[boost] Re: live_ptr (Was: Libraries requested or under construction)

From: Alan Griffiths (alan_at_[hidden])
Date: 1999-07-27 16:51:34

In message <v04020a00b3c2c5483805@[206.224.81.146]>, Chelly <postmast.ro
ot.admi.gov_at_[hidden]> writes
>At 08:43 PM +0100 1999.07.26, Alan Griffiths wrote:
>>I too have something similar - see:
>>
>>    http://www.octopull.demon.co.uk/arglib/uncounted_ptr.html
>
>(I fixed your URL)

Thanks. Sorry for the typo.

>An interesting property of these pointers is the shift in responsibility.
>With a shared pointer, all parties who may destroy the object must be set
>up to use one of the smart pointer types, whereas with one of these, only
>the party who needs to *know* of the deleted object needs to use a smart
>pointer.
>
>    struct Foo : live_base {
>        // ...
>    };

Of course there is the trade off that "live_base" is incorporated into
Foo - an intrusive mechanism which limits applicability. (The
alternative of incorporating Foo into a template that aggregates it with
the live_base is less flexible than using pointer types.)

[snip]
>
>live_ptr isn't about ownership, but about monitoring. 

Both our solutions (live_ptr and uncounted_ptr) are about this, the
difference is in implementation pattern.  You gain flexibility in
ownership mechanism by constraining Foo to incorporate "live_base". I
impose an ownership mechanism but remove the constraints on Foo.

You are clearly able to address a different set of problems, which means
that we are in "gut feel" territory in assessing the more general
approach.

>However, it can be
>used in similar ways - there could be multiple monitoring parties that all
>"own" the object, and when any of them delete the object, all the others
>know. 

This idea feels like a risky design - unless there is some inter-owner
protocol in place then a live_ptr could become dead(?) under code that
has checked it for use. I much prefer the simplicity of "single owner"
model.

>A variant of the live_ptr could be created that deletes the monitored
>object when it goes out of scope, if it hasn't already been deleted.

But don't return this variant from a function. :)
-- 
Alan Griffiths  (alan_at_[hidden])  http://www.octopull.demon.co.uk/
ACCU Chairman   (chair_at_[hidden])             http://www.accu.org/