[Boost-bugs] [Boost C++ Libraries] #2338: sp_collector.cpp reports reachable objects as unreachable

Subject: [Boost-bugs] [Boost C++ Libraries] #2338: sp_collector.cpp reports reachable objects as unreachable
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2008-09-17 22:12:18

#2338: sp_collector.cpp reports reachable objects as unreachable
--------------------------+-------------------------------------------------
 Reporter:  pdimov        |       Owner:  pdimov   
     Type:  Bugs          |      Status:  new      
Milestone:  Boost 1.37.0  |   Component:  smart_ptr
  Version:  Boost 1.36.0  |    Severity:  Problem  
 Keywords:                |  
--------------------------+-------------------------------------------------
 Jim Barry in http://lists.boost.org/boost-users/2008/09/40507.php:


 {{{
 I am trying to detect circular references using the
 find_unreachable_objects function in sp_collector.cpp. However, it reports
 false positives in the simple case of one object being owned by another.
 To illustrate:

   #define BOOST_SP_ENABLE_DEBUG_HOOKS
   #include <boost/shared_ptr.hpp>

   // sp_collector.cpp exported functions
   std::size_t find_unreachable_objects(bool report);

   struct X {};

   struct Y
   {
     boost::shared_ptr<X> pX;

     Y() : pX(new X) {}
   };

   int main()
   {
     boost::shared_ptr<Y> y(new Y);
     find_unreachable_objects(true);
     return 0;
   }

 The output is as follows:

   ... 2 objects in m.
   ... 1 objects in m2.
   ... 1 objects in open.
   Unreachable object at 0036BA60, 12 bytes long.

 Clearly this is incorrect as there are no circular references. Looking at
 the code, there is what appears to be a typo at line 124:

   std::cout << "... " << m2.size() << " objects in open.\n";

 should be:

   std::cout << "... " << open.size() << " objects in open.\n";

 But the real problem seems to be at line 121:

   if(p->use_count() != i->second) open.push_back(p);

 I'm not entirely sure what's going on in this section of code, but it ends
 up determining (incorrectly) that the "X" object is unreachable.
 }}}

-- 
Ticket URL: <http://svn.boost.org/trac/boost/ticket/2338>
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