("Harry Potter", "J.K. Rowling"));
   v.push_back(new Film("The Godfather", "Francis Ford Coppola"));
   v.push_back(new Music("Blowing in the wind", "Bob Dylan"));

   for(Publisher *ptr: v) {
        ptr->publish();
        ptr->use();
        std::cout << '\n';
        delete ptr;
   }
}

void test2() {
    std::vector<std::unique_ptr<Publisher>> v;

    v.push_back(std::make_unique<Book>("Harry Potter", "J.K. Rowling"));
    v.push_back(std::make_unique<Film>("The Godfather", "Francis Ford Coppola"));
    v.push_back(std::make_unique<Music>("Blowing in the wind", "Bob Dylan"));

    for(const auto &ptr: v) {
        ptr->publish();
        ptr->use();
        std::cout << '\n';
    }
}

void test3() {
    Book book("A Philosophy of Software Design", "John Ousterhout");
    book.publish();
    book.use();
}

int main() {
    std::cout << "运行时多态：纯虚函数、抽象类\n";

    std::cout << "\n测试1: 使用原始指针\n";
    test1();

    std::cout << "\n测试2: 使用智能指针\n";
    test2();

    std::cout << "\n测试3: 直接使用类\n";
    test3();
}

运行结果测试截图：

 回答问题：

问题1：抽象类机制

（1）是什么决定了 Publisher 是抽象类？用一句话说明，并指出代码中的具体依据。

答：vitual声明Publisher中成员函数为纯虚函数。

具体依据：

virtual void publish() const = 0;
virtual void use() const = 0;

（2）如果在 main.cpp 里直接写 Publisher p; 能否编译通过？为什么？

答：不能。因为Publisher的成员函数publish()、use()是纯虚函数，让类无法实例化。纯虚函数没有实现，如果创建对象，调用这些函数时没有可执行的代码。所以必须通过继承并实现所有纯虚函数后，才能创建派生类的对象。

问题2：纯虚函数与接口继承

（1） Book 、 Film 、 Music 必须实现哪两个函数才能通过编译？请写出其完整函数声明。

答：必须实现Publish() const、use() const。

public:
    void publish() const override;        // 接口
    void use() const override;            // 接口

（2） 在 publisher.cpp 的 Film 类实现中，把两个成员函数实现里的 const 去掉（保持函数体不变），重新编译，报错信息是什么？

答：函数声明、实现不匹配，声明中有const而实现中没有const。

问题3：运行时多态与虚析构

（1）在 test1() 里， for (Publisher *ptr : v) 中，ptr 的声明类型是什么？

答：Publisher *

（2）当循环执行到 ptr->publish(); 时 ptr 实际指向的对象类型分别有哪些？（按循环顺序写出）

答：Book、Film、Music

（3）基类 Publisher 的析构函数为何声明为virtual ？若删除 virtual ，执行 delete ptr; 会出现什么问题？

答：基类的析构函数声明为virtual是为了实现多态析构。因为ptr是指向派生类对象的基类指针，当基类的析构函数为虚函数时，编译器会先调用派生类对象的析构函数再调用基类析构函数，以确保派生类对象资源被正确释放。若删除virtual执行delete ptr，只调用基类析构函数不会调用派生类析构函数，派生类对象资源不能够被正确释放，造成内存泄漏。

实验任务二：

源码：

#pragma once
#include <string>

// 图书描述信息类Book: 声明
class Book {
public:
    Book(const std::string &name_, 
         const std::string &author_, 
         const std::string &translator_, 
         const std::string &isbn_, 
         double price_);

    friend std::ostream& operator<<(std::ostream &out, const Book &book);

private:
    std::string name;        // 书名
    std::string author;      // 作者
    std::string translator;  // 译者
    std::string isbn;        // isbn号
    double price;        // 定价
};

#include <iomanip>
#include <iostream>
#include <string>
#include "book.hpp"


// 图书描述信息类Book: 实现
Book::Book(const std::string &name_, 
          const std::string &author_, 
          const std::string &translator_, 
          const std::string &isbn_, 
          double price_):name{name_}, author{author_}, translator{translator_}, isbn{isbn_}, price{price_} {
}

// 运算符<<重载实现
std::ostream& operator<<(std::ostream &out, const Book &book) {
    using std::left;
    using std::setw;
    
    out << left;
    out << setw(15) << "书名:" << book.name << '\n'
        << setw(15) << "作者:" << book.author << '\n'
        << setw(15) << "译者:" << book.translator << '\n'
        << setw(15) << "ISBN:" << book.isbn << '\n'
        << setw(15) << "定价:" << book.price;

    return out;
}

#pragma once

#include <string>
#include "book.hpp"

// 图书销售记录类BookSales：声明
class BookSale {
public:
    BookSale(const Book &rb_, double sales_price_, int sales_amount_);
    int get_amount() const;   // 返回销售数量
    double get_revenue() const;   // 返回营收
    
    friend std::ostream& operator<<(std::ostream &out, const BookSale &item);

private:
    Book rb;         
    double sales_price;      // 售价
    int sales_amount;       // 销售数量
};

