（1）分析相关对象的需求，设计出一组实现公共功能的函数。
（2）将这些函数作为基类的虚函数（或纯虚函数），它们定义了一个
统一的公共接口。
（3）由该类基类派生出若干子类，在各子类中实现这些虚函数。
#include<iostream>
using namespace std;
class Container //抽象类
{
protected:
double radius;
public:
Container(double radius){
Container::radius = radius;
}
virtual double surface_area()=0;//纯虚函数
virtual double volume()=0;//纯虚函数
};
class Cube:public Container
{
public:
Cube(double radius):Container(radius){}
double surface_area(){return radius*radius*6;}
double volume(){return radius*radius*radius;}
};
class Sphere:public Container
{
public:
Sphere(double radius):Container(radius){}
double surface_area(){return radius*radius*3.14;}
double volume(){return 3.1416*radius*radius*radius*4/3;}
};
class Cylinder:public Container
{
double height;
public:
Cylinder(double radius, double height):Container(radius){
Cylinder::height = height;
}
double surface_area(){
return 2*3.1416*radius*(height+radius);
}
double volume(){return 3.1416*radius*radius*height;}
};
int main()
{
Container *p;
Cube obj1(5);
Sphere obj2(5);
Cylinder obj3(5,5);
p = &obj1;
cout<<"正方体表面积:"<<p->surface_area()<<endl;
cout<<"正方体体积:"<<p->volume()<<endl;
p = &obj2;
cout<<"球体表面积:"<<p->surface_area()<<endl;
cout<<"球体体积:"<<p->volume()<<endl;
p = &obj2;
cout<<"圆柱体表面积:"<<p->surface_area()<<endl;
cout<<"圆柱体体积:"<<p->volume()<<endl;
return 0;
}