Re: [Boost-users] [SmartPtr] Memory Leak Perhaps

Subject: Re: [Boost-users] [SmartPtr] Memory Leak Perhaps
From: Benjamin Sobotta (mayday_at_[hidden])
Date: 2010-09-20 09:08:56

On Monday 20 September 2010 02:44:12 pm you wrote:
> Dear Benjamin,
>
> > shared_ptr to this are dangerous. As soon as you do that
> > twice you already have a problem.
>
> Oh, that's interesting. Could you please explain why?

Well because you can only erase 'this' once. When you have two _independent_ 
smart pointers, both will attempt to delete the object they're pointing to.

>
> > If you want to create shared_ptrs to this, use
> > shared_from_this.
> >
> > http://www.boost.org/doc/libs/1_44_0/libs/smart_ptr/enable_shared_from_th
> >is.html
>
> Thank you for the reference. At the very bottom of the documentation, I see
> "There must exist at least one shared_ptr instance p that owns t" (in the
> requirements of template<class T> shared_ptr<T const>
> enable_shared_from_this<T>::shared_from_this() const). So, does the
> following code not meet this requirement?
>
> class Y: public T, public boost::enable_shared_from_this<Y>
> {
> public:
>
>     boost::shared_ptr<Y> f() const
>     {
>         return shared_from_this();
>     }
> };
>
> const T* tp = new Y;//where Y also derives from T
> boost::shared_ptr<T> q = tp->f();

Yes, as far as I know this won't work. But why would you want that?

boost::shared_ptr<T> q = boost::make_shared<Y>();

is the safe way to do it. No explicit 'new', no explicit 'delete', no 
problems.

Call your insert_me_into routine through the shared_ptr. Then, inside 
insert_me_into, after you're done with your computations, use shared_from_this 
to get a shared_ptr to stick into the container.

> TIA,
> --Hossein

HTH,

Benjamin