Re: Containers for derived classes

This is a reply to a post on C++Sig..

At 18:07 2003-08-08, David Abrahams wrote:
>[email protected] writes:
>
> > I.e. the identity is not preserved. Could this be a problem with MinGW,
> > which I am using?
> > To be sure, here is the complete code again:
> >
> > #include <boost/shared_ptr.hpp>
> > #include <boost/python.hpp>
> >
> > using namespace boost;
> >
> > class A {
> >      public:
> >       A(int i) : val(i) { }
> >       int val;
> > };
> > typedef shared_ptr<A> A_ptr;
> >
> > class C {
> >      public:
> >       void set(A_ptr a) { this->a=a; }
> >       A_ptr get() { return this->a; }
> >       void f() { a.get()->val *= 2; }
> >      private:
> >       A_ptr a;
> > };
> >
> > BOOST_PYTHON_MODULE(ptr_test)
> > {
> >     using namespace boost::python;
> >     class_<A, A_ptr>("A", init< int >() )
> >       .def_readwrite("val", &A::val)
> >     ;
> >     class_<C>("C" )
> >       .def("set", &C::set )
> >         .def("get", &C::get )
> >         .def("f",   &C::f )
> >     ;
> > }
>
>OK, I understand the problem now.  It's not a MinGW issue.  The
>problem is that when you ask for A to be held by A_ptr, an A_ptr
>lvalue converter gets registered, since there is actually an A_ptr
>object inside the Python wrapper.  Lvalue converters can be used where
>only an rvalue is needed, as in:
>
>      void set(A_ptr a)
>
>and they get special priority over rvalue converters.  The only way to
>get the behavior you want is actually to NOT hold A objects by A_ptr:
>
>     class<A>("A", init<int>() )
>        ...
>
>so that the rvalue converter that gets registered can be found.
>
>Clearly, we need special rules for shared_ptr, so that this won't
>happen.  I'm just not sure whether we need special rules for all
>smart pointer types, yet.  Any thoughts?

I don't think any other smart pointer types need this special handling,
although I guess it could simply be solved more generally by introducing
a real priority value for converters.

IIRC boost.python already has a specialization for C++ -> Python conversions
of shared_ptr's, so introducing one for Python -> C++ would also solve this 
right?

---
Daniel Wallin



-------------------------------------------------------
This SF.Net email sponsored by: Free pre-built ASP.NET sites including
Data Reports, E-commerce, Portals, and Forums are available now.
Download today and enter to win an XBOX or Visual Studio .NET.
http://aspnet.click-url.com/go/psa00100003ave/direct;at.aspnet_072303_01/01