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1 Chapter 13 Object-Oriented Programming: Polymorphism 杨明

2 OBJECTIVES What polymorphism( 多态 ) is, how it makes programming more convenient, and how it makes systems more extensible and maintainable. To declare and use virtual functions( 虚函数 ) to effect polymorphism. To declare pure virtual functions( 纯虚函数 ) to create abstract classes. The distinction between abstract and concrete classes( 抽象类和具体类 ). 2

3 13.1 Introduction Topics 13.2 Relationships Among Objects in an Inheritance Hierarchy 13.3 Abstract Classes and Pure virtual Functions 13.4 Case Study: Payroll System Using Polymorphism 3

4 13.1 Introduction --- 基础 class B 继承 class A, 即 B is-a A 1. B big; 2. A small = big; 3. A &refsmall = big; 4. A *psmall = &big; 正确 1. big = small; 2. B *pbig = &small; 错误 x y class A x y z class B 基类部分派生部分 4

5 13.1 Introduction --- 需求画板 Tool 基类 Shape: 派生出长方形椭圆形三角形菱形等用 vector 或者 array 来保存指针基类指针 vs 派生类指针当需刷新画板时, 枚举指针并调用各自的 draw 函数 5

6 13.1 Introduction --- 需求用户通过键盘输入多个员工信息, 统计收入数据 : (1) CommissionEmployee name, ssn, grosssales, commisionrate (2) BasePlusCommissionEmployee name, ssn, grosssales, commisionrate, basesalary 用 vector 或者 array 来保存指向员工对象的指针 CommissionEmployee Pointer 希望通过这些指针来调用各自的 earnings() 函数以进行统计 6

7 13.1 Introduction --- 需求 7

8 13.1 Introduction Polymorphism( 多态 ): 建立在继承基础上的一种面向对象语言的特性通过指向派生类的基类指针, 调用派生类的函数 ; 将不同类型的派生类对象都当作基类来处理, 可以屏蔽不同派生类对象之间的差异, 写出通用的代码, 进行泛型编程, 以适应需求的不断变化 Virtual Function( 虚函数 ) Pure Virtual Function( 纯虚函数 ): 没有给出实现代码的虚函数 Abstract Class( 抽象类 ) vs Concrete Class( 具体类 ) 8

9 13.1 Introduction Topics 13.2 Relationships Among Objects in an Inheritance Hierarchy 13.3 Abstract Classes and Pure virtual Functions 13.4 Case Study: Payroll System Using Polymorphism 9

10 13.2 Relationships Among Objects in an Inheritance Hierarchy Invoking Base-Class Functions from Derived- Class Objects ( 基类指针指向派生类, 调用基类函数 ) Aiming Derived-Class Pointers at Base-Class Objects ( 派生类指针指向基类, 错误 ) Derived-Class Member-Function Calls via Base-Class Pointers ( 基类指针指向派生类, 调用派生类函数, 错误 ) Virtual Functions ( 应用虚函数, 解决上述问题 ) Summary of the Allowed Assignments Between Base-Class and Derived-Class Objects and Pointers ( 基类 / 派生类对象和指针之间的赋值 ) 10

11 Invoking Base-Class Functions from Derived-Class Objects 基类 CommissionEmployee void print() const; 派生类 BasePlusCommissionEmployee void print() const; 通过指向派生类的基类指针, 调用的是基类的函数结论 : 对于普通成员函数, 调用基类还是派生类的函数, 取决于句柄的类型, 而不是句柄指向的实际对象类型 11

12 Aiming Derived-Class Pointers at Base-Class Objects 派生类指针指向基类对象 Compilation Error 赋值 : Right is-a left? right 必须 is-a left, 才能进行赋值! left = right; 12

13 Derived-Class Member-Function Calls via Base-Class Pointers 通过指向派生类的基类指针, 调用的是基类的 print() 函数可否调用派生类特有的 BaseSalary 相关函数? Compilation Error 结论 : 通过对象句柄, 仅能调用该句柄类型对应的成员函数 13

