[C++-sig] Best way to register shared_ptr<const T>

[Alex Mohr]

> It looks you know a lot about it... would you be able to help me on the other issue I posted recently:
> http://mail.python.org/pipermail/c++-sig/2008-January/013254.html
> 
> somehow boost::python is able to tell when 2 different instances of a boost::shared_ptr<> point to 
> the same C++ object.
> 
> This is not the case for custom smart pointers.
> 
> I'm happy to investigate, but I could not find where to start. Do you know how it works?

Boost.python has special-case code for shared_ptr.  When boost.python 
creates a shared_ptr for a class, it supplies a special custom deleter 
that tracks the python object identified with the c++ object.

When a shared_ptr is converted to_python, boost.python checks to see if 
it has its special deleter, and if so, it can get the existing python 
object from it.  That's how "object identity" is maintained for shared_ptr.

It is possible to make object identity work for custom smart pointers, 
but it is hard.  What we're doing is totally gross.  After a class_<> is 
  registered with boost.python, I manually replace the to_python 
converter that boost.python registered with my own that wraps it but 
also does custom object-identity mapping.  This requires digging into 
the conversion registry using private api, and doing nasty stuff, but 
you *can* make it work.

Alex