>> And what's wrong with just shared_ptr<B>? Why do you need the
>> additional level of indirection?
>>
> For example
>
> int main()
> {
>     shared_ptr<B> p(new D1);
>     shared_ptr<B> user1 = p;  // User1 uses p
>     p.reset(new D2); // p changes pointed type to D2, user1 still points to
> D1
>     return 0;
> }
>
>
> or
>
> int main()
> {
>     shared_ptr<B> p(new D1);
>     shared_ptr<B>* user2 = &p;  // User2 uses p, but the p.use_count() is
> still 1.
>     p.reset(new D2); // p changes pointed  type to D2, *user2 points to D1
>     return 0;
> }

Ok, so you actually want the following:
1) The container itself shouldn't have "strong" references to your objects
2) An object can be recreated/replaced when it doesn't have any strong
references.
Am I right?

If so, store weak_ptr<Base> in your container:

// pseudo-code
int main()
{
 multi_index_container<
   weak_ptr<B>,
  // some indices
 > container;
 shared_ptr<B> p(new D1);
 container.insert(p);

 // at some futher stage:
 auto iterator = container.find(someKey);
 auto p = iterator->lock();
 // if the weak_ptr expired, i.e. no other clients use object, you
may re-create it
 if (!p)
   recreateIt();
}