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From: Larry Evans (jcampbell3_at_[hidden])
Date: 2003-05-29 09:13:50

Peter Dimov wrote:
> Larry Evans wrote:
>>Chuck Messenger wrote:
>>>The basic idea is to intercept all memory allocations -- p = new X;
>>>saving the info with "map[p] = sizeof(X);". To find the
>>>interconnections between objects, you do this:
>>> for (map_type::iterator it = map.begin(); it != map.end(); ++it)
>>> { char *p = it->first;
>>> unsigned size = it->second;
>>> for (char *v = p; v + 4 <= p + size; v += 4) {
>>> char *q = *(reinterpret_cast<char **>(v));
>>> if (map.count(q)) {
>>> // connection detected from object p to object q
>>> }
>>> }
>>> }
>>This is similar to Mirek Fidler's code:
> If by "similar to" you mean "derived from" or "inspired by" (an euphemism
> for "plagiarized" or "borrowed" which is itself an e. f. "stolen") then no,
> it is not "similar". sp_collector.cpp is written entirely from scratch. It
I very much apologize for the possible implication made by my poor choice
of words.
> is not even derived from or inspired by (no quotes) Greg's cyclic_ptr
> although of course as the implemented algorithm is basically the same there
> are obvious similarities.
> FWIW, it is not similar (no quotes) to Mirek Fidler's code, either. I just
> checked.
By similarity, I was referring to the fact that each allocated object
had it's size stored somewhere (it->second in the above) and that
the memory was scanned in each object to see if it contained
another pointer ( or a word that could be interpreted as pointer. e.g.
in sp_collector this, I believe, the:
   reinterpret_cast<shared_ptr_layout const *>(p)

in scan_and_count). Fidler also used a similar method to cylic_ptr's
(and Christopher's "global rc mark-scan") to determine roots by
finding all objects pointed to by root pointers (by subtracting the count
due to pointers from the heap). This is again, similar to sp_collector
as you noted above. Of course this is again based on an incomplete
understanding of sp_collector. After a closer look, I didn't notice
any comment about finding roots, as with Fidler's method. This make quess
that sp_collector avoided actually decrementing the reference counts, as
in Christopher's and cyclic_ptr) and only stored the count's in
a map (the map2_type). Then, during the heap scan, a check was made to
see if the reference count was == that in the map2_type, and if so, then
the object was garbage (since map2_type contains internal counts). OK,
now I'm realizing I need to spend more time understanding sp_collector.
Maybe you could clarify. Maybe Fidler could make any corrections
of my understanding of his code too.

I in no way meant to imply anything negative. I apologize for any

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