[C++-sig] Exposing simple polymorphism with boost python

[Stefan Seefeld]

On 23/10/14 03:38 AM, Erik Türke wrote:
> Hi Python-Experts :-)
>
>
> i am starting to get really frustrated trying to expose a simple C++
> polymorphism to python with boost::python.
> I do have the following structure in C++:
>
>     struct Base {
>         int typeID;
>     };
>
>     struct Derived : public Base {
>         int derivedProperty;
>     }
>
>     //and some more from base derived types....    
>
>     Base *returnSomethingDerivedFromBase(...) {
>         Derived *ret = new Derived;
>         ret->derivedProperty = 1234;
>         return ret;
>     }
>
>     BOOST_PYTHON_MODULE(foo)
>     {
>         class_<Base>("Base")
>             .add_property("baseProperty", &Base::baseProperty);
>
>         class_<Derived, bases<Base> >("Derived")
>             .add_property("derivedProperty", &Derived::derivedProperty);
>
>         def("returnSomethingDerivedFromBase",
> returnSomethingDerivedFromBase);    
>     }
>
>
> And in Python i just want to have the following:
>
>     object = returnSomethingFromDerived() #object is of type Base
>     if object.typeID = 1:

Typo: you likely mean '==', not '='.

>         #here i want to cast to Derived and access "derivedProperty"
>         #but this is not working :-( :
>         object.__class__ = Derived

That's indeed  not possible. But I also don't know why you want that.
The objet presumably already is of type 'Derived', so there is nothing
to cast. (In Python all attribute lookups happen at call-time.)

>
> Is there a way to accomplish this at all? Or isn´t this possible as it
> would be in C++ ?

Both C++ and Python are strongly typed (i.e., all objects have a
specific type, with specific methods bound to them). But contrary to
C++, Python also is dynamically typed, so you can reassign a new object
with a different type to an existing variable. In that context, casting
isn't of any use.

Or perhaps a more complete example would be useful to demonstrate your
needs, so we can look into how best to support that in Python.
HTH,
        Stefan

-- 

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