14 Derived-Class Member-Function Calls via Base-Class Pointers 解决办法 : downcasting If the address of a derived-class object ( 派生类对象地址 ) has been assigned to a pointer of one of its direct or indirect base classes ( 基类指针 ), it is acceptable to cast that base-class pointer back to a pointer of the derived-class type. 14

15 class B 继承 class A, 即 B is-a A Downcasting 1. B big; 2. A small = big; 3. A &refsmall = big; 4. A *psmall = &big; 正确 From A to B B &refbig = (B &) refsmall; B *pbig = (B *) psmall; x y class A x y z class B 基类部分派生部分 15

16 Type Fields and switch Statements 如何知道 vector 中当前的基类指针需要 downcast 为哪种派生类指针? 增加数据成员扩展 Shape 类 : enum ShapeType{ 长方形, 三角形, 椭圆形, 菱形 }; ShapeType shapetype; // Shape 类 + 数据成员保存指针额外信息 : Shape *pshape = &rect; pshape->shapetype = 长方形 ; 根据类型信息 downcast 指针 : switch (pshape->shapetype) case 长方形 : downcast pshape to prectangle; 16

17 Virtual Functions With virtual functions, the type of the object being pointed to, not the type of the handle, determines which version of a virtual function to invoke. 虚函数 : 调用哪个 ( 基类 / 派生类 ) 虚函数, 由实际对象类型而不是句柄类型决定. 17

18 基类 1. class Shape{ 2. public: 3. virtual void draw() const; 4. }; 派生类 Virtual Functions --- 语法 1. class Rectangle : public Shape{ 2. virtual void draw() const; 3. }; 可省略. 只要基类声明函数为虚函数, 则所有派生类的该函数均为虚函数 18

19 基类 1. class Shape{ 2. public: 3. virtual void draw() const; 4. }; 派生类 Virtual Functions --- 语法 1. class Rectangle : public Shape{ 2. virtual void draw() const; 3. }; Override( 覆盖 ) 19

20 Virtual Functions 虚函数用于继承关系中的基类和派生类, 以实现多态. 派生类中覆盖 (Overridden) 的虚函数和基类中的虚函数必须函数签名和返回值均相同. 函数定义时不需要 virtual 关键词. 1. Rectangle rect; 2. Shape *p = &rect; 3. p->draw(); 20

21 Virtual Functions 虚函数的调用由内存中的对象决定, 两种情况 : 通过指向派生类的基类指针和引用调用, 程序会在执行时 (execution time) 根据对象类型动态选择合适的派生类函数动态绑定 ( dynamic binding ) 或延迟绑定 ( late binding ). 通过对象名和点操作符调用, 程序在编译时 (compile time) 即根据对象类型确定函数静态绑定 ( static binding ). 21

22 Virtual Functions --- 若干限制只有类成员函数才能声明为虚函数静态成员函数不能是虚函数构造函数不能是虚函数析构函数通常声明为虚函数 ( 注意基类和派生类的析构函数不同名, BOOK 13.9) Small *p = new Big; delete p; // 调用 Big 的析构函数 22

23 Summary of the Allowed Assignments Between Base-Class and Derived-Class Objects and Pointers base-class pointer derived-class pointer base-class object OK ERROR derived-class object OK OK 23

24 13.1 Introduction Topics 13.2 Relationships Among Objects in an Inheritance Hierarchy 13.3 Abstract Classes and Pure virtual Functions 13.4 Case Study: Payroll System Using Polymorphism 24

25 13.3 Abstract Classes and Pure virtual Functions --- 需求 1. class Shape{ 2. public: 3. virtual void draw() const; 4. }; 一些成员函数对于基类来说是没有意义的, 将其声明为虚成员函数的目的是要求派生类给出其实现并实现多态. 25

26 13.3 Abstract Classes and Pure virtual Functions --- 纯虚函数 Pure Virtual Function( 纯虚函数 ) A pure virtual function is specified by placing "= 0" in its declaration, as in virtual void draw() const = 0; 对于纯虚函数, 不需要在类源码中给出其实现代码. 26

