Re: boost::python abstract base class

Hi John, Usually the best place for Boost.Python questions is: http://www.python.org/sigs/c++-sig/ John Hunter writes:

I am trying to get raw pointers and abstract base classes working properly with boost::python v2. I am working on an example inspired by the tutorial http://www.boost.org/libs/python/doc/tutorial/doc/inheritance.html.

BTW, more up-to-date docs are here: http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/boost/boost/libs/p...

In my case, the base class has a pure virtual function, but I don't need to override it in python and call it from c++, so I don't think I need to do a BaseWrap.

The problem is that if I try and expose Base to python, I get the compiler (gcc 3.2) error:

g++ -isystem/usr/local/include -I/usr/local/include/python2.2 -fPIC -ftemplate-depth-100 -c rawpointer.cpp /usr/local/include/boost/python/object/value_holder.hpp: In instantiation of `boost::python::objects::value_holder<Base>': /usr/local/include/boost/type_traits/alignment_of.hpp:52: instantiated from `boost::detail::alignment_of_impl' /usr/local/include/boost/python/object/instance.hpp:29: instantiated from `boost::alignment_of' ...snip... rawpointer.cpp:47: instantiated from here /usr/local/include/boost/python/object/value_holder.hpp:36: cannot declare field `boost::python::objects::value_holder<Base>::m_held' to be of type ` Base' /usr/local/include/boost/python/object/value_holder.hpp:36: because the following virtual functions are abstract: rawpointer.cpp:15: virtual const std::string& Base::get_name() rawpointer.cpp:14: virtual void Base::sayit() const make: *** [rawpointer.o] Error 1

Yeah, it's trying to instantiate the copy constructor for your base type.

BOOST_PYTHON_MODULE_INIT(rawpointer) { class_<Base>("Base", no_init) .def("sayit", &Base::sayit);

Try class_("Base", no_init) .def("sayit", &Base::sayit); instead.

class_("Foo") .def("sayit", &Foo::sayit); class_("Bar") .def("sayit", &Bar::sayit);

def("foofactory", foofactory, return_value_policy()); def("barfactory", barfactory, return_value_policy());

def("simonsays", simonsays); }

-- David Abrahams dave@boost-consulting.com * http://www.boost-consulting.com Boost support, enhancements, training, and commercial distribution