Problem: Given a vector of pointers to Shape objects, print the area of each of the shape.

Transcription

1 Problem: Given a vector of pointers to Shape objects, print the area of each of the shape. 1

2 Problem: Given a vector of pointers to Shape objects, sort the shape objects by areas in decreasing order. Std++sort: genericity ( 泛型 ) 2

3 Object-Oriented Programming C++ Inheritance and Polymorphism CSIE Department, NTUT Woei-Kae Chen 3

4 Introduction to inheritance Inheritance Derived class inherits data members and member functions from base class Single inheritance inherits from one base class Multiple inheritance inherits from multiple base classes Types of inheritance public private protected Is a relationship Alternative to composition Base Derived 4

5 Inheritance Examples 1 Base class Student Shape Loan Employee Account Derived classes GraduateStudent UndergraduateStudent Circle Triangle Rectangle CarLoan HomeImprovementLoan MortgageLoan FacultyMember StaffMember CheckingAccount SavingsAccount 5

6 Inheritance Examples 2 Base class ( 或 super class) CommunityMember Employee Student Alumnus Single inheritance Faculty Staff Single inheritance Administrator Teacher Single inheritance Derived class ( 或 subclass) AdministratorTeacher Multiple inheritance 6

7 Inheritance Examples 3 Shape TwoDimensionalShape ThreeDimensionalShape Circle Square Triangle Sphere Cube Tetrahedron 7

8 is-a is-a vs. has-a relationship Inheritance Derived class object treated as base class object Example: Car is a vehicle has-a Composition Object contains one or more objects of other classes as members Example: Car has a steering wheel 8

9 UML Example (C++): Inheritance X Shape UML: Class UML: Inheritance Y Circle class X { class Y : public X { 9

10 UML Example (C++): Composition X Employee Y Date class Employee { Date d; // Composition 10

11 Introduction to polymorphism Problem: A list of unknown shapes? Shape Circle Rectangle Triangle 11

12 Declaring a list of unknown shapes Shape s[100]; Circle s[100]; Shape s[100]; Shape *s[100]; // Bad // Bad // Bad 1. 混合 type 的 shape 怎麼儲存在 array? 2. 怎麼呼叫? // OK, 但怎麼呼叫? Solution: 設成 virtual function Shape s[0] = new Circle; s[1] = new Triangle; for (i=0;i<100;i++) s[i]->;//??? Circle Rectangle Triangle Does NOT work 12

13 Effect of Virtual Functions Example (non-virtual) class X { public: void f() { cout << x ; } class Y : public X { public: void f() {cout<< y ;} X a; a.f(); // x Y b; b.f(); // y X *p=&a; p->f(); X *p=&b; p->f(); // x // x Example (virtual) class X { public: virtual void f(){ cout << x ; } class Y : public X { public: void f() {cout<< y ;} X a; a.f(); // x Y b; b.f(); // y X *p=&a; p->f(); X *p=&b; p->f(); // x // y virtual 只對 pointer 或 reference 有效 (dynamic binding) 13

14 Virtual Functions for the shape problem 設成 virtual function Shape Java? Circle Rectangle Triangle Shape *s; s = new Circle; s->; // 呼叫 circle 的 s = new Triangle; s->; // 呼叫 Triangle 的 Shape *s[100]; // 設定 s[0..99] 分別指向哪一種物件 for (int i=0;i<100;i++) s[i]->; // 依據實際的 type 呼叫 14

15 Virtual Functions & Polymorphism Polymorphism Ability for objects of different classes to respond differently to the same function call Implemented through virtual functions C++ 允許使用 base class pointer( 或 reference) 呼叫 derived class 的 member function C++ chooses the correct overridden function in object Dynamic binding Function determined during execution time ShapePtr->Draw(); Static binding 已知 derivedobject Function determined during compile-time ShapeObject.Draw(); Pointer 可能指向任何 derivedobject 15

16 Dynamic Binding Static Binding Dynamic binding (run-time) 用 base class 的 pointer 或 reference 呼叫 virtual function Static binding (compile time) 其他 ( 用 object 呼叫 virtual/non-virtual function, 用 pointer 或 reference 呼叫 non-virtual function class X { public: void f1() { cout << xf1 ; } virtual void f2() { cout << xf2 ; } class Y : public X { public: void f1(){cout<< yf1 ;} void f2(){cout<< yf2 ;} X a; a.f1(); // xf1 static a.f2(); // xf2 static Y b; b.f1(); // yf1 static b.f2(); // yf2 static X *p=&a; p->f1(); // xf1 static p->f2(); // xf2 dynamic X *pb=&b; pb->f1(); // xf1 static pb->f2(); // yf2 dynamic 16

17 Abstract and Concrete Classes Abstract classes To provide a base class for other classes No objects can be instantiated Declare one or more pure virtual functions Example virtual void = 0; Concrete classes Objects can be instantiated No unimplemented virtual functions Implement all unimplemented virtual functions 17

18 Abstract and Concrete Classes C++ example class Shape { public: virtual void = 0; void setcolor(int c) { color = c; } private: int color; virtual is inherited Abstract class Concrete class class Rectangle : public Shape { public: void { void getarea() { virtual void f() { Shape setcolor() Rectangle Not pure virtual 18

19 Abstract and Concrete Classes UML example 斜體字 pure virtual function 正體字有 implementation Shape setcolor() 斜體字 abstract class 正體字 concrete class Circle Rectangle Triangle 19

20 Virtual Destructors Problem: How to delete an object with a base class pointer? Solution: declare a virtual (base-class) destructor class X { public: virtual ~X(); // virtual destructor private: class Y : public X { X *pa = new Y; // use *pa delete pa; 20

21 How does compiler handle virtual functions? virtual functions overheads? memory speed How it works? dynamic binding vtable Abstract class Shape Point Circle Cylinder 21

22 vtable virtual functions Shape area() volume() PSN() print() Point x y PSN() print() functions 每個 class 都有一個 vtable 每個 object 都帶著 vtable 的 pointer array Circle radius area() PSN() print() Cylinder height area() volume() PSN() print() bp->psn(); 22