C++_day8pm_多态

1.什么是多态

　　

　　

2.

　　

　　示例代码：　　

#include <iostream>

using namespace std;

//形状:位置、绘制
//+--圆形:半径、（绘制）
//+--矩形:长宽、（绘制）
//形状
class Shape{
public:
        Shape(int x, int y): m_x(x), m_y(y){}
        virtual void draw(void) const
        {
            cout << "形状(" << m_x << ',' << m_y << ')' << endl;
        }
    
protected:
    int m_x;
    int m_y;
};

//圆形
class Circle: public Shape{
public:
    Circle(int x, int y, int r): Shape(x, y), m_r(r){}
    void draw(void) const
    {
        cout << "圆形（" << m_x << ',' << m_y << ',' << m_r << ')' << endl;
    }
private:
    int m_r;
};

//矩形
class Rectangle: public Shape{
public:
    Rectangle(int x, int y, int w, int h): Shape(x, y), m_w(w), m_h(h){}
    void draw(void) const
    {
        cout << "矩形(" << m_x << ',' << m_y << ',' << m_w << ',' << m_h << ')' << endl;
    }
private:
    int m_w;
    int m_h;
};

void render(Shape* shapes[])
{
    for(size_t i = 0; shapes[i]; ++i)
        shapes[i]->draw();
}

void drawAny(Shape const& shape)
{
    shape.draw();
}

int main(void)
{
    Shape* shapes[10] = {0};
    shapes[0] = new Circle (1,2,3);
    shapes[1] = new Circle(4,5,6);
    shapes[2] = new Rectangle(7,8,9,10);
    shapes[3] = new Rectangle(11,12,13,14);
    shapes[4] = new Circle(15,16,17);
    render(shapes);
    /*
    Circle c(18,19,20);
    Shape& r = c;
    r.draw();
    */
    Circle cr(18,19,20);
    drawAny(cr);
    Rectangle rc(21,22,23,24);
    drawAny(rc);
    
    return 0;
}

 协变类型只能是指针或者引用，不能是对象。

 

纯抽象类：

　　全部由纯虚函数构成的抽象类称为纯抽象类或者接口。

动态： 编译阶段产生，运行阶段执行。

//模板方法模式---好莱坞模式
#include <iostream>

using namespace std;

class Parser{
public:
    void parse(char const* filename)
    {
        cout << "开始解析" << filename << "文件..." << endl; 
        drawText();
        drawCircle();
        drawRectangle();
        drawImage();
        cout << "解析" << filename << "文件完成！" << endl;
    }
    
private:
    virtual void drawText(void) = 0;
    virtual void drawCircle(void) = 0;
    virtual void drawRectangle(void) = 0;
    virtual void drawImage(void) = 0;

};

class Render: public Parser{
private:
    void drawText(void)
    {
        cout << "绘制文字..." << endl;
    }
    void drawCircle(void) 
    {
        cout << "绘制圆形..." << endl;
    }
    void drawRectangle(void)
    {
        cout << "绘制矩形..." << endl;
    }
    void drawImage(void)
    {
        cout << "绘制图像..." << endl;
    }
};

int main(void) 
{
    Render render;
    render.parse("stdc++.pdf");
    
    return 0;
}

 注：虚表放在类中，虚表指针放在对象中。