[C++-sig] Polymorphism between Python and C++

[David Abrahams]

Patrick Hartling <patrick at vrac.iastate.edu> writes:

> I'm doing some experimenting with Boost.Python v2, and I have run into
> a problem I just can't seem to solve.  I want to define an abstract
> base class in C++, derive a Python class from that base, and then use
> the derived Python class polymorphically where the abstract C++ class
> can be used.  I have attached the simplest code I could come up with
> that demonstrates the problem.  When I run test.py, I get the
> following output:
>
> I'm Derived1!
> Traceback (most recent call last):
>    File "./test.py", line 18, in ?
>      handler.setObj(derived2)
> TypeError: bad argument type for built-in operation
>
> I have been over the documentation (both on boost.org and on the
> PythonInfo Wiki)

Wow, I forgot about that Wiki. Is it worth linking
http://www.python.org/cgi-bin/moinmoin/boost_2epython into the Boost
web pages? Most of the info appears to be outdated...

>  but I can't figure out what I am doing wrong.  It
> seems like I am close, but there must be some detail I'm missing.

Yep. The detail is that Derived2.__init__() needs to call
Base.__init__(). Otherwise there will be no C++ Base object in the
Python object, and the conversion to Base* will fail.


-- 
                       David Abrahams
   dave at boost-consulting.com * http://www.boost-consulting.com
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