c++之NVI手法

non-virtual interface(NVI)手法:令用户通过public non-virtual成员函数间接调用private virtual函数，将这个non-virtual函数称为virtual函数的wrapper.wrapper确保得以在一个virtual函数被调用之前设定好适当场景，并在调用结束之后清理场景.注意此时用指针或引用调用该non-virtual成员函数时，该non-virtual成员函数调用的virtual函数取决于指针或引用的动态类型，即存在多态

enum ShapeColor { Red, Green, Blue };
class Shape
{
public:
    void draw(ShapeColor color = Red) const
    {
        doDraw(color);
    }
private:
    virtual void doDraw(ShapeColor color) const = ;
};
 
class Rectangle :public Shape
{
public:
 
private:
    virtual void doDraw(ShapeColor color) const;
};
 
void Rectangle::doDraw(ShapeColor color) const
{
    cout << "Rectangle's doDraw" << "color= " << color << endl;
}
 
void main()
{
    Shape*p = new Rectangle;
        ////输出:Rectangle's doDrawcolor= 0
    p->draw();
    system("pause");
}

---

enum ShapeColor { Red, Green, Blue };
class Shape
{
public:
    void draw(ShapeColor color = Red)const
    {
        doDraw(color);
    }
private:
    virtual void doDraw(ShapeColor color) const
    {
        cout << "Shape's doDraw" << "color= " << color << endl;
    }
};
 
class Rectangle :public Shape
{
public:
 
private:
    virtual void doDraw(ShapeColor color) const;
};
 
void Rectangle::doDraw(ShapeColor color) const
{
    cout << "Rectangle's doDraw" << "color= " << color << endl;
}
 
/*
输出:
Shape's doDrawcolor= 0
Rectangle's doDrawcolor= 0
*/
void main()
{
    Shape*p = new Shape;
    p->draw();
    delete p;
    p = NULL;
    Shape*q = new Rectangle;
    q->draw();
    delete q;
    q = NULL;
 
    system("pause");
}

---

enum ShapeColor { Red, Green, Blue };
class Shape
{
public:
 
    void draw(ShapeColor color = Red)const
    {
        doDraw(color);
    }
private:
    void doDraw(ShapeColor color) const
    {
        cout << "Shape's doDraw" << "color= " << color << endl;
    }
};
 
class Rectangle :public Shape
{
public:
 
private:
    void doDraw(ShapeColor color) const;
};
 
void Rectangle::doDraw(ShapeColor color) const
{
    cout << "Rectangle's doDraw" << "color= " << color << endl;
}
/*
输出:
Shape's doDrawcolor= 0
Shape's doDrawcolor= 0
*/
int main()
{
    Shape*p = new Shape;
    p->draw();
    delete p;
    p = NULL;
    Shape*q = new Rectangle;
    q->draw();
    delete q;
    q = NULL;
 
    system("pause");
    return ;
}