RTTI (Run-time type information) in C++

　　In C++, RTTI (Run-time type information) is available only for the classes which have at least one virtual function.

　　For example, dynamic_cast uses RTTI and following program fails with error “cannot dynamic_cast `b’ (of type `class B*’) to type `class D*’ (source type is not polymorphic) ” because there is no virtual function in the base class B.

 1 #include<iostream>
 2
 3 using namespace std;
 4
 5 class B
 6 {
 7 };
 8 class D: public B
 9 {
10 };
11
12 int main()
13 {
14     B *b = new D;
15     D *d = dynamic_cast<D*>(b);
16     if(d != NULL)
17     {
18         cout<<"works";
19     }
20     else
21     {
22         cout<<"cannot cast B* to D*";
23     }
24     getchar();
25     return 0;
26 }

　　Adding a virtual function to the base class B makes it working.

 1 #include<iostream>
 2
 3 using namespace std;
 4
 5 class B
 6 {
 7 public:
 8     virtual void fun()
 9     {
10     }
11 };
12 class D: public B
13 {
14 };
15
16 int main()
17 {
18     B *b = new D;
19     D *d = dynamic_cast<D*>(b);
20     if(d != NULL)
21     {
22         cout<<"works";
23     }
24     else
25     {
26         cout<<"cannot cast B* to D*";
27     }
28     getchar();
29     return 0;
30 }

　　

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　　2013-11-26　　21:13:16