Question

I’m having trouble with virtual methods. When I call f it doesn’t work. Why?

#include <iostream>

struct A {
   virtual void f() const { std::cout << "In A"; }
   virtual ~A() {};
};

struct B : A {
   void f() const { std::cout << "In B"; }
};

int main()
{
   A* a = new A();

   B* b = dynamic_cast<B*>(a);

   (*b).f();


   delete a;
}

It doesn’t print anything at all, and I don’t get any errors. What did I do wrong?

Answer 1

What is wrong is you did not check if retuned pointer is NULL.

dynamic_cast tells you if the actual object pointed by a is of the type b, which it is obviously not. And in such a scenario it will return you a NULL.

Basically, You are dereferencing a NULL pointer, causing an Undefined Behavior which unluckily for you doesn’t crash.

When you use a language provided feature it should be used in the way mandated by the standard. The use of dynamic_cast warrants a NULL check of the returned pointer.

Your pointer a should actually point to an derived class object b. You need:

   A* a = new B();
   B* b = dynamic_cast<B*>(a);

Also, your code must check the returned pointer:

   if(b != NULL)
       (*b).f();