[C++-sig] Bug and patch for boost.python with enable_shared_from_this

[John Trimble]

It worked! That problem had been driving me crazy for the past 2 days.
I don't know why I couldn't find the answer via Google. Perhaps its
because of all the brain damage from my Java programming. Thank you so
much.

- John

2008/2/10 Chad Austin <chad at imvu.com>:
> Hi all,
>
> A month ago or so we discovered a bug in Boost.Python that manifests when
> using enable_shared_from_this.  Here is a test case that demonstrates the
> bug:
>
>
> #include <boost/python.hpp>
>  #include <boost/enable_shared_from_this.hpp>
>
>  using namespace boost;
>  using namespace boost::python;
>
>  class Test;
>  typedef shared_ptr<Test> TestPtr;
>
>  class Test : public enable_shared_from_this<Test> {
>  public:
>      static TestPtr construct() {
>          return TestPtr(new Test);
>      }
>
>      void act() {
>          TestPtr kungFuDeathGrip(shared_from_this());
>      }
>
>      void take(TestPtr t) {
>      }
>  };
>
>  void Export_test() {
>      class_<Test, TestPtr, noncopyable>("Test")
>          .def("construct", &Test::construct)
>          .staticmethod("construct")
>
>          .def("act", &Test::act)
>          .def("take", &Test::take)
>          ;
>  }
>
>  And here is the Python:
>
>          x = Test.construct()
>          x.take(x)
>          x.act()
>
>  The bug (as I understand it) is that the shared_ptr_from_python converter
> creates a new shared_ptr for the Test instance to pass it into C++.  This
> new shared_ptr has a nop deleter, except that it keeps the Python object
> alive as long as the new shared_ptr is alive.  The problem here is that
> creating a shared_ptr to an object of type T when T is
> enable_shared_from_this<T> resets the weak pointer inside of
> enable_shared_from_this<T>.  (See shared_ptr's constructors for the details
> of the implementation.)  A fix that passes the test above and has worked for
> us is to change shared_ptr's constructor to accept a
> dont_enable_shared_from_this argument and pass that in from the
> shared_ptr_from_python converter.  The patch is attached (against boost
> 1.33, but the bug didn't look fixed in 1.34 either).
>
> Cheers,
> Chad
>
> p.s. I have seen other people with this problem as well.  Googling for
> bad_weak_ptr turns up several results.
>
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