#include <iostream>
using namespace std;

struct A
{
   virtual void f() {std::cout << "A\n";}
};
struct B : public A
{
   virtual void f() {std::cout << "B\n";}
};

int main()
{
    A* a = new B;
    
    B* b = dynamic_cast<B*>(a);
    
    b->f();
    
    B& br = dynamic_cast<B&>(*a);
    
    br.f();
    
    try
    {
        A* a2 = new A;
        B& fail = dynamic_cast<B&>(*a2);
    }
    
    catch(std::exception& e)
    {
        cout << e.what() << "\n";
    }
    
    return 0;
}