C++语言基础 例程 抽象类

贺老师的教学链接  本课讲解

实例：顶层的Shape作为抽象类

#include <iostream>
using namespace std;
//声明抽象基类Shape
class Shape
{
public:
    virtual float area( ) const
    {
        return 0.0;   //虚函数
    }
    virtual float volume() const
    {
        return 0.0;   //虚函数
    }
    virtual void shapeName() const =0;              //纯虚函数
};

//声明Point类
class Point:public Shape//Point是shape的公用派生类
{
public:
    Point(float=0,float=0);
    void setPoint(float,float);
    float getX( ) const
    {
        return x;
    }
    float getY( ) const
    {
        return y;
    }
    virtual void shapeName( ) const
    {
        cout<<"Point:";   //对虚函数进行再定义
    }
    friend ostream & operator<<(ostream &,const Point &);
protected:
    float x,y;
};
//定义Point类成员函数
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,const Point &p)
{
    output<<"["<<p.x<<","<<p.y<<"]";
    return output;
}

//声明Circle类
class Circle:public Point
{
public:
    Circle(float x=0,float y=0,float r=0);
    void setRadius(float);
    float getRadius( ) const;
    virtual float area( ) const;
    virtual void shapeName( ) const
    {
        cout<<"Circle:";   //对虚函数进行再定义
    }
    friend ostream &operator<<(ostream &,const Circle &);
protected:
    float radius;
};
//声明Circle类成员函数
Circle::Circle(float a,float b,float r):Point(a,b),radius(r) { }

void Circle::setRadius(float r)
{
    radius=r;
}

float Circle::getRadius( ) const
{
    return radius;
}

float Circle::area( ) const
{
    return 3.14159*radius*radius;
}

ostream &operator<<(ostream &output,const Circle &c)
{
    output<<"["<<c.x<<","<<c.y<<"], r="<<c.radius;
    return output;
}

//声明Cylinder类
class Cylinder:public Circle
{
public:
    Cylinder (float x=0,float y=0,float r=0,float h=0);
    void setHeight(float);
    virtual float area( ) const;
    virtual float volume( ) const;
    virtual void shapeName( ) const
    {
        cout<<"Cylinder:";   //对虚函数进行再定义
    }
    friend ostream& operator<<(ostream&,const Cylinder&);
protected:
    float height;
};
//定义Cylinder类成员函数
Cylinder::Cylinder(float a,float b,float r,float h)
    :Circle(a,b,r),height(h) { }

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

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,const Cylinder& cy)
{
    output<<"["<<cy.x<<","<<cy.y<<"], r="<<cy.radius<<", h="<<cy.height;
    return output;
}

//main函数
int main( )
{
    Point point(3.2,4.5);//建立Point类对象point
    Circle circle(2.4,1.2,5.6);                     //建立Circle类对象circle
    Cylinder cylinder(3.5,6.4,5.2,10.5);            //建立Cylinder类对象cylinder
    point.shapeName();                              //静态关联
    cout<<point<<endl;

    circle.shapeName();                             //静态关联
    cout<<circle<<endl;
    cylinder.shapeName();                           //静态关联
    cout<<cylinder<<endl<<endl;

    Shape *pt;                                      //定义基类指针
    pt=&point;                                      //指针指向Point类对象
    pt->shapeName( );                                //动态关联
    cout<<"x="<<point.getX( )<<",y="<<point.getY( )<<"\narea="<<pt->area( )
        <<"\nvolume="<<pt->volume()<<"\n\n";

    pt=&circle;                                     //指针指向Circle类对象
    pt->shapeName( );                                //动态关联
    cout<<"x="<<circle.getX( )<<",y="<<circle.getY( )<<"\narea="<<pt->area( )
        <<"\nvolume="<<pt->volume( )<<"\n\n";

    pt=&cylinder;                                   //指针指向Cylinder类对象
    pt->shapeName( );                                //动态关联
    cout<<"x="<<cylinder.getX( )<<",y="<<cylinder.getY( )<<"\narea="<<pt->area( )
        <<"\nvolume="<<pt->volume( )<<"\n\n";
    return 0;
}