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From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-06-16 07:16:57

On 06/16/2005 06:38 AM, Peter Dimov wrote:
> Larry Evans wrote:
>>to find the roots, at least AFAICT. Is this intentional, ...
> Yes, this is intentional, as reset_and_collect does not require any help
> from the implementation of shared_ptr, so it can be used with
> tr1::shared_ptr as well. It has been developed in response to Eric Niebler's
> scenario where the roots that form cycles are known to the programmer.

Ah! My mind's fog is lifting slightly ;)

> It's possible to combine the two approaches and perhaps provide a default
> enable_tracking that does a memory scan as in sp_collector. I think that in

I would be nice if somehow the method for finding a referent's contained
smart_ptr's could be a determined by a policy.

> most typical cyclic shared_ptr situations the problematic roots are known in
> advance, though, so a reset_and_collect-type solution may be enough.

Hadn't thought of that.

> Note that reset_and_collect is perfectly capable of figuring out on its own
> that unknown roots to a node exist, it only needs a starting point (and can
> be modified to work with several starting points).

Hmm... is this anything like the 'local mark scan' in [mart90] cited on
Jones page? Give a node, call it the subroot node, it traverses nodes
reachable from that while decrementing the refcounts. If the subroot
node still has a positve refcount, it can't be in a cycle; so, it
undoes the decrementing. I'm guessing that the difference is that:

   1) reset_and_collect doesn't decrement, it only increments the value
      associated with the node's key in a map_type.

   2) instead of user selection of the staring point, the selection is
      made each time a decrement is made, and the starting point is
      the smart_ptr making the decrement.

The modification to several starting points sounds like it may be a
variation of the method in [lins90a], where a decrement doesn't
immediately scan the subnodes, but waits until a que of decremented
smart_ptr's is filled, and then scan's nodes reachable from each
element in the que. Is that, at least approximately, what's happening
in reset_and_collect?

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