Re: [Boost-users] [smart_ptr] Sole ownership of shared_ptr?

From: Richard L. Aurbach (rick_at_[hidden])
Date: 2008-04-07 12:33:31

On Apr 7, 2008, at 11:00 AM, Joseph Thomson wrote:

> Actually, I think I have a better way which uses auto_ptr (or the  
> boost ptr
> containers) as you suggested.  It still isn't thread safe, but this  
> isn't of
> immediate concern, and there are other methods for this anyway.
>
> Before, I was storing a map of shared_ptrs in the ObjectManager and a
> weak_ptr reference in any ObjectHandles to check if the object is  
> still
> valid.  Instead:
>
> - A map of auto_ptrs (or a ptr_map of Objects) would be stored in the
> ObjectManager.
> - On its creation, Object would create and store a  
> shared_ptr<ObjectHandle>,
> passing a raw pointer to it in the process (don't worry, this is still
> safe).
> - Any user wanting to get access to the Object would request an  
> ObjectHandle
> from ObjectManager, which would in turn request a copy of Object's
> shared_ptr<ObjectHandle>.
> - On its destruction (by ObjectManager), Object would inform its
> ObjectHandle of its destruction, and ObjectHandle would set an  
> internal flag
> to indicate that it is expired.
> - ObjectHandle would handle any requests from the user by first  
> checking the
> expiry flag, and then forwarding the call to Object using its stored  
> raw
> pointer.  The expiry flag is set by the Object itself, so there is  
> no chance
> of the pointer being invalid when called.
>
> Any advice/thoughts are appreciated.
>
> Cheers,
>
> Joe

Joe,

Suppose that: (a) ObjectManager uses shared_ptrs, not auto_ptrs,
and (b) the base class of all Objects derives from  
boost::tr1::enable_shared_from_this.

Then suppose the logic of ObjectHandle was something like:

        shared_ptr<Object>		ptr = Object->shared_from_this();
        if (ptr) {
                ptr->perform-requested-action();
        }
        // shared ptr is reset when it goes out of scope

Wouldn't this:
 >> preserve the single point of deletion (since the shared_ptr above  
is local).
 >> be thread-safe (because 'ptr' prevents the object from being  
deleted until the operation is complete.

Or am I overlooking something really obvious in my (perhaps  
overly-)quick response?

Cheers,

Rick