{key}面向对象程序设计-C++ polymorphism 【第十三次上课笔记】

Peronal Link: http://segmentfault.com/a/1190000002464822

 

这节课讲了本门课程 面向对象程序设计中最为重要的一个部分 - 多态

 1 /*************************************************************************

 2     > File Name: polymorphism.cpp

 3     > Author: Jeremy Wu

 4     > Created Time: Mon 25 May 2015 10:04:07 AM CST

 5     > Function: tune every instrument

 6  ************************************************************************/

 7 

 8 //多态：一个接口，多种实现

 9 //发生多态现象的条件：

10 //    １．通过调用虚函数(基类中必须要有，派生类中可有可无建议加上)

11 //    ２．借助指针或者引用调用该虚函数

12 //

13 //虚函数只能是 非静态 成员函数

14 //如果类中有虚函数，最好把 析构函数 也定义为虚函数

15 //

16 //如果基类的析构函数是虚函数，由它派生的所有的派生类的析构函数都 自动 成为虚函数

17 // ·构造函数无法定义为虚函数

18 //

19 //基类可以指向它的所有的派生类

20 //相当于基类给所有的派生类提供了统一的接口

21 //一般不会直接创建基类的对象

22 //

23 //{ 抽象基类 }

24 //如果一个类中包含纯虚函数，就是抽象类

25 //抽象类无法创建对象

26 //如果需要 避免 创建基类对象，应该使用纯虚函数

27 

28 

29 

30 //接下来是一个例子，有一个基类叫做乐器类

31 //由它派生出Wind ， Brass 和 Gitar 三个类

32 

33 #include <iostream>

34 

35 using namespace std;

36 

37 class Instrument {

38 public:

39     virtual void play ();    //virtual 关键字

40     Instrument () { cout << "Instement () " << endl; }

41     virtual ~Instrument () { cout << "~Instement () " << endl; }

42 };

43 

44 void Instrument::play () {    //类的外部定义前不用加　virtual 关键字

45     cout << "Instrument::play ()" << endl;

46 }

47 

48 class Wind : public Instrument{

49     virtual void play () { cout << "Wind::play () " << endl; }    //派生类中 virtual 关键字可加可不加

50     ~Wind () { cout << "Wind::~Wind ()" << endl; }

51 };

52 

53 class Brass : public Instrument {

54     virtual void play () { cout << "Brass::play ()" << endl; }

55     ~Brass () { cout << "Brass::~Brass () " << endl; }

56 };

57 

58 class Gitar : public Instrument {

59     virtual void play () { cout << "Gitar::play ()" << endl; }

60     ~Gitar () { cout << "Gitar::~Gitar ()" << endl; }

61 };

62 

63 //inst 指针在定义的时候，是静态类型

64 //在程序运行过程中，它的动态类型就会根据指向对象的类型作改变

65 //

66 //有时候把动态绑定称为运行时绑定，也称后继绑定

67 //run-time binding

68 

69 void tune (Instrument * inst) {    //引用传递 或者用指针

70     inst->play ();                //Dynamic binding

71                                 //在编译时进行　动态绑定

72 }

73 

74 int main () {

75 

76     Instrument * p = new Gitar;

77     Instrument * p2 = new Wind;

78 

79     delete p;

80     delete p2;

81 

82     return 0;

83 }

---

接下来老师出了一道题目

描述如下：

以下是我的 Solution:

 1 /*************************************************************************

 2     > File Name: ployPROB.cpp

 3     > Author: Jeremy Wu

 4     > Created Time: Mon 25 May 2015 10:50:07 AM CST

 5  ************************************************************************/

 6 

 7 //定义一个形状类的继承结构

 8 //

 9 //Shape -> Triangle

10 //        -> Rectangle

11 //        -> Circle

12 //

13 //给形状定义两个操作：

14 //    getArea ()　求面积

15 //    getLength () 求周长

16 

17 #include <iostream>

18 #include <cmath>

19 

20 const double PI = 3.1415926;

21 

22 using namespace std;

23 

24 class Shape {

25 public:

26     virtual double getArea () = 0;    

27     virtual double getLength () = 0;

