Boost::python and iterators from base class virtual functions

Question

So I'm trying to write a base class to handle classes that wrap std::vector, which I have working up to defining the __iter__ function. My first approach, and the one I wish to get working, is to have the begin() and end() functions in the base class. Doing this compiles fine, but when I run the code in python I get an error that looks simiar to:

Boost.Python.ArgumentError: Python argument types in

Container.__iter__(Container)

did not match C++ signature:

__iter__(boost::python::back_reference< stl_iter< std::vector< std::shared_ptr< K > > std::allocator< std::shared_ptr< K > > > >&>)

The following sample extension can be tested with

from test import *

c = Container()

for i in range(10):
    c.append(K(str(i)))

for i in c:
    print i

Sample extension:

#include <memory>
#include <vector>
#include <string>
#include <boost/python.hpp>

template<class T> 
T* get_pointer(std::shared_ptr<T> const& p) { return p.get(); }

template<class T>
class stl_iter {
protected:
    typedef typename T::value_type V;
    typedef typename T::iterator iter;
    virtual T& get_vector()=0;
public:
    virtual ~stl_iter() {}

    virtual void append(V item) {
        get_vector().push_back(item);
    }

    virtual iter begin() {
        return get_vector().begin();
    }

    virtual iter end() {
        return get_vector().end();
    }
};

class K {
    std::string val;
public:
    K(std::string s) : val(s) {}

    std::string get_val() const { return val; }
};
typedef std::shared_ptr<K> pK;
typedef std::vector<pK> vK;

class Container : public stl_iter<vK> {
    vK items;
protected:
    vK& get_vector() { return items; }

public:
    // Works if I uncomment these
    //vK::iterator begin() { return get_vector().begin(); }
    //vK::iterator end() { return get_vector().end(); }


public:
    virtual ~Container() {}
};
typedef std::shared_ptr<Container> pContainer;
typedef std::vector<pContainer> vContainer;

BOOST_PYTHON_MODULE_INIT(test) {
    using namespace boost::python;

    class_<K, pK>("K", init<std::string>())
        .def("__str__", &K::get_val)
        ;

    class_<Container, pContainer>("Container")
        .def("append", &Container::append)
        .def("__iter__", range(&Container::begin, &Container::end))
        ;
}

Do I understand correctly that the point of this is to expose vector types using boost python? And to do this in a general way? — Matthew Scouten
– Matthew Scouten, Commented Nov 21, 2011 at 19:40
I get this error trying to run a (non-c++11) adapted version of your code: TypeError: __init__() should return None, not 'NoneType' if I try to instantiate a Container. — James
– James, Commented Nov 27, 2011 at 20:52

Naddiseo · Accepted Answer · 2012-06-01 22:33:44Z

The answer was actually quite simple: make the container behave like an stl container, and then to use boost's iterator<>() function instead of range().

To do this, I had to make the typedefs in stl_iter public, and to rename them to value_type and iterator.

Heres the updated C++ code.

#include <memory>
#include <vector>
#include <string>
#include <boost/python.hpp>

template<class T> 
T* get_pointer(std::shared_ptr<T> const& p) { return p.get(); }

template<class T>
class stl_iter {
protected:
    virtual T& get_vector()=0;
public:
    // Next two lines changed, and made public
    typedef typename T::value_type value_type;
    typedef typename T::iterator iterator;

    virtual ~stl_iter() {}

    virtual void append(value_type item) {
        get_vector().push_back(item);
    }

    virtual iterator begin() {
        return get_vector().begin();
    }

    virtual iterator end() {
        return get_vector().end();
    }
};

class K {
    std::string val;
public:
    K(std::string s) : val(s) {}

    std::string get_val() const { return val; }
};
typedef std::shared_ptr<K> pK;
typedef std::vector<pK> vK;

class Container : public stl_iter<vK> {
    vK items;
protected:
    vK& get_vector() { return items; }

public:
    virtual ~Container() {}
};
typedef std::shared_ptr<Container> pContainer;
typedef std::vector<pContainer> vContainer;

BOOST_PYTHON_MODULE_INIT(test) {
    using namespace boost::python;

    class_<K, pK>("K", init<std::string>())
        .def("__str__", &K::get_val)
        ;

    class_<Container, pContainer>("Container")
        .def("append", &Container::append)
        // Use iterator() instead of range()
        .def("__iter__", iterator<Container>())
        ;
}

luart · Accepted Answer · 2014-01-11 06:14:35Z

1

range() also can be used fine, but underlying iterator anyway should support STL semantic, here is sample:

...
.def("__iter__"
     , range<return_value_policy<copy_non_const_reference> >(
           &my_sequence<heavy>::begin
         , &my_sequence<heavy>::end))

answered Jan 11, 2014 at 6:14

luart

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