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From: Peter Dimov (pdimov_at_[hidden])
Date: 2003-05-29 06:06:52

Chuck Messenger wrote:
> boost/libs/smart_ptr/src/sp_collector.cpp:
> There is no sample program to compile/run, so I have to guess somewhat
> at how to use this one.

There is libs/smart_ptr/test/collector_test.cpp. You need to #define

The basic idea of the algorithm is to keep track of all heap objects managed
by a shared_ptr. When the collector is invoked, it can use this information,
coupled with a memory scan in these heap blocks, to compute the reference
counts that would have occured if not for the external shared_ptr instances
that are unknown to the collector.

Using the difference between the computed counts and the actual counts, the
collector can infer which blocks are reachable (they have a larger real

Greg's cyclic_ptr uses the same algorithm except that his code decrements
the counts in place instead of maintaining a parallel count structure. It
also uses x = x; assignments (a clever trick) to discover the pointer

Both pointer discovery methods are flawed, although in different ways. :-)
It is not possible to write a portable collector using this algorithm
without compiler support.

Another class of garbage collected smart pointers keeps track of every smart
pointer _instance_ using a global registry (sp_collector.cpp keeps track of
every _object_). The upside is perfect discovery. The downside is that every
smart pointer copy incurs much larger overhead, esp. in multithreaded code
where the central registry becomes a bottleneck.


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