C++多态性

C++多态

1.1多态性(polymorphism)是面向对象程序设计的一个重要特征。
1.2从系统实现的角度看，多态性分两类：静态多态性与动态多态性。
静态多态性是通过函数重载(运算符重载也是函数重载)来实现的。它又称为编译时多态。
动态多样性是在程序运行过程中才动态确定操作所针对的对象。它又称为运行时多态，动态多态性是通过虚函数来实现的。
1.3一段代码：

#include <iostream>

using namespace std;

class Point

{

public:

Point(float a=0,float b=0);

void setPoint(float a,float b);

float getX() const {return x;}

float getY() const {return y;}

friend ostream& operator << (ostream& output,Point& p);

protected:

float x,y;

};

Point::Point(float a, float b)

{

x=a;

y=b;

}

void Point::setPoint(float a, float b)

{

x=a;

y=b;

}

ostream& operator << (ostream& output,Point& p)

{

output << "[" << p.getX() << "," << p.getY() << "]" << endl;

return output;

}

class Circle:public Point

{

public:

Circle(float a,float b,float r):Point(a,b),radius(r){}

void setRadius(float r);

float getRadius() const {return radius;}

float area() const;

friend ostream& operator << (ostream& output,Circle &c);

protected:

float radius;

};

void Circle::setRadius(float r)

{

radius=r;

}

float Circle::area() const

{

return 3.14159*radius*radius;

}

ostream& operator << (ostream& output,Circle &c)

{

output << "Center [" << c.x << "," << c.y << "] radius=" << c.radius

<< " area=" << c.area() << endl;

return output;

}

class Cylinder:public Circle

{

public:

Cylinder(float a,float b,float r,float h):Circle(a,b,r),height(h){}

void setHeight(float h){height=h;}

float getHeight() const{return height;}

float area() const;

float volume() const;

friend ostream& operator << (ostream& output,Cylinder& cy);

protected:

float height;

};

float Cylinder::area() const

{

return 2*Circle::area()+2*3.14159*radius*height;

}

float Cylinder::volume() const

{

return Circle::area()*height;

}

ostream& operator << (ostream& output,Cylinder& cy)

{

output << "Center [" << cy.getX() << "," << cy.getY() << "] r="

<< cy.radius << "\narea=" << cy.area() << " volume=" << cy.volume()

<< endl;

return output;

}

int main()

{

Cylinder cy(1,2,3,4);

cout <<"Cylinder\n" << "Center [" << cy.getX() << "," << cy.getY() << "] r="

<< cy.getRadius() << "\narea=" << cy.area() << " volume=" << cy.volume()

<< endl;

cy.setHeight(1);

cy.setPoint(2,3);

cy.setRadius(4);

cout << "Cylinder\n" << cy << endl;

Point &p=cy;

cout << "Point\n" << p;

Circle &c=cy;

cout << "Circle\n" << c;

return 0;

}

1.4在本例中存在静态多态性，这是由运算符重载引起的，可以看到，在编译时编译系统即可以判定应调用哪个运算符函数。