{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 0;
}

---

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 () = 0;    
    virtual double getLength () = 0;
    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 << '	' << "Area is " << sp.getArea () << endl;
    cout << '	' << "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 0;
}

---

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 () = 0;    //不实现基类可以这么写，表示定义一个纯虚函数
    virtual double zhou () = 0; //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 (6, 7, 8);
    p2 = new Rectangle (10, 4.6);
    p3 = new Circle (5.8);

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

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

    return 0;
}