C++_day8pm_多态

1.什么是多态

　　示例代码：　　

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 //形状:位置、绘制
 6 //+--圆形:半径、（绘制）
 7 //+--矩形:长宽、（绘制）
 8 //形状
 9 class Shape{
10 public:
11         Shape(int x, int y): m_x(x), m_y(y){}
12         virtual void draw(void) const
13         {
14             cout << "形状(" << m_x << ',' << m_y << ')' << endl;
15         }
16     
17 protected:
18     int m_x;
19     int m_y;
20 };
21 
22 //圆形
23 class Circle: public Shape{
24 public:
25     Circle(int x, int y, int r): Shape(x, y), m_r(r){}
26     void draw(void) const
27     {
28         cout << "圆形（" << m_x << ',' << m_y << ',' << m_r << ')' << endl;
29     }
30 private:
31     int m_r;
32 };
33 
34 //矩形
35 class Rectangle: public Shape{
36 public:
37     Rectangle(int x, int y, int w, int h): Shape(x, y), m_w(w), m_h(h){}
38     void draw(void) const
39     {
40         cout << "矩形(" << m_x << ',' << m_y << ',' << m_w << ',' << m_h << ')' << endl;
41     }
42 private:
43     int m_w;
44     int m_h;
45 };
46 
47 void render(Shape* shapes[])
48 {
49     for(size_t i = 0; shapes[i]; ++i)
50         shapes[i]->draw();
51 }
52 
53 void drawAny(Shape const& shape)
54 {
55     shape.draw();
56 }
57 
58 int main(void)
59 {
60     Shape* shapes[10] = {0};
61     shapes[0] = new Circle (1,2,3);
62     shapes[1] = new Circle(4,5,6);
63     shapes[2] = new Rectangle(7,8,9,10);
64     shapes[3] = new Rectangle(11,12,13,14);
65     shapes[4] = new Circle(15,16,17);
66     render(shapes);
67     /*
68     Circle c(18,19,20);
69     Shape& r = c;
70     r.draw();
71     */
72     Circle cr(18,19,20);
73     drawAny(cr);
74     Rectangle rc(21,22,23,24);
75     drawAny(rc);
76     
77     return 0;
78 }

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。

 1 //模板方法模式---好莱坞模式
 2 #include <iostream>
 3 
 4 using namespace std;
 5 
 6 class Parser{
 7 public:
 8     void parse(char const* filename)
 9     {
10         cout << "开始解析" << filename << "文件..." << endl; 
11         drawText();
12         drawCircle();
13         drawRectangle();
14         drawImage();
15         cout << "解析" << filename << "文件完成！" << endl;
16     }
17     
18 private:
19     virtual void drawText(void) = 0;
20     virtual void drawCircle(void) = 0;
21     virtual void drawRectangle(void) = 0;
22     virtual void drawImage(void) = 0;
23 
24 };
25 
26 class Render: public Parser{
27 private:
28     void drawText(void)
29     {
30         cout << "绘制文字..." << endl;
31     }
32     void drawCircle(void) 
33     {
34         cout << "绘制圆形..." << endl;
35     }
36     void drawRectangle(void)
37     {
38         cout << "绘制矩形..." << endl;
39     }
40     void drawImage(void)
41     {
42         cout << "绘制图像..." << endl;
43     }
44 };
45 
46 int main(void) 
47 {
48     Render render;
49     render.parse("stdc++.pdf");
50     
51     return 0;
52 }

注：虚表放在类中，虚表指针放在对象中。