28     virtual ~Shape () { cout << "~Shape ()" << endl; }

29     Shape () { cout << "Shape ()" << endl; }

30 };

31 

32 class Triangle : public Shape {

33 private:

34     double x, y, z;

35 public:

36     double getArea () { 

37         double p = (x + y + z) / 2.0;

38         return sqrt (p * (p - x) * (p - y) * (p - z));

39     }

40     double getLength () { return x + y + z; }

41     Triangle (double ix = 0.0, double iy = 0.0, double iz = 0.0) { 

42         x = ix, y = iy, z = iz;

43     }

44     ~Triangle () { cout << "~Triangle () " << endl; }

45 };

46 

47 class Rectangle : public Shape {

48 private:

49     double x, y;

50 public:

51     double getArea () { return x * y; }

52     double getLength () { return 2.0 * (x + y); }

53     Rectangle (double ix = 0.0, double iy = 0.0) {

54         x = ix, y = iy;

55     }

56     ~Rectangle () { cout << "~Rectangle () " << endl; }

57 };

58 

59 class Circle : public Shape {

60 private:

61     double r;

62 public:

63     double getArea () { return PI * r * r; }

64     double getLength () { return 2.0 * PI * r; }

65     Circle (double ir = 0.0) {

66         r = ir;

67     }

68     ~Circle () { cout << "~Circle ()" << endl; }

69 };

70 

71 void test (Shape & sp) {

72     cout << "/***********************************/" << endl;

73     cout << '\t' << "Area is " << sp.getArea () << endl;

74     cout << '\t' << "Length is " << sp.getLength () << endl;

75     cout << "/***********************************/" << endl;

76 }

77 

78 int main () {

79 

80     Circle p1 (3.0);

81     Rectangle p2 (3.0, 6.0);

82     Triangle p3 (1.0, 2.0, 1.5);

83 

84     test (p1);

85     test (p2);

86     test (p3);

87 

88     return 0;

89 }

---

以下是官方版 Solution:

 

 1 /*************************************************************************

 2     > File Name: example.cpp

 3     > Author: Jeremy Wu

 4     > Created Time: Mon 25 May 2015 11:34:47 AM CST

 5  ************************************************************************/

 6 

 7 #include <iostream>

 8 #include <cmath>

 9 

10 using namespace std;

11 

12 class Shape {

13 public:

14     virtual double area () = 0;    //不实现基类可以这么写，表示定义一个纯虚函数

15     virtual double zhou () = 0; //pure virtual function

16 };

17 

18 class Triangle : public Shape {

19     double a, b, c;

20 public:

21     Triangle (double aa, double bb, double cc) : a (aa), b (bb), c (cc) {}

22     virtual double area () {

23         double p = 0.5 * (a + b + c);

24         return sqrt (p * (p - a) * (p - b) * (p - c));

25     }

26     virtual double zhou () {

27         return a + b + c;

28     }

29 };

30 

31 class Rectangle : public Shape {

32     double a, b;

33 public:

34     Rectangle (double aa, double bb) : a (aa), b (bb) {}

35     virtual double area () {

36         return a * b;

37     }

38     virtual double zhou () {

39         return 2.0 * (a + b);

40     }

41 };

42 

43 class Circle : public Shape {

44     double r;

45 public:

46     Circle (double rr) : r (rr) {}

47     virtual double area () {

48         return 3.14159 * r * r;

49     }

50     virtual double zhou () {

51         return 2.0 * 3.14159 * r;

52     }

53 };

54 

55 void printArea (Shape * p) { 

56     cout << p->area () << endl; 

57 }

58 

59 void printZhou (Shape * p) {

60     cout << p->zhou () << endl;

61 }

62 

63 int main () {

64 

65     Shape * p1, * p2, * p3;

66     

67     //Shape sp;

68 

69     p1 = new Triangle (6, 7, 8);

70     p2 = new Rectangle (10, 4.6);

71     p3 = new Circle (5.8);

72 

73     printArea (p1);

74     printArea (p2);

75     printArea (p3);

76 

77     printZhou (p1);

78     printZhou (p2);

79     printZhou (p3);

80 

81     return 0;

82 }