a drawing program

data
- center
operatons
- render
- move
- resize

Inheritance in C++

can define one class in term of another
can capture the notion that
- an ellipse is a shape
- a circle is a special kind of ellipse
- a rectangle is a different shape
- circles,ellipses,and rectangles share common
  - attributes
  - services
- circles,ellipses,and rectangles are not identical（同一的）

conceptual（概念） model

?render 不同

shape

define the general properties of shape

class XYPos{...};//x,y point
class shape{
public:
    Shape();
    virtual ~Shape();
    virtual void render(); // virtual 子类和父类的同名函数联系在一起
    void move(const XYPos&);
    virtual void resize();
protected:
    XYPos center;
}

add new shapes

class Ellipse:public Shape{
public:
    Ellipse(float maj,float mint);
    virtual void render();//will define own  virtual不加也是virtual的，好习惯，不用看shape
protected:
    float major_axis,minor_axis; 
};

class Circls:public Ellipse{
public:
	Circle(float radius):Ellispse(radius,radius){}
    virtual void render();
};

Example

void render(Shape* p){ // 通用函数
    p->render();	//calls correct render function for given shape
}	
// p的静态类型是 shape 的指针；p的动态类型指的是 p当时指的对象的类型是什么。如果render是virtual 是动态绑定，取决于render；不是virtual是静态绑定。
void func(){
    Ellipse ell(10,20);
    ell.render();
    
    Circle circ(40);
    circ.render();
    
    render(&ell); // virtual作用：运行时候确定调用哪个函数，这里调用 ell 的函数
    reder(&circ);
}

p是多态的，p指的谁，变成谁的形态。

poly多morphism形态（多态性）

upcast: take an object of the derived class as an object of base one
- Ellipse can be treaded as a Shape

upcast：把子类对象当作父类看待

dynamic binging(动态绑定)
- binding:which function to be called
  - static binding:call the function as the code
  - dynamic binding:call the function of the object

绑定：调用函数时候应调用哪个函数，叫做绑定

静态绑定：调的函数在编译时候确定

动态绑定：运行时候才知道调用哪个函数，根据指针所指的对象决定