Re: [boost] Re: Cyclic pointers - a comparison of known Boost efforts

From: Gregory Colvin (gregory.colvin_at_[hidden])
Date: 2003-05-30 14:06:40

On Friday, May 30, 2003, at 12:38 America/Denver, Chuck Messenger wrote:

> Gregory Colvin wrote:
>> Thanks, but your description of cyclic_ptr is pretty far off the mark.
>
> Sorry -- hopefully you'll forgive my ignorance. I'm just throwing out 
> what I understand to be the case, in the hopes that someone (such as 
> yourself) would set me straight...
>
>> It does not maintain a global map, and copying cyclic_ptr cost the 
>> same
>> as copying shared_ptr.
>
> OK, I'll have to review the old messages again.  I thought someone had 
> said cyclic_ptr used a global map.  Sorry.  I suppose better yet would 
> be for me to study your actual *code*...

Larry's analysis gives a better explanation than I could.

> Do you do a raw scan of memory, like sp_collector, then?  (i.e. not 
> perfect discovery)

No.  The self-assignment trick is precise, unless it is foiled by a
user-written operator=.

>> The special assignment mode is used only during
>> the mark phase of a collection, and costs more or less the same as any
>> other discovery method.
>
> Suppose I have the structure:
>
>     struct Image {
>         char huge_image[ONE_ZILLION];
>         cyclic_ptr<whatever> ptr;
>     };
>
> As I understand it, in order to discover 'ptr', you'd invoke 
> operator=() on each Image structure you came to.  Correct, or 
> incorrect?

Correct.  So it would be smart to write an operator= for Image that
avoids copying huge_image to itself.  And it would best if the
uber-pointer used a less intrusive discovery method.  I think
sp_collector is on the right track, if it could be made precise.