Effective C++ 37. Never redefine a function's inherited default parameter value

ruihuank

于 2017-11-03 10:42:48 发布

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分类专栏： Effective-c++-笔记文章标签： effective-c++

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本文探讨了C++中虚函数的动态绑定与默认参数的静态绑定特性，通过具体示例说明了如何避免由默认参数引起的派生类行为不一致问题，并提出了一种非虚接口(NVI)的设计模式来解决该问题。

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virtual functions are dynamically bound , but default parameters are statically bound.

class Shape {
public:
    enum ShapColor (Red, Green, Blue);
    virtual void draw(ShapeColor color = Red) const = 0;
};

class Rectangle: public Shape {
public:
    virtual void draw(ShapeColor color = Green) const;
};
Shape *pr = new Rectangle;

the default parameter value for this function call is taken from the Shape class, not the Rectangle class!

avoid confusion
I.

class Shape {
public:
    enum ShapColor (Red, Green, Blue);
    virtual void draw(ShapeColor color = Red) const = 0;
};

class Rectangle: public Shape {
public:
    virtual void draw(ShapeColor color = Red) const;
};
Shape *pr = new Rectangle;

but if the default paramter value is changed in Shape, all derived classes that repeat it most also be changed.

II. NVI idiom(non-virtual interface )

class Shape{
public:
    enum ShapeColor{ Red, Green, Blue };
    void draw(ShapeColor color = Red) const {
        doDraw(color);
    }
private:
    virtual void doDraw(color) const = 0;
};

class Rectangle: public Shape {
public:

private:
    virtual void doDraw(ShapeColor color) const;
};