#include <iostream>
#include <string>

class B
{
public:
  void say(const std::string& msg) const { std::cout << "\n  " << msg; }
  B() { say("B default ctor"); }
  B(const B& b)
  {
    say("B copy ctor");
  }
  virtual ~B() {}
  B& operator=(const B& b)
  {
    say("B assignment");
    if(&b == this) return *this;
    // no state to assign
    return *this;
  }
};

class D : public B
{
public:
  D() : B() { say("D default ctor"); }
  D(const D& d) : B(d) { say("D copy ctor"); }
  D& operator=(const D& d)
  {
    say("D assignment");
    if(&d == this) return *this;
    B::operator=(d);
    i = d.i;
    return *this;
  }
private:
  int i;
};

B* f(const B& obj)
{
  obj.say("f(const B& obj)");

  // The next statement will never compile.
  // You can't promote base to derived.

  //  return new D(obj);

  // this works:
  const D* pD = dynamic_cast<const D*>(&obj);
  if(pD)
    return new D(*pD);
  else
    throw std::exception("can't promote base to derived");
}

void main()
{
  D d;
  B b;
  try {
    // attempting to assign to d

    D* pD = dynamic_cast<D*>(f(d));
    if(pD)
      d = *pD;

    // attempting to assign to b

    b = *f(d);
  }
  catch(std::exception& ex)
  {
    std::cout << "\n  " << ex.what();
  }
  std::cout << "\n\n";
}