Re: [Boost-users] How to make multi_index_container work withstd::unique_ptr?

Subject: Re: [Boost-users] How to make multi_index_container work withstd::unique_ptr?
From: Jayden Shui (jayden.shui_at_[hidden])
Date: 2011-12-21 07:51:01

On Wed, Dec 21, 2011 at 4:54 AM, Igor R <boost.lists_at_[hidden]> wrote:

> >> 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();
> }
>

Hi, Igore,

I'd like to first thank you for keeping helping me one this problem. I'd
like to try to make my requirements clear by some pseudo codes.

int main()
{
   multi_index_container<
       shared_ptr<scoped_ptr<B>>,
       ..named_index..
   > container;     // container own the objects.

   // Insert object M of type D1 into container
   container.insert(shared_ptr<new scoped_ptr<B>(new D1("M")));
   // more inserts follows.

   // A user uses an object in the container.
   User user;
   user.Use(*container.find("M"));

   // Erase object M may be invalid if A is used. Thus we count its number
of uses
   if (container.find("M")->use_count() == 1)
   {
       container.erase("M");
   }

  // Replace object M in the container to N of type D2. user use object N
now since it share info with container.
  itorator it = container.find("M");
  shared_ptr<scoped_ptr<B>>& ppb = *it;
  (*ppb).reset(new D2("N"));
  container.replace(it, ppb);

}

I am look for simple ways (not double pointer) to implement those
requirements.

Best regards,

Jayden