[C++-sig] shared_ptr trouble

[Ralf W. Grosse-Kunstleve]

I'm having terrible trouble understanding how Boost.Python interacts with
shared_ptr. I'm trying to implement a small tree node class with parent-child
relationships via weak_ptr:

  class node : boost::noncopyable
  {
    private:
      node() {}

      boost::weak_ptr<node> parent_;

    public:
      std::string id;

    // ...
  };

To be used eventually like this:

  node1 = node.create(id="a")
  node2 = node.create(id="b", parent=node1)

node1 has no parent, node2 should use node1 as the parent. The equivalent C++
code is:

  std::string
  exercise_child_parent_id()
  { 
    boost::shared_ptr<node> node1 = node::create("a");
    boost::shared_ptr<node> node2 = node::create("b", node1);
    return node2->get_parent_id();
  }

A minimal, self-contained example is attached. The test is simply:

from boost_adaptbx_tree_ext import node, exercise_child_parent_id

def exercise():
  print "C++:", exercise_child_parent_id()
  node1 = node.create(id="a")
  node2 = node.create(id="b", parent=node1)
  print "Python:", node2.get_parent_id()

if (__name__ == "__main__"):
  exercise()

Output:

C++: a
Python: no parent

The C++ code produces the desired result, but the Python version fails. By
adding print statements to this function of the node class

      static
      boost::shared_ptr<node>
      create(
        std::string const& id,
        boost::shared_ptr<node> const& parent)
      {
        boost::shared_ptr<node> result(new node);
        result->id = id;
        result->parent_ = parent;
        return result;
      }

I found out that result->parent_->id is in fact "a" right before the return,
but back in Python the parent is lost. Since node is noncopyable I cannot
figure out how this can be, and what I need to do to fix the problem. I'd be
grateful for advice!

This is with Python 2.4, gcc 3.4 under Fedora Core 3 x86_64, using the Boost
CVS from yesterday.

Cheers,
        Ralf

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