{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

 /*************************************************************************
     > File Name: polymorphism.cpp
     > Author: Jeremy Wu
     > Created Time: Mon 25 May 2015 10:04:07 AM CST
     > Function: tune every instrument
  ************************************************************************/

 //多态：一个接口，多种实现
 //发生多态现象的条件：
 //    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)
 //    ２．借助指针或者引用调用该虚函数
 //
 //虚函数只能是 非静态 成员函数
 //如果类中有虚函数，最好把 析构函数 也定义为虚函数
 //
 //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
 // ·构造函数无法定义为虚函数
 //
 //基类可以指向它的所有的派生类
 //相当于基类给所有的派生类提供了统一的接口
 //一般不会直接创建基类的对象
 //
 //{ 抽象基类 }
 //如果一个类中包含纯虚函数，就是抽象类
 //抽象类无法创建对象
 //如果需要 避免 创建基类对象，应该使用纯虚函数

 //接下来是一个例子，有一个基类叫做乐器类
 //由它派生出Wind ， Brass 和 Gitar 三个类

 #include <iostream>

 using namespace std;

 class Instrument {
 public:
     virtual void play ();    //virtual 关键字
     Instrument () { cout << "Instement () " << endl; }
     virtual ~Instrument () { cout << "~Instement () " << endl; }
 };

 void Instrument::play () {    //类的外部定义前不用加　virtual 关键字
     cout << "Instrument::play ()" << endl;
 }

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

 class Brass : public Instrument {
     virtual void play () { cout << "Brass::play ()" << endl; }
     ~Brass () { cout << "Brass::~Brass () " << endl; }
 };

 class Gitar : public Instrument {
     virtual void play () { cout << "Gitar::play ()" << endl; }
     ~Gitar () { cout << "Gitar::~Gitar ()" << endl; }
 };

 //inst 指针在定义的时候，是静态类型
 //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
 //
 //有时候把动态绑定称为运行时绑定，也称后继绑定
 //run-time binding

 void tune (Instrument * inst) {    //引用传递 或者用指针
     inst->play ();                //Dynamic binding
                                 //在编译时进行　动态绑定
 }

 int main () {

     Instrument * p = new Gitar;
     Instrument * p2 = new Wind;

     delete p;
     delete p2;

     return ;
 }

---

接下来老师出了一道题目

描述如下：

以下是我的 Solution:

 /*************************************************************************
     > File Name: ployPROB.cpp
     > Author: Jeremy Wu
     > Created Time: Mon 25 May 2015 10:50:07 AM CST
  ************************************************************************/

 //定义一个形状类的继承结构
 //
 //Shape -> Triangle
 //        -> Rectangle
 //        -> Circle
 //
 //给形状定义两个操作：
 //    getArea ()　求面积
 //    getLength () 求周长

 #include <iostream>
 #include <cmath>

 const double PI = 3.1415926;

 using namespace std;

 class Shape {
 public:
     virtual double getArea () = ;
     virtual double getLength () = ;
     virtual ~Shape () { cout << "~Shape ()" << endl; }
     Shape () { cout << "Shape ()" << endl; }
 };

 class Triangle : public Shape {
 private:
     double x, y, z;
 public:
     double getArea () {
         double p = (x + y + z) / 2.0;
         return sqrt (p * (p - x) * (p - y) * (p - z));
     }
     double getLength () { return x + y + z; }
     Triangle (double ix = 0.0, double iy = 0.0, double iz = 0.0) {
         x = ix, y = iy, z = iz;
     }
     ~Triangle () { cout << "~Triangle () " << endl; }
 };

 class Rectangle : public Shape {
 private:
     double x, y;
 public:
     double getArea () { return x * y; }
     double getLength () { return 2.0 * (x + y); }
     Rectangle (double ix = 0.0, double iy = 0.0) {
         x = ix, y = iy;
     }
     ~Rectangle () { cout << "~Rectangle () " << endl; }
 };

 class Circle : public Shape {
 private:
     double r;
 public:
     double getArea () { return PI * r * r; }
     double getLength () { return 2.0 * PI * r; }
     Circle (double ir = 0.0) {
         r = ir;
     }
     ~Circle () { cout << "~Circle ()" << endl; }
 };

 void test (Shape & sp) {
     cout << "/***********************************/" << endl;
     cout << '\t' << "Area is " << sp.getArea () << endl;
     cout << '\t' << "Length is " << sp.getLength () << endl;
     cout << "/***********************************/" << endl;
 }

 int main () {

     Circle p1 (3.0);
     Rectangle p2 (3.0, 6.0);
     Triangle p3 (1.0, 2.0, 1.5);

     test (p1);
     test (p2);
     test (p3);

     return ;
 }

---

以下是官方版 Solution:

 /*************************************************************************
     > File Name: example.cpp
     > Author: Jeremy Wu
     > Created Time: Mon 25 May 2015 11:34:47 AM CST
  ************************************************************************/

 #include <iostream>
 #include <cmath>

 using namespace std;

 class Shape {
 public:
     virtual double area () = ;    //不实现基类可以这么写，表示定义一个纯虚函数
     virtual double zhou () = ; //pure virtual function
 };

 class Triangle : public Shape {
     double a, b, c;
 public:
     Triangle (double aa, double bb, double cc) : a (aa), b (bb), c (cc) {}
     virtual double area () {
         double p = 0.5 * (a + b + c);
         return sqrt (p * (p - a) * (p - b) * (p - c));
     }
     virtual double zhou () {
         return a + b + c;
     }
 };

 class Rectangle : public Shape {
     double a, b;
 public:
     Rectangle (double aa, double bb) : a (aa), b (bb) {}
     virtual double area () {
         return a * b;
     }
     virtual double zhou () {
         return 2.0 * (a + b);
     }
 };

 class Circle : public Shape {
     double r;
 public:
     Circle (double rr) : r (rr) {}
     virtual double area () {
         return 3.14159 * r * r;
     }
     virtual double zhou () {
         return 2.0 * 3.14159 * r;
     }
 };

 void printArea (Shape * p) {
     cout << p->area () << endl;
 }

 void printZhou (Shape * p) {
     cout << p->zhou () << endl;
 }

 int main () {

     Shape * p1, * p2, * p3;

     //Shape sp;

     p1 = new Triangle (, , );
     p2 = new Rectangle (, 4.6);
     p3 = new Circle (5.8);

     printArea (p1);
     printArea (p2);
     printArea (p3);

     printZhou (p1);
     printZhou (p2);
     printZhou (p3);

     return ;
 }