[C++-sig] Cross Module Inheritance and Downcast

[Roman Yakovenko]

On Jan 11, 2008 8:28 PM, Joseph Lisee <jlisee at gmail.com> wrote:
> Hello All,
>
> I am having trouble getting cross module inheritance/downcasting to work.  All
> of the following types are wrapped up in shared_ptrs.  Here is the code layout:
>
> Shared Library A: Has the Base class.  It implements virtual functions of the
> Base class.  It also has a factory which produces objects referred to by a Base
> class shared_ptr.  This includes the ability to produce Derived class objects
> referred to by the Base class shared_ptr. It *does not* link to "Shared Library
> B", or know anything of the derived types.
>
> Shared Library B: Has the Derived class and links to "Shared Library A".  It
> implements virtual functions of the Derived class.
>
> Extension Module A: This is linked to "Shared Library A".  The base class is
> wrapped like so:
>
> class BaseWrapper: public Base, public bp::wrapper< Base > {
>    ... more code ...
> }
>
> void registerBaseClass() {
>     bp::class_<BaseWrapper,
>         bp::bases< ... more code ...>,
>         boost::noncopyable >( ... more code ...).
>     ... more code ...
> }
>
>
>
> Extension Module B: This is linked to both "Shared Library A", and "Shared
> Library B".  It wraps the derived class like so:
>
> struct Derived_wrapper : Derived, bp::wrapper< Derived > {
>    ... more code ...
> }
>
> void register_Derived_class(){
>     bp::class_< Derived_wrapper,
>         bp::bases< Base >,
>         boost::noncopyable >( ... more code ...).
>     ... more code ...
> }
>
>
> This issue is that when I call the factory in "Extension Module A" from python
> it only gives me the Base class objects.  It does not automatically downcast.  I
> do have both the Base and Derived class extension modules imported, and I
> imported the Base class one first.  I get this same behavior on Windows, Linux
> and Mac.
>
> So to fix this I tried to create a manual cast function:
> boost::shared_ptr<Derived> castTo(boost::shared_ptr<Base> from) {
>     return boost::dynamic_pointer_cast<Derived>(from);
> }
>
> Now the cast function works on Linux just fine, but on Windows and Mac it just
> returns the same python object I give it.
>
> Can anyone point me to how I should be doing this?  Has anyone got a system like
> this to work?

I could be wrong, but there is a known problem with the library. You
can search this mailing list and find the patch.

-- 
Roman Yakovenko
C++ Python language binding
http://www.language-binding.net/