Re: boost::python abstract base class

From: David Abrahams (dave_at_[hidden])
Date: 2002-12-20 11:44:36

Hi John,

Usually the best place for Boost.Python questions is:

     http://www.python.org/sigs/c++-sig/

John Hunter <jdhunter_at_[hidden]> 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/python/doc/index.html

> 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<boost::python::objects::value_holder<Base> >'
> /usr/local/include/boost/python/object/instance.hpp:29:   instantiated from `boost::alignment_of<boost::python::objects::value_holder<Base> >'
> ...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, noncopyable>("Base", no_init)
         .def("sayit", &Base::sayit);

instead.

>     class_<Foo, bases<Base> >("Foo")
> 	.def("sayit", &Foo::sayit);
>     class_<Bar, bases<Base> >("Bar")
> 	.def("sayit", &Bar::sayit);
> 	        
>     def("foofactory", foofactory, return_value_policy<manage_new_object>());
>     def("barfactory", barfactory, return_value_policy<manage_new_object>());
>
>     def("simonsays", simonsays);
> }
>

-- 
                       David Abrahams
   dave_at_[hidden] * http://www.boost-consulting.com
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