[C++-sig] Abstract class instances to-python conversion

[Valentin Perrelle]

Hi,

I'm currently embedding and extending Python using Boost.Python. I'm 
trying to expose some c++ library (OIS) classes to my Python scripts. 
This library mainly exposes abstract classes since the actual 
implementation are derived classes specialized for each operating system.

So, i have to wrap those abstract classes. Then, eventually i will 
convert some existing C++ object of this class to its Python equivalent. 
Ideally, I would think that the HeldType is a pointer to the existing 
C++ object. There is no pointer managment problem since the lifetime of 
the object is greater than the execution time of the script.

There is probably something i don't understand in the design of 
Boost.Python. At this point, my problem is that i need a to_python 
conversion which requires the abscence of noncopyable attribute which 
then implies to be able to build an instance of the object (which i 
cannot provide since the class is abstract, and i don't have any 
concrete derived class). I don't yet understand why holding a pointer in 
the Python object requires the ability to build instances of the wrapped 
class. The exact compile-time error i get is:

boost_1_44/boost/python/object/pointer_holder.hpp:194:14: error: cannot 
allocate an object of abstract type 'OIS::Keyboard'

I'm using boost 1.44 (required by some other library) Python 3.2 and 
MinGW/Msys. Module declaration:

BOOST_PYTHON_MODULE(OIS)
{
	class_<Keyboard, Keyboard*>("Keyboard", no_init);
}

Python init:

try {
	PyImport_AppendInittab("OIS", PyInit_OIS);
	Py_InitializeEx(0);

	object main(import("__main__"));
	dict globals(main.attr("__dict__"));
	globals["keyboard"] = ptr(keyboard);
}
catch (error_already_set&) {
	PyErr_Print();
}

Did i choose the wrong desing ? Did i do something wrong ? What should i 
do to solve this problem ?


Valentin Perrelle.