[C++-sig] [Py++] boost::shared_ptr casting

[peoro]

Yes, having all of the objects at their most-derived type would be
what I need, but it's not what happens.

I posted a piece of C++ code that shows what I do (I exported it using
py++ without any further modification): all of the pointers my
functions return are wrapped in boost::shared_ptr's, but from Python I
see them at the "derivation level" they were when I passed them to
python the first time (look at the summarized output in my previous
message)...


On Sat, Mar 20, 2010 at 10:06 PM, Jim Bosch <talljimbo at gmail.com> wrote:
> On Sat, 2010-03-20 at 21:31 +0100, peoro wrote:
>> Hello,
>>
>> I'm having some issues with shared_ptr's in a Python environment: it
>> looks like once a shared_ptr enters Python, it cannot be upcasted nor
>> downcasted.
>>
>> Here's a brief C++ source code to explain better the issue:
>>
>>
>> #include <boost/shared_ptr.hpp>
>>
>> class Base { };
>> class Derived : public Base { };
>>
>> boost::shared_ptr<Base> base1( ) {
>>   return boost::shared_ptr<Base>( new Base );
>> }
>> boost::shared_ptr<Base> base2( ) {
>>   return boost::shared_ptr<Base>( new Derived );
>> }
>> boost::shared_ptr<Base> base3( boost::shared_ptr<Derived> derived ) {
>>   return boost::shared_ptr<Base>( derived );
>> }
>> boost::shared_ptr<Derived> derived1( ) {
>>   return boost::shared_ptr<Derived>( new Derived );
>> }
>> boost::shared_ptr<Derived> derived2( boost::shared_ptr<Base> base ) {
>>   return boost::static_pointer_cast<Derived>( base );
>> }
>>
>>
>> then, after building the library and importing it into Python, I would
>> expect something like this:
>>
>>
>> from shared_ptr_inheritance import *
>> type( base1() ) -> Base
>> type( base2() ) -> Base
>> type( derived1() ) -> Derived
>> type( base3( derived1() ) ) -> Base
>> type( derived2( base2() ) ) -> Derived
>>
>>
>> but instead, this is the actual behaviour:
>>
>>
>> from shared_ptr_inheritance import *
>> type( base1() ) -> Base
>> type( base2() ) -> Base
>> type( derived1() ) -> Derived
>> type( base3( derived1() ) ) -> Derived (!!!)
>> type( derived2( base2() ) ) -> Base (!!!)
>>
>>
>> am I doing something wrong?
>> why does this happen, and, mostly, how can I cast a shared_ptr<Base>
>> containing a Derived pointer, to shared_ptr<Derived> ?
>> ______________________________________________
>
> If you've wrapped all classes with shared_ptr storage, to-python
> converts should always produce the most-derived type in Python, and I
> think that's what you want on the Python side.  This may not work if you
> can't use shared_ptr storage for some reason, or if you return an
> intermediate class you haven't wrapped.
>
> Are you using shared_ptr storage?
>
>
> Jim Bosch
>
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