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POHow to expose C++ classes with const_iterator
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  1. POHow to expose C++ classes with const_iterator
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    copied!<p>I am using <a href="http://www.boost.org/doc/libs/1_53_0/libs/python/doc/index.html" rel="nofollow">Boost.Python</a> to expose a 3rd party C++ API.</p> <p>A header file I've come to declares an iterable class (has <code>begin</code> and <code>end</code> methods), and a custom iterator class with which to do the iteration:-</p> <pre><code>// File: data.hpp #include &lt;utility&gt; // for std::pair #include &lt;cstring&gt; // for size_t namespace notmylib { // forward declaration class DataIterator; // Storage for arbitrary data class Data { public: Data(void); virtual ~Data(void); // ... typedef DataIterator const_iterator; const_iterator begin(void) const; const_iterator end(void) const; typedef std::pair&lt;int, int&gt; TRange; private: TRange m_data; }; // Data iterator class class DataIterator { public: // constructors DataIterator(void); DataIterator(const Data); ~DataIterator(void); // copy constructor DataIterator(const DataIterator&amp; iter); // assignment operator DataIterator&amp; operator=(const DataIterator&amp; iter); // Comparison operators bool operator==(const DataIterator&amp; iter) const; bool operator!=(const DataIterator&amp; iter) const; // Data Range typedef TData::TRange TRange; TRange GetRange(void) const; Data GetRangeAsData(void) const; // Go backwards one step void Rewind(void); private: Data m_data; size_t m_index; }; } </code></pre> <p>I've created minimal wrapper classes deriving from these:-</p> <pre><code>// File: pydata.hpp #include "data.hpp" namespace mylib { class PyData : public notmylib::Data { public: PyData(void); virtual ~PyData(void); }; class PyIterator : public notmylib::DataIterator { public: PyIterator(void); PyIterator(const notmylib::Data); PyIterator(const PyIterator&amp; iter); ~PyIterator(void); }; } </code></pre> <p>And the Boost.Python declaration:-</p> <pre><code>// File: pydata.cpp #include "pydata.hpp" #include &lt;boost/python/class.hpp&gt; #include &lt;boost/python/iterator.hpp&gt; #include &lt;boost/python/module.hpp&gt; BOOST_PYTHON_MODULE(const_iterator) { using namespace boost::python; class_&lt;mylib::PyData, boost::noncopyable&gt;("Data", "Iterable Data storage class.") .def("__iter__", iterator&lt;const mylib::PyData&gt;() /*, // */ ) ///&lt; iterator / range / iterators ? // boost::python::return_value_policy&lt; ///&lt; This CallPolicy // boost::python::copy_const_reference&gt; &gt;() ) /// doesn't work either ; class_&lt;mylib::PyIterator&gt;("DataIterator", "Iterator for Data class. Don't actually need to expose...") .def("__eq__", &amp;mylib::PyIterator::operator==) .def("__neq__", &amp;mylib::PyIterator::operator!=) // ... ; } </code></pre> <p>The only oddity here seems to be the use of the <code>const_iterator</code> typedef. This is described in the Boost.Python <a href="http://www.boost.org/doc/libs/1_53_0/libs/python/doc/v2/iterator.html#iterator-spec" rel="nofollow">iterator documentation</a>, and from what I gather, all that it needs is a <code>const</code> qualifier in <code>iterator</code>'s template parameters. i.e.</p> <pre><code>boost::python::iterator&lt;const mylib::PyData&gt;() </code></pre> <p>However, that leads to the following compiler error, using GCC 4.7.2 on Linux x86_64:-</p> <pre><code>g++ -std=gnu++98 -c -g -O1 -fPIC -I/usr/include/python2.7 pydata.cpp -o pydata.o </code></pre> <pre><code>In file included from /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_algobase.h:66:0, from /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/memory:64, from /usr/include/boost/config/no_tr1/memory.hpp:21, from /usr/include/boost/get_pointer.hpp:14, from /usr/include/boost/python/object/pointer_holder.hpp:11, from /usr/include/boost/python/to_python_indirect.hpp:10, from /usr/include/boost/python/converter/arg_to_python.hpp:10, from /usr/include/boost/python/call.hpp:15, from /usr/include/boost/python/object_core.hpp:14, from /usr/include/boost/python/object/class.hpp:9, from /usr/include/boost/python/class.hpp:13, from pydata.cpp:4: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h: In instantiation of 'struct std::iterator_traits&lt;notmylib::DataIterator&gt;': /usr/include/boost/detail/iterator.hpp:81:8: required from 'struct boost::detail::iterator_traits&lt;notmylib::DataIterator&gt;' /usr/include/boost/python/object/iterator.hpp:58:17: required from 'struct boost::python::objects::iterator_range&lt;boost::python::return_value_policy&lt;boost::python::return_by_value&gt;, notmylib::DataIterator&gt;::next' /usr/include/boost/python/object/iterator.hpp:127:45: required from 'boost::python::api::object boost::python::objects::detail::demand_iterator_class(const char*, Iterator*, const NextPolicies&amp;) [with Iterator = notmylib::DataIterator; NextPolicies = boost::python::return_value_policy&lt;boost::python::return_by_value&gt;]' /usr/include/boost/python/object/iterator.hpp:167:11: required from 'boost::python::objects::iterator_range&lt;NextPolicies, Iterator&gt; boost::python::objects::detail::py_iter_&lt;Target, Iterator, Accessor1, Accessor2, NextPolicies&gt;::operator()(boost::python::back_reference&lt;Target&amp;&gt;) const [with Target = const mylib::PyData; Iterator = notmylib::DataIterator; Accessor1 = boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;; Accessor2 = boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;; NextPolicies = boost::python::return_value_policy&lt;boost::python::return_by_value&gt;]' /usr/include/boost/python/detail/invoke.hpp:75:82: required from 'PyObject* boost::python::detail::invoke(boost::python::detail::invoke_tag_&lt;false, false&gt;, const RC&amp;, F&amp;, AC0&amp;) [with RC = boost::python::to_python_value&lt;const boost::python::objects::iterator_range&lt;boost::python::return_value_policy&lt;boost::python::return_by_value&gt;, notmylib::DataIterator&gt;&amp;&gt;; F = boost::python::objects::detail::py_iter_&lt;const mylib::PyData, notmylib::DataIterator, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::python::return_value_policy&lt;boost::python::return_by_value&gt; &gt;; AC0 = boost::python::arg_from_python&lt;boost::python::back_reference&lt;const mylib::PyData&amp;&gt; &gt;; PyObject = _object]' /usr/include/boost/python/detail/caller.hpp:223:13: required from 'PyObject* boost::python::detