懂c++的帮帮忙。。。

#include <iostream>
using namespace std;
class Shape
{
public:
Shape(){}
virtual ~Shape(){}
virtual long GetArea()=0;
virtual void Draw()=0;
};
void Shape::Draw()
{
cout<<"Abstract drawing mechanism!\n";
}
class Circle:public Shape
{
public:
Circle(int radius):itsRadius(radius){}
virtual ~Circle(){}
long GetArea(){return 3.14*itsRadius*itsRadius;}
long GetPrim(){return 2*3.14*itsRadius;}
void Draw();
private:
int itsRadius;
int itsCircumference;
};
void Circle::Draw()
{
cout<<"Circle drawing routine here!\n";
Shape::Draw();
}
class rectangle:public Shape
{
public:
rectangle(int len,int width):
itsLength(len),itsWidth(width){}
virtual ~rectangle(){}
long GetArea(){return itsLength*itsWidth;}
long GetPerim(){return 2*itsLength+2*itsWidth;}
virtual int GetLength(){return itsLength;}
virtual int GetWidth(){return itsWidth;}
void Draw();
private:
int itsWidth;
int itsLength;
};
void rectangle::Draw()
{
for (int i=0;i<itsLength;i++)
{
for (int j=0;j<itsWidth;j++)
{
cout<<"x";
}
cout<<"\n";
}
Shape::Draw();
}
class Square:public rectangle
{
public:
Square(int len);
Square(int len,int width);
virtual ~Square(){}
long GetPerim(){return 4*GetLength();}
};
Square::Square(int len):
rectangle(len,len)
{}

Square::Square(int len,int width):
rectangle(len,width)
{
if (GetLength()!=GetWidth())
{
cout<<"Error,not a square... a Rectangle??\n";
}

}
int main()
{
int choice;
bool fQuit=false;
Shape *sp;
while (fQuit==false)
{
cout<<"(1)Circle (2)Rectangle (3)Square (0)Quit:";
cin>>choice;

switch (choice)
{
case 1:sp=new Circle(5);
break;
case 2:sp=new rectangle(4,6);
break;
case 3:sp=new Square(5);
break;
default:fQuit=true;
break;
}
if (fQuit==false)
{
sp->Draw();
delete sp;
cout<<endl;
}
}
return 0;
}

我真想不明白，Draw函数是怎么调用的。。。。

请高手帮忙解说，谢谢。。。。。

square和rectangle公用一个draw函数，因为画的方法都是一样的，shapre::draw()是调用基类的draw函数，circle里那个draw这样调用估计是console下不方便画圆吧，所以只做个示范。大概就是示范了下抽象基类和虚函数调用，没什么特别的。