#include <iomanip>
#include <iostream>
#include <string>
#include "booksale.hpp"

// 图书销售记录类BookSales：实现
BookSale::BookSale(const Book &rb_, 
                   double sales_price_, 
                   int sales_amount_): rb{rb_}, sales_price{sales_price_}, sales_amount{sales_amount_} {
}

int BookSale::get_amount() const {
    return sales_amount;
}

double BookSale::get_revenue() const {
    return sales_amount * sales_price;
}

// 运算符<<重载实现
std::ostream& operator<<(std::ostream &out, const BookSale &item) {
    using std::left;
    using std::setw;
    
    out << left;
    out << item.rb << '\n'
        << setw(15) << "售价:" << item.sales_price << '\n'
        << setw(15) << "销售数量:" << item.sales_amount << '\n'
        << setw(15) << "营收:" << item.get_revenue();

    return out;
}

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <string>
#include <vector>
#include "booksale.hpp"

// 按图书销售数量比较
bool compare_by_amount(const BookSale &x1, const BookSale &x2) {
    return x1.get_amount() > x2.get_amount();
}

void test() {
    using std::cin;
    using std::cout;
    using std::getline;
    using std::sort;
    using std::string;
    using std::vector;
    using std::ws;

    vector<BookSale> sales_records;         // 图书销售记录表

    int books_number;
    cout << "录入图书数量: ";
    cin >> books_number;

    cout << "录入图书销售记录\n";
    for(int i = 0; i < books_number; ++i) {
        string name, author, translator, isbn;
        double price;
        cout << string(20, '-') << "第" << i+1 << "本图书信息录入" << string(20, '-') << '\n';
        cout << "录入书名: "; getline(cin>>ws, name);
        cout << "录入作者: "; getline(cin>>ws, author);
        cout << "录入译者: "; getline(cin>>ws, translator);
        cout << "录入isbn: "; getline(cin>>ws, isbn);
        cout << "录入定价: "; cin >> price;

        Book book(name, author, translator, isbn, price);

        double sales_price;
        int sales_amount;

        cout << "录入售价: "; cin >> sales_price;
        cout << "录入销售数量: "; cin >> sales_amount;

        BookSale record(book, sales_price, sales_amount);
        sales_records.push_back(record);
    }

    // 按销售册数排序
    sort(sales_records.begin(), sales_records.end(), compare_by_amount);

    // 按销售册数降序输出图书销售信息
    cout << string(20, '=') <<  "图书销售统计" << string(20, '=') << '\n';
    for(auto &record: sales_records) {
        cout << record << '\n';
        cout << string(40, '-') << '\n';
    }
}

int main() {
    test();
}

运行结果测试截图：

回答问题：

问题1：重载运算符<<

（1）找出<<被重载了几处？用于哪些类型？

答：两处。用于Book和Booksale类型。

friend std::ostream& operator<<(std::ostream &out, const Book &book);

friend std::ostream& operator<<(std::ostream &out, const BookSale &item);

（2）找出使用重载<<输出对象的代码，写在下面。

task2.cpp中：输出BookSale类对象 record

for(auto &record: sales_records) {
        cout << record << '\n';
        cout << string(40, '-') << '\n';
    }

Booksale.cpp中：输出Book类对象rb

out << item.rb << '\n'
        << setw(15) << "售价:" << item.sales_price << '\n'
        << setw(15) << "销售数量:" << item.sales_amount << '\n'
        << setw(15) << "营收:" << item.get_revenue();

问题2：图书销售统计

（1）"按销售数量降序排序"，描述降序排序实现方式。

答：实现方式是先排序，再按序遍历输出。先定义自定义比较函数compare_by_amount，表示amount较大优先，降序逻辑。然后使用std::sort函数，并使用使用vector中的begin()、end()迭代器对vector进行排序。

// 按图书销售数量比较
bool compare_by_amount(const BookSale &x1, const BookSale &x2) {
    return x1.get_amount() > x2.get_amount();
}

// 按销售册数排序
    sort(sales_records.begin(), sales_records.end(), compare_by_amount);

// 按销售册数降序输出图书销售信息
    cout << string(20, '=') <<  "图书销售统计" << string(20, '=') << '\n';
    for(auto &record: sales_records) {
        cout << record << '\n';
        cout << string(40, '-') << '\n';
    }

（2）拓展（选答*）：如果使用lambda表达式，如何实现？

答：lambda表达式的基本语法为[capture外部变量] (parameters参数列表) {函数体}，相当于通过lambda表达式内嵌降序排序函数逻辑。

// 使用lambda表达式替代compare_by_amount函数
std::sort(sales_records.begin(), sales_records.end(),
    [](const BookSale &x1, const BookSale &x2) {
        return x1.get_amount() > x2.get_amount();// 降序排序
    }
);

实验任务三：

（无需写入实验博客文档）

实验任务四：

源码：

#pragma once

#include<string>
#include<iostream>

//抽象类定义
class MachinePet{
    public:
        MachinePet(const std::string &name);
        virtual ~MachinePet() = default;
    
    public:
        std::string get_nickname() const;
        virtual std::string talk() const = 0;//纯虚函数，多态返回叫声

    private:
        std::string nickname;
};

