struct A {
  int c = -1;
  virtual ~A() {}
};

struct B : A {int aa = 0;};

const A& getA(){
  std::unique_ptr<A> ap(new B);
  return *ap;
}


int main()
{
  const A& a = getA();
  A acopy = a;
  acopy.c = -2;
  A& acopyr = acopy;
  std::cout << a.c << std::endl;
  try{
    B& b = dynamic_cast<B&>(acopyr);
    std::cout << b.aa << std::endl;
  }catch(std::bad_cast b){
    std::cout << "bad" << std::endl;
  }
}

-1
bad