Mailman 3 [Boost Python] Cannot call base method from within python extended class method - Cplusplus-sig

Jan. 8, 2015

      Hello,

My problem is the following:

I am trying to bind a third party C++ library to python using
boost-python.
This library has a class Element and a class Owner (the later takes
ownership 
of an Element object).

To make things interesting, Element defines a pure virtual method
"get_float".
The Element should be sub-classed in python. Here is a simplified
version of the
library:

// m1.hpp
---------------------------------------------------------------------
// m1.hpp

class Element {
public:
    int get_int() { return -1; }
    virtual float get_float() = 0;
};

class Owner {
    Element *m_element;

public:

    Owner(): m_element(NULL) {}
    ~Owner() { delete m_element; }

    void set_element(Element* e) {
	    delete m_element;
        m_element = e;
    };

    Element* get_element() { return m_element; }
};

and here is how I am binding to python:

// m1_py.cpp
----------------------------------------------------------------

class ElementWrap : public Element,
		    public wrapper<Element>
{
public:
    PyObject* self;

    ElementWrap(PyObject* self_): Element(), self(self_)
    { Py_INCREF(self); } 

    ~ElementWrap() { Py_DECREF(self); }

    virtual float get_float() { return call_method<float>(self,
"get_float"); }

};

void set_element(Owner& owner, auto_ptr<Element> e) {
    owner.set_element(e.get());
    e.release();
}

BOOST_PYTHON_MODULE(m1) {

    class_<Element, auto_ptr<ElementWrap>,
boost::noncopyable>("Element", init<>())
	.def("get_int", &Element::get_int)
	.def("get_float", &Element::get_float)
	;
    register_ptr_to_python<auto_ptr<Element> >();
    implicitly_convertible<auto_ptr<ElementWrap>, auto_ptr<Element> >();

    class_<Owner>("Owner", init<>())
	.def("set_element", &set_element)
	.def("get_element", &Owner::get_element, return_internal_reference<>())
    ;
}

... but when I use it like this:

# m2.py
-----------------------------------------------------------------------

import m1

class SubElement(m1.Element):

    def get_float(self):
        return 4.5678 + self.get_int()

element = SubElement()
owner = m1.Owner()
owner.set_element(element)
element = owner.get_element()

# ops! 
print (element.get_float())

I get an exception in the last line:
Traceback (most recent call last):
  File "m2.py", line 16, in <module>
    print (element.get_float())
  File "m2.py", line 8, in get_float
    return 1.0 + self.get_int()
Boost.Python.ArgumentError: Python argument types in
    Element.get_int(SubElement)
did not match C++ signature:
    get_int(Element {lvalue})

It seems that when I passed ownership to python I lost the ability to
call
a method from the base class.

Can anyone help me?

Thanks in advance

Cheers
Tiago

[Boost Python] Cannot call base method from within python extended class method

Tiago Coutinho

Paul Omta

Holger Joukl

Tiago Coutinho

Holger Joukl

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