类的继承

#include<iostream.h>
const double PI=3.14159265;
class Circle
{
protected:
    double radius;
public:
    
    Circle(double r){radius=r;};
    virtual double area(){return 0.0; };
    virtual double volume(){return 0.0;};
};

class Sphere:public Circle   //qiu ti
{public:
    
    Sphere(double r):Circle(r){}
    double area(){return 4*PI*radius*radius;};
    double volume(){return 4*PI*radius*radius/3.0; };
};

class Column:public Circle  //zhu ti
{ public:
    Column(double r,double h):Circle(r){ height=h;}
    double area(){return 2*PI*radius*(radius+height);};
    double volume(){ return PI*radius*radius*height;};
 private:
     double height;

};


void main()
{
Circle *p;
Sphere subj(1);
p=&subj;
cout<<"球体面积:";
cout<<p->area()<<endl;
cout<<"球体体积:";
cout<<p->volume()<<endl;

Column object(2,3);
p=&object;
cout<<"柱体面积:";
cout<<p->area()<<endl;
cout<<"柱体体积:";
cout<<p->volume()<<endl;

}

这是我写的段代码一段关于类的普通继承的一个例子：经过我自身学习并打出这段代码是一般会遇到一下这些错误：

1、把这些代码： Circle(double r){radius=r;};
                       virtual double area(){return 0.0; };
                       virtual double volume(){return 0.0;};

          写成这样： Circle(double r){radius=r;};
                         virtual double area(){};
                         virtual double volume(){}；这会显示error 因为没有返回值 return；有时候会因为lose {} 显示error；还望各位与我一样的初学者要注意一下！

2、    在继承父类的时候 class Sphere:public Circle  这语句 要彻记 没有括号的，还有这两句： Sphere subj(1); 取地址符这没括号                                                                                                                                            p=&subj;

3、