Effective C++ Item 37 绝不又一次定义继承而来的缺省參数值

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经验：绝对不要又一次而来的缺省參数值。由于缺省參数值都是静态绑定，而 virtual 函数 -- 你唯一应该覆写的东西 -- 却是动态绑定

class Shape{
public:
	enum ShapeColor {Red, Green, Blue};
	virtual void draw(ShapeColor color = Red) const = 0;
};
class Rectangle: public Shape{
public:
	virtual void draw(ShapeColor color = Green) const; // 赋予不同缺省參数值
}
class Circle: public Shape{
public:
	virtual void draw(ShapeColor color) const;
	//以对象调用此函数。一定要指定參数值
	//以指针(reference)调用此函数，能够不指定參数值，从base 继承缺省參数值
};
//下面三个变量的静态类型都是 Shape *
Shape *ps;
Shape *pc = new Circle;
Shape *pr = new Rectangle;

pr->draw(); 

class Shape{
public:
	enum ShapeColor {Red, Green, Blue};
	void draw(ShapeColor color = Red) const{
		doDraw(color);
	}
private:
	virtual void doDraw(ShapeColor color) const = 0;//
};
class Rectangle: public Shape{
public:
	...
private:
	virtual void draw(ShapeColor color) const; // 赋予不同缺省參数值
}