细读《Effective C++》之九

Chapter 6. Inheritance and Object-Oriented Design

条款36：Never redefine an inherited non-virtual function

class B {
public:
void mf();
...
};

class D: public B {
public:
void mf();
...
};

D x; // x is an object of type D
B *pB = &x; // get pointer to x
pB->mf(); // calls B::mf
D *pD = &x; // get pointer to x
pD->mf(); // calls D::mf

non-virtual functions是statically bound，此处mf的调用取决于其声明类型而非所指对象；virtual functions是dynamically bound，如果mf是virtual function，其调用将取决于其所指对象，即D。

这儿，出于以下两点原因，Never redefine an inherited non-virtual function：

1) public inheritance应保证is-a关系。

2) non-virtual function的不变性（invariant）应凌驾于特异性（specialization）。

Things to Remember

1) Never redefine an inherited non-virtual function.

条款37：Never redefine a function's inherited default parameter value

redefine a function，那么这个function一定是virtual function了，virtual functions are dynamically bound, but default parameter values are statically bound。

// a class for geometric shapes
class Shape {
public:
enum ShapeColor { Red, Green, Blue };
// all shapes must offer a function to draw themselves
virtual void draw(ShapeColor color = Red) const = 0;
...
};

class Rectangle: public Shape {
public:
// notice the different default parameter value — bad!
virtual void draw(ShapeColor color = Green) const;
...
};

Shape *ps = new Rectangle; // static type = Shape*, dynamic type = Rectangle*
ps->draw(Shape::Red); // calls Rectangle::draw(Shape::Red)
ps->draw(); // calls Rectangle::draw(Shape::Red)!

由于ps的dynamic type是Rectangle，所以解析为Rectangle::draw()，由于ps的static type是Shape，所以解析为Rectangle::draw(Shape::Red)。

因此，应将带default parameter value的函数声明为non-virtual，其实现用一个private virtual function完成：

class Shape {
public:
enum ShapeColor { Red, Green, Blue };
void draw(ShapeColor color = Red) const // now non-virtual
{
doDraw(color); // calls a virtual
}
...

private:
virtual void doDraw(ShapeColor color) const = 0; // the actual work is
}; // done in this func

class Rectangle: public Shape {
public:
...

private:
virtual void doDraw(ShapeColor color) const; // note lack of a
... // default param val.
};

Things to Remember

Never redefine an inherited default parameter value, because default parameter values are statically bound, while virtual functions — the only functions you should be overriding — are dynamically bound.

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