main.cpp

#include <iostream>
#include <typeinfo>
#include <vector>
#include <tr1/memory>

struct Base
{
        virtual void set(const Base &b)
        { throw "not implemented"; }

        virtual Base& index() const
        { throw "not implemented"; }

        virtual Base* clone() const
        { throw "not implemented"; }
};

struct Derived : Base
{
        virtual Base* clone() const
        { return new Derived(*this); }
};

struct List : Base
{
        List()
        {}
        List(const List &orig)
        { _x.push_back(std::tr1::shared_ptr<Base>(orig._x[0]->clone()));}

        virtual void set(const Base &b)
        { _x.push_back(std::tr1::shared_ptr<Base>(b.clone())); }

        virtual Base& index() const
        {
                std::vector<std::tr1::shared_ptr<Base> >::const_iterator it = _x.begin();
                return *((*it).get());
        }

        virtual Base* clone() const
        { return new List(*this); }

        std::vector<std::tr1::shared_ptr<Base> > _x;
};


int main()
{

        try
        {
                Derived d;
                List sublist;
                sublist.set(d);

                List l;
                l.set(sublist);

                Base &b = l.index();
                std::cout << "b knows that it is a list: " << typeid(b).name() << std::endl;

                std::cout << "but index is not implemented???" << std::endl;
                l.index().index();
        }
        catch(const char *e)
        {
                std::cerr << "ERROR: " << e << std::endl;
        }
        return 0;
}