C++ dynamic_cast issue

Question

class C1
{
   public:
   C1* A;

   void SomeMethod()
   {
       class C2;
       C2* c2 = dynamic_cast<C2*>(A);
    }
};

class C2 : public C1 {};

In gcc i'm getting "target is not a pointer or reference to complete type" when dynamic_cast is invoked. What's wrong?

Answer 1

The following compiles:

class C2;

class C1
{
   virtual ~C1() { } // <--- NOTE MUST BE polymorphic to use dynamic_cast
   public:
   C1* A;

   void SomeMethod();
};

class C2 : public C1 {};

void C1::SomeMethod()
{
       C2* c2 = dynamic_cast<C2*>(A); // <=== USED after C2 definition 
}

int main() {

}

Two problems:

1. C2 was only forward declared, thus an incomplete type (need complete declaration of the class)
2. C1 was not polymorphic (dynamic_cast only applies to polymorphic types)