27 13.3 Abstract Classes and Pure virtual Functions --- 抽象类和具体类 Abstract Class( 抽象类 ): 包含一个或多个纯虚函数的类. 无法实例化, 但可以声明指针和引用, 可以用于继承. 1. Shape obj; // Error, 不能实例化 2. Rectangle objrectangle; 3. Shape *ptr = &objrectangle; // OK, 可指针 4. Shape &ref = objrectangle; // OK, 可引用 Concrete Class( 具体类 ): 不包含纯虚函数, 可以实例化 27

28 13.3 Abstract Classes and Pure virtual Functions 成员函数是否声明为虚函数, 取决于是否需要多态性支持虚函数是否声明为纯虚函数, 取决于该函数对于当前类是否有意义, 以及当前类是否需要实例化基类虚函数 has an implementation 纯虚函数 does not provide an implementation 派生类 gives the derived class the option of overriding the function requires the derived class to override the function ( for that derived class to be concrete; otherwise the derived class remains abstract ) 28

29 13.1 Introduction Topics 13.2 Relationships Among Objects in an Inheritance Hierarchy 13.3 Abstract Classes and Pure virtual Functions 13.4 Case Study: Payroll System Using Polymorphism 29

30 13.4 Case Study: Payroll System Using Polymorphism 目的 : 输出各类员工的基本信息和薪金信息 Salaried employees ( 普通薪金制员工 ) Name, SSN, Weekly Salary Commission employees ( 佣金制员工 ) Name, SSN, Gross sales amount, Commission rate Base-salary-plus-commission employees ( 带底薪的佣金制员工 ) Name, SSN, Gross sales amount, Commission rate, Base Salary 30

31 13.4 Case Study: Payroll System Using Polymorphism 为何设计基类 Employee? 尽可能多地抽取共有属性和操作代码重用多态和泛型编程, 需要设计共同的基类 31

32 Creating Abstract Base Class Employee Employee Class Name, SSN: 各类员工的共有属性 print(): 输出 Name, SSN 等基本信息虚函数 earnings(): 没有意义, 要求派生类实现纯虚函数 32

33 Creating Abstract Base Class Employee 33

34 Creating Concrete Derived Class SalariedEmployee SalariedEmployee, 继承 Employee weeklysalary: 普通薪金制员工的独有属性 print(): Override 基类函数, 输出基本信息和薪酬信息 earnings(): 必须 Override 基类的纯虚函数, 计算薪酬 34

35 Creating Concrete Derived Class CommissionEmployee CommissionEmployee, 继承 Employee grosssales, commisionrate: 佣金制员工的独有属性 print(): Override 基类函数, 输出基本信息和薪酬信息 earnings(): 必须 Override 基类的纯虚函数, 计算薪酬 35

36 Creating Indirect Concrete Derived Class BasePlusCommissionEmployee BasePlusCommissionEmployee, 继承 CommissionEmployee Class, 间接继承 Employee Class basesalary: 带底薪的佣金制员工的独有属性 print(): Override 基类 CommissionEmployee 函数, 输出基本信息和薪酬信息 earnings(): 选择 Override 基类的虚函数, 计算薪酬 36

37 Demonstrating Polymorphic Processing 实例化三种类型员工, 建立三个对象通过对象名调用 print 和 earnings 函数 ( 静态绑定 ) 通过基类指针调用 print 和 earnings 函数 ( 动态绑定 ) 通过基类引用调用 print 和 earnings 函数 ( 动态绑定 ) 37

38 Summary 虚函数和多态静态绑定和动态绑定纯虚函数抽象类和具体类 38

39 Homework 实验必选题目 ( 交实验报告 ): 13.12, 实验任选题目 ( 不交实验报告 ): 作业题目 (Homework): 13.5, 13.8,

Microsoft PowerPoint - L17_Inheritance_v4.pptx

Microsoft PowerPoint - L17_Inheritance_v4.pptx C++ Programming Lecture 17 Wei Liu ( 刘威 ) Dept. of Electronics and Information Eng. Huazhong University of Science and Technology May. 2015 Lecture 17 Chapter 20. Object-Oriented Programming: Inheritance