Boost.python inheriting from wrapped classes

Question

I have run into a problem when creating python bindings for an existing library with boost.python. The scenario is as follows:

#include<boost/python.hpp>

namespace bp = boost::python;

struct Base {
    std::stringstream _myString;
    Base() { };
    Base(const Base& base) { _myString<<base._myString.str(); }

    void getString(std::stringstream& some_string) {
        _myString.str("");
        _myString<<some_string.str();
        std::cout<<"Got string: \""<<_myString.str()<<"\""<<std::endl;
    }
};

struct BaseWrapper : Base,
                     bp::wrapper<Base>
{
    BaseWrapper() :
        Base(),
        bp::wrapper<Base>() { };

    BaseWrapper(const Base& base) :
        Base(base),
        bp::wrapper<Base>() { };

    void getString(bp::object pyObj) {
        std::string strLine = bp::extract<std::string>(pyObj);
        std::stringstream sstrLine;
        sstrLine<<strLine;
        Base::getString(sstrLine);
    }
};

struct Derived : Base
{
    Derived() : Base() { };
    Derived(const Derived& derived) : Base() { _myString<<derived._myString.str(); };
};

struct DerivedWrapper : Derived,
                        bp::wrapper<Derived>
{
    DerivedWrapper() :
        Derived(),
        bp::wrapper<Derived>() { };

    DerivedWrapper(const Derived derived) :
        Derived(derived),
        bp::wrapper<Derived>() { };
};

BOOST_PYTHON_MODULE(testInheritance){
    bp::class_<BaseWrapper>("Base")
        .def("getString", &BaseWrapper::getString);

    bp::class_<DerivedWrapper, bp::bases<Base> >("Derived");
}

(Sorry for the long code block, it was the minimum example I could think of.)

You can see that I had to override getString() method in the BaseWrapper so that it would work with Python strings and this part works fine:

>>> import testInheritance
>>> base = testInheritance.Base()
>>> base.getString("bla")
Got string: "bla"
>>>

The problem appears as soon as I try to call getString from a instance of Derived :

>>> derived = testInheritance.Derived()
>>> derived.getString("bla")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Boost.Python.ArgumentError: Python argument types in
    Base.getString(Derived, str)
did not match C++ signature:
    getString(BaseWrapper {lvalue}, boost::python::api::object)
>>>

I can understand what is going wrong here, but I have no idea how to fix that. I would appreciate any help!

Best Regards, eDude

Answer 1

The problem is, that DerivedWrapper has no relationship to BaseWrapper . Thus DerivedWrapper needs to provide its own python adapted implementation of void getString(bp::object pyObj) .

So one way to make it work is like this:

struct DerivedWrapper : Derived,
                        bp::wrapper<Derived>
{
    DerivedWrapper() :
        Derived(),
        bp::wrapper<Derived>() { };

    DerivedWrapper(const Derived derived) :
        Derived(derived),
        bp::wrapper<Derived>() { };

    void getString(bp::object pyObj) {
        std::string strLine = bp::extract<std::string>(pyObj);
        std::stringstream sstrLine;
        sstrLine<<"(from DerivedWrapper) "<<strLine;
        Derived::getString(sstrLine);
    }
};

[...]

    bp::class_<DerivedWrapper, bp::bases<Base> >("Derived")
        .def("getString", &DerivedWrapper::getString);

And the output of

base = testInheritance.Base()
base.getString("bla")

derived = testInheritance.Derived()
derived.getString("blub")

is as expected

Got string: "bla"
Got string: "(from DerivedWrapper) blub"

Answer 2

I got exactly the same problem which I manage to solve, but solution has some internal faults in some special cases. There is a known problem about using boost::shared_ptr in python when it is necessary to create boost::weak_ptr references from values passed by boost::python. I won't go into details while it is not related to the post. Anyway I needed to wrap boost::shared_ptr into another class (I called it PythonSharedPtr) to hide boost::shared_ptr from boost::python and then I get similar problem. Consider following setup: class A is used on c++ side as boost::shared_ptr, class B (inherited from A) as boost::shared_ptr and in python both shared_ptr are wrapped into another class (for further info why I needed to make it, is explained at: boost::python and weak_ptr : stuff disappearing http://mail.python.org/pipermail/cplusplus-sig/2009-November/014983.html

So then I need to write proper export to the boost python: class_, wrapped_shared_ptr, noncopyable> and class_, wrapped_shared_ptr, noncopyable>

I think till now it is similar with your code. The tricky part is how to allow in boost::python usage of bases for the exporting of B (while shared_ptr is not related to shared_ptr). After some research in boost::python sources and came with the solution which is pretty nice, but it works only in cases, that there is no multi-inheritance for class B.

Here is the code:

namespace boost { namespace python { namespace objects
{
    template<typename Source, typename Target>
    struct shared_ptr_cast_generator
    {
        static void* execute(void* source)
        {
            const boost::shared_ptr<Source>* sourcePtr = static_cast<boost::shared_ptr<Source>*>(source);
            const boost::shared_ptr<Target> target     = boost::dynamic_pointer_cast<Target>(*sourcePtr);
            if(reinterpret_cast<size_t>(target.get()) == reinterpret_cast<size_t>(sourcePtr->get()))
                return source;
            else
            {
                // assertion which is triggered when multi-inheritance is used for Source type
                // in this case it is necessary to create new instance of shared_ptr<Target> but
                // it is not possible to make it in-place due to memory leak
                assert(!"Wrong cast");
                return nullptr;
            }
        }
    };

    template<typename Source, typename Target>
    struct cast_generator<boost::shared_ptr<Source>, boost::shared_ptr<Target> >
    {
        typedef shared_ptr_cast_generator<Source, Target> type;
    };
}}}

By providing such code, it is possible to adjust second export to the python: class_, wrapped_shared_ptr, noncopyable, bases >

Just be carefull with the conversion stored in execute function - if conversion between Source and Target exists, it returns same address - so it must be also valid if you just reinterpret Source* to Target* (data in both classes must be stored at exactly same places).

Maybe such solution is not sufficient in your case, but at least it can give you some ideas.