Re: [boost] BPL: Passing Python subclass instance of C++ abstract class to C++

From: Ullrich Koethe (koethe_at_[hidden])
Date: 2000-12-11 12:39:16

John Barnard wrote:
> 
> Before I ask my question about BPL, let me thank David, Ullrich, and
> all of the contributors and testers of BPL. I've been very impressed
> with it in the short time of being using it. 

Thanks a lot.

> Now on to my question/problem: I'm trying to pass an Python subclass
> instance of a C++ abstract class back to C++. I have a Python class
> testPI which is a subclass of the wrapped C++ abstract class LogPI. I
> wish to pass an instance of testPI to a method draw of the wrapped C++
> class StochasticGrid (all of this in Python, of course), where the
> method draw expects a reference to a LogPI object. When I attempt to
> this I get the error message:
> 
> RuntimeError: __init__ function for extension class 'LogPI' was never
> called.
> 
> indicating the BPL was expecting a constructor for the abstract class
> LogPI. Can you do this in BPL or have I made an error. I've attached
> the relevant C++ and Python code below. I'm using Microsoft VC++ with
> the latest patches on MS 98SE.
> 

I think you simply must wrap a constructor for 'LogPI' that can be
called from within Python. This works because BPL will actually
instantiate a 'LogPI_callback' which must not be an abstract class and
thus can be instantiated. (Note that in your example code,
'LogPI_callback' lacked a constructor. Note also that this constructor
must be called from within the Python-constructor - not doing this
caused the error message in your code.) Here is a simple example that
works for me:

    #include "boost/python/class_builder.hpp"
    #include <string>

    struct Abstract
    {
        virtual std::string test() = 0;
    };

    struct Abstract_callback: Abstract
    {
        Abstract_callback(PyObject * self)
        : m_self(self)
        {}

        std::string test()
        {
            return
boost::python::callback<std::string>::call_method(m_self, "test");
        }

        PyObject * m_self;
    };

    extern "C" void initabstract()
    {
        boost::python::module_builder a("abstract");

        boost::python::class_builder<Abstract, Abstract_callback> 
            a_class(a, "Abstract");
        a_class.def(boost::python::constructor<>()); // wrap a
constructor
        a_class.def(&Abstract::test, "test");
    }

Then in Python

    >>> from abstract import *
    >>> class A(Abstract):
    ...     def __init__(self, text):
    ...             Abstract.__init__(self)  # call the base class
constructor
    ...             self.text = text
    ...     def test(self):                  # implement abstract
function
    ...             return self.text
    ... 
    >>> a = A("Hello")
    >>> a.test()
    'Hello'
    >>> 

Ulli
-- 
 ________________________________________________________________
|                                                                |
| Ullrich Koethe  Universität Hamburg / University of Hamburg    |
|                 FB Informatik / Dept. of Computer Science      |
|                 AB Kognitive Systeme / Cognitive Systems Group |
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