How to create a Python Object from a C++ Object using Boost.Python

Question

I have a C++ project that use a lot of classes. The main one is 'sso::Object' (every classes are in the 'sso' namespace) and this class is derived into some other classes but one which is abstract: 'sso::Drawable'.

This class has two pure virtual methods 'sso::Drawable::set_opacity' and 'sso::Drawable::raw_draw' and it is derived into other classes like 'sso::View' which implement these two methods.

The whole project works fine when it is used in C++, but I would like to use it in Python too, so I created a Boost.Python module like that:

class DrawableWrapper : public sso::Drawable , public wrapper<sso::Drawable> {
public:
    void set_opacity(byte opacity) { this->get_override("set_opacity")(opacity); }

    void raw_draw(const sso::Rect &rect,sso::Target &target,const sso::Position &position) const {
        this->get_override("raw_draw")(rect,target,position);
    }
};

BOOST_PYTHON_MODULE(sso) {

    class_<DrawableWrapper,boost::noncopyable> ("Drawable",init<>())
        .add_property ("opacity",&sso::Drawable::get_opacity)
        // python_sso_getter & python_sso_setter_* are only used for an easier access to accessors
        .add_property ("position",python_sso_getter<sso::Drawable,sso::Position,&sso::Drawable::get_position>,python_sso_setter_1_const<sso::Drawable,sso::Position,&sso::Drawable::set_position>)
        .def("raw_draw",pure_virtual(&sso::Drawable::raw_draw))
   ;

    class_<sso::View,bases<sso::Drawable> >                 ("View",init<>())
        .def("insert",python_sso_setter_1<sso::View,sso::Drawable*,&sso::View::insert>)
        .def("remove",&sso::View::erase)
    ;

}

This code compile without errors but when I execute these lines in Python:

myview = sso.View()
print myview

I get this output:

<sso.View object at 0x7f9d2681a4b0>

But my C++ debugger tell me that the variable 'v' (the python 'myview') is a 'sso::Object' instance, not a 'sso::View' one. sso::View::View() is called but the variable type is not a view and I don't know why. Do you have any idea about that ? Did you do something like that and found a way to make it work ?

I use Python2.7 and Boost.Python1.49 and gcc version 4.6.1

EDIT: I've made a mistake: sso::Drawable does not inherit from sso::Object, but sso::View does (=multiple inheritance).

obmarg · Accepted Answer · 2012-07-03 11:50:48Z

1

The output <sso.View object at 0x7f9d2681a4b0> from python is just python telling you what it thinks the object type is called, it has nothing to do with the actual type of object created at the C++ level.

As you've told boost python to expose the object as sso.View then that is what python will see it as. If you were to change your code to expose std::string as sso.View then python would still report <sso.View object at 0x7f9d2681a4b0> after you've created it.

Similarly, if you were to change "View" in your C++ to "BlahDeBlah" then python would report the object as <sso.BlahDeBlah object at 0x7f9d2681a4b0> (of course you'd also have to create it via sso.BlahDeBlah()

That aside, I can't see anything wrong with the code you have posted. Does sso::View inherit from sso::Object? If that's the case, and you've witnessed sso::view::View() being called, then I think you might just be misinterpreting the debugger when it tells you the object is of type sso::Object. Perhaps it has a pointer to the base class or something similar at the point you are debugging?

What happens when you call myview.insert or myview.remove from python?

EDIT: I suspect (though I could be wrong) that you might not have RTTI turned on in your compiler, so typeid() is just returning the type that implemented the function you are in when you called typeid(). That would certainly explain why you get sso::View when in insert but a different answer in other functions.

Anyway, I've been looking into the documentation a little bit more and I think your problem is actually that you've provided the ability to override the raw_draw method but you haven't actually overridden it with anything.

If you have a look at the decleration of the pure_virtual boost::python function at the bottom of this file you'll see a comment:

//
// Passed a pointer to member function, generates a def_visitor which
// creates a method that only dispatches to Python if the function has
// been overridden, either in C++ or in Python, raising a "pure
// virtual called" exception otherwise.
//

So what you're seeing is just expected behaviour. If you provide an override for raw_draw in sso::View or a python class that inherits from it then you should no longer get this error.

edited Jul 3, 2012 at 11:50

answered Jul 3, 2012 at 10:57

obmarg

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7 Comments

Zaghp Over a year ago

sso::View inherit from sso::Drawable which inherit from sso::Object, so indirectly, yes, View inherit from Object (all the inheritances are public in this case). myview.insert is already used in my script and works fine (the sso::View::insert() method is called). I think the problem is that sso::Drawable is abstract and Boost.Python don't know to make an object from a class inherited from an abstract one.

Zaghp Over a year ago

I used typeid().name() to get the object type: in the insert() method I got N3sso4ViewE (=sso::View) and in an other method where I make my tests, I got N3sso6ObjectE (=sso::Object)

Zaghp Over a year ago

This error make my program crash because the sso::Object class does not have the raw_draw method so I get an Abord signal with the message : "pure virtual method called"

obmarg Over a year ago

@Wysiag I think I've figured out your problem, and I've updated my answer accordingly.

Zaghp Over a year ago

I saw, thanks. I'll modify my code later to test that. There are something that I don't understand: my sso::View class override the raw_draw method and I don't have any problem using this class in my C++ programs. Do you think I need to redeclare this method in the Python definition ?

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