能力查询

时间：2011-05-25 来源：Mr.Late

本文内容来源：《C＋＋必知必会》条款27

能力查询用于判断一个对象是否支持一个特定的操作，它是通过对“不相关”的类型进行dynamic_cast转换而表达的，这种dynamic_cast用法通常被称为"cross-cast"。

 1 #include <stdio.h>
 2 
 3 class Rollable{
 4 public:
 5     virtual ~Rollable(){
 6     }
 7     virtual void roll() = 0;
 8 };
 9 
10 class Shape{
11 public:
12     virtual ~Shape(){
13     }
14     virtual void draw() const = 0;
15 };
16 
17 class Circle: public Shape, public Rollable{
18 public:
19     void draw() const {
20         printf("A circle is drawing!\r\n");
21     }
22     void roll(){
23         printf("A circle is rolling!\r\n");
24     }
25 };
26 class Square: public Shape{
27 public:
28     void draw() const{
29         printf(" A square is drawing!");
30     }
31 };
32 int main(int argc, char **argv)
33 {
34     Shape *pShape = new Circle(); 
35     pShape->draw();
36     Rollable *pRoller = dynamic_cast<Rollable *>(pShape);
37     if(pRoller){
38         //the Circle class has implemented the Rollable interface, thus pRoller is not NULL
39         pRoller->roll();
40     }
41     Shape *pShape2 = new Square();
42     pShape2->draw();
43     Rollable *pRoller2 = dynamic_cast<Rollable *>(pShape2);
44     if(pRoller2){
45         //the Square class hasn't implemented the Rollable interface,thus pRoller isNULL
46         pRoller2->roll();
47     }
48     return 0;
49 }