下列代码中, s1-> draw(); 和 s2-> draw(); 输出不同结果的主要原因是 ( ) 。

class Shape  {
public :
    virtual void draw () {
        cout << " 绘制图形 " << endl;
    }
    virtual ~Shape () {}
};

class Circle  :  public Shape  {
public :
    void draw ()  override  {
        cout << " 绘制圆形 " << endl;
    }
};

class Rectangle  :  public Shape  {
public :
    void draw ()  override  {
        cout << " 绘制矩形 " << endl;
    }
};

int main () {
    Shape * s1 = new Circle ();
    Shape * s2 = new Rectangle ();

    s1 -> draw ();
    s2 -> draw ();

    delete s1 ;
    delete s2 ;
    return 0 ;
}