c++ - Inheritance, selective method execution -

i watched video can found @ https://www.youtube.com/watch?v=4f72vulwfvc , liked of concept cases presented. working linked list , need selective method execution, example:

#include <stdio.h> class {  public:     a() : next(0) {         if (head == 0) {             head = this;         } else {             a* step = head;             while (step->next != 0) {                 step = step->next;             }             step->next = this;         }     }     virtual ~a() {         if (head == this) {             head = 0;         } else {             a* step = head;             while (step->next != this) {                 step = step->next;             }             step->next = next;         }     }     virtual void foo() {         // nothing...     }     static a* head;     a* next; };  class b : public {  public:     b() {}     virtual ~b() {}     virtual void foo() {         printf("function foo\n");     } };  a* a::head = 0;  int main() {     a_cls;     b b_cls;      a* step = a::head;      while (step != 0) {         step->foo();         step = step->next;     }      return 0; } 

after instantiating of objects, method foo() of objects of class b need execute. achieve this, virtual method foo() added class a, empty body, virtual void foo() {}, , in class b, code added method foo() body.

it works not it, in main function looks doing @ each node, not, feels null pointer. there creative solution this?

note: using c++03.

check out dynamic_cast way check particular derived type , call foo on objects of class b (or class derived b):

int main() {     a_cls;     b b_cls;      a* step = a::head;     b* step_b = 0;      while (step != 0) {         step_b = dynamic_cast<b *>(step);         if (step_b != 0) {             step_b->foo();         }         step = step->next;     }      return 0; } 

this way, there's no need define empty foo method on a. try out on ideone.