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

From: Chuck Messenger (chuckm_at_[hidden])
Date: 2003-05-30 14:43:27

Gregory Colvin wrote:

>> 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.

OK -- so I see why you're saying it's the same as sp_collector -- 
because it covers the same amount of memory.  Of course, sp_collector
is merely scanning the memory, while you're constructing objects
throughout the memory.  Also, sp_collector fails to scan alot of
memory which it needs to -- cyclic_ptr never fails to "scan" the
required memory.

>  And it would best if the
> uber-pointer used a less intrusive discovery method. 

Ah yes - the old "uber-pointer" from way back (maybe yesterday...).  I 
ended up tossing its intrusive discovery method, as it was inefficient.

However, that led me to my new-and-improved "juju_pointer".  It achieves
perfect discovery, but without any (significant) overhead.

> I think
> sp_collector is on the right track, if it could be made precise.

"precise" could mean 2 things here.  sp_collector has the obvious
problem that it fails to examine all the memory which could contain
"internal" pointers (that is, pointers which will be destroyed when
a shared_ptr-owned object is destroyed).  In particular, it won't
examine the contents of any STL containers.

Another meaning of "precise" would be in the direction of "perfect 
discovery" -- i.e. knowing exactly where in each structure the pointers 
are, so that an sp_collector-style raw memory scan, or cyclic_ptr's
"*p = *p", isn't necessary.  (Am I using the phrase "perfect discovery" 
correctly?)

Which did you mean?

Do you believe (my definition of) "perfect discovery" is a worthy design 
objective, if it can be done without significant overhead?

     - Chuck Messenger