Re: [Boost-users] shared_ptr sharing the same ref count

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-04-12 12:49:16

Loïc Joly wrote:

> Hello,
>
> I have the following classes:
>
> class Owned;
> class Owner
> {
>    unknownType getOwned(int i);
>    vector<Owned> owned;
> };
>
> Owner will be handled in the client code by shared_ptr. At some
> places, the client code will only know one owned object, obtained
> through getOwned, therefore, the owned object should contain a
> shared_ptr to its owner, to make it survive. Now, my question is:
> What should be unknownType?
>
> If I make unknownType a classic shared_ptr<Owned>, I have a circular
> reference. What I would like would be a way to have a kind of
> shared_ptr<Owned> that would use the ref count of the owner. Is this
> possible to do?

If you are using a Boost release, you can do:

shared_ptr<Owned> getOwned( shared_ptr<Owner> p, int i )
{
    return shared_ptr<Owned>( &p->owned[ i ], boost::bind( 
&shared_ptr<Owner>::reset, p ) );
}

which is essentially your:

> - Make unknownType a class that contain a shared_ptr<Owner> and a
> Owned*, and forward all its functions to Owned.

Of course you need to keep the vector<> from reallocating since your pointer 
will dangle.

If you are using the very latest CVS version that includes explicit support 
for shared_ptr aliasing, it's even easier:

shared_ptr<Owned> getOwned( shared_ptr<Owner> p, int i )
{
    return shared_ptr<Owned>( p, &p->owned[ i ] );
}

PS:

I can't resist the opportunity to correct the spelling of that function:

shared_ptr<Pwnt> getPwnt( shared_ptr<Pwn3rer> p, int i );

:-)