//电子宠物猫类
class PetCat:public MachinePet{
    public:
        PetCat(const std::string &name);
    
    public:
        std::string talk() const override;//多态返回叫声
};

//电子宠物狗类
class PetDog:public MachinePet{
public:
        PetDog(const std::string &name);
    
    public:
        std::string talk() const override;//多态返回叫声
};

#include<iostream>
#include<string>
#include"pet.hpp"

//MachinePet类实现
MachinePet::MachinePet(const std::string &name):nickname{name}{
}

std::string MachinePet::get_nickname() const{
    return nickname;
}

//PetCat类实现
PetCat::PetCat(const std::string &cat_name):MachinePet(cat_name){
}

std::string PetCat::talk() const {
    return "miao wu~";
}

//PetDog类实现
PetDog::PetDog(const std::string &dog_name):MachinePet(dog_name){
}

std::string PetDog::talk() const {
    return "wang wang~";
}

#include <iostream>
#include <memory>
#include <vector>
#include "pet.hpp"

void test1() {
    std::vector<MachinePet *> pets;

    pets.push_back(new PetCat("miku"));
    pets.push_back(new PetDog("da huang"));

    for(MachinePet *ptr: pets) {
        std::cout << ptr->get_nickname() << " says " << ptr->talk() << '\n';
        delete ptr;  // 须手动释放资源
    }   
}

void test2() {
    std::vector<std::unique_ptr<MachinePet>> pets;

    pets.push_back(std::make_unique<PetCat>("miku"));
    pets.push_back(std::make_unique<PetDog>("da huang"));

    for(auto const &ptr: pets)
        std::cout << ptr->get_nickname() << " says " << ptr->talk() << '\n';
}

void test3() {
    // MachinePet pet("little cutie");   // 编译报错：无法定义抽象类对象

    const PetCat cat("miku");
    std::cout << cat.get_nickname() << " says " << cat.talk() << '\n';

    const PetDog dog("da huang");
    std::cout << dog.get_nickname() << " says " << dog.talk() << '\n';
}

int main() {
    std::cout << "测试1: 使用原始指针\n";
    test1();

    std::cout << "\n测试2: 使用智能指针\n";
    test2();

    std::cout << "\n测试3: 直接使用类\n";
    test3();
}

运行结果测试截图：

实验任务五：

源码：

#pragma once

#include<iostream>
#include<string>
#include<algorithm>

//定义类模板
template<typename T>
class Complex{
    public:
        Complex();
        Complex(T x,T y);
        Complex(const Complex&other);
        T get_real() const;
        T get_imag() const;

        //成员函数:
        Complex& operator+=(const Complex& other){
            real = real + other.real;
            imag = imag + other.imag;
            return *this;
        }

        bool operator==(const Complex& other) const{
            return (real==other.real&&imag==other.real);
        }

        //非成员函数:
        friend Complex<T> operator+(const Complex<T>& a1,const Complex<T>& a2){
            return Complex<T>(a1.real+a2.real,a1.imag+a2.imag);
        }

        friend std::ostream& operator<<(std::ostream& out,const Complex<T>& complex){
            if(complex.imag>0)
                out << complex.real << " + " << abs(complex.imag) << "i";
            else if(complex.imag<0)
                out << complex.real << " - " << abs(complex.imag) << "i";
            else
                out << " 0 ";
            return out;
        }

        friend std::istream& operator>>(std::istream& in,Complex<T>& complex){
            in >> complex.real >> complex.imag;
            return in;
        }
    private:
        T real,imag;
};

//类实现
template<typename T>
Complex<T>::Complex():real{0},imag{0}{
}

template<typename T>
Complex<T>::Complex(T x,T y):real{x},imag{y}{
}

template<typename T>
Complex<T>::Complex(const Complex&other){
    real = other.real;
    imag = other.imag;
}

template<typename T>
T Complex<T>::get_real() const{
    return real;
}

template<typename T>
T Complex<T>::get_imag() const{
    return imag;
}

#include <iostream>
#include "Complex.hpp"

void test1() {
    using std::cout;
    using std::boolalpha;
    
    Complex<int> c1(2, -5), c2(c1);

    cout << "c1 = " << c1 << '\n';
    cout << "c2 = " << c2 << '\n';
    cout << "c1 + c2 = " << c1 + c2 << '\n';
    
    c1 += c2;
    cout << "c1 = " << c1 << '\n';
    cout << boolalpha << (c1 == c2) << '\n';
}

void test2() {
    using std::cin;
    using std::cout;

    Complex<double> c1, c2;
    cout << "Enter c1 and c2: ";
    cin >> c1 >> c2;
    cout << "c1 = " << c1 << '\n';
    cout << "c2 = " << c2 << '\n';

    const Complex<double> c3(c1);
    cout << "c3.real = " << c3.get_real() << '\n';
    cout << "c3.imag = " << c3.get_imag() << '\n';
}

int main() {
    std::cout << "自定义类模板Complex测试1: \n";
    test1();

    std::cout << "\n自定义类模板Complex测试2: \n";
    test2();
}

运行结果测试截图：