#include <iostream>
using namespace std;


class D : public B {

public:

   D() {};
   
   D(std::string a);

   virtual ~D() {};

   virtual void aB();
   
   std::string m_string; 
   
   template<class Archive> void serialize(Archive &ar, const unsigned int version)
   {
       ar & boost::serialization::base_object<B>(*this);
	   ar & m_string;
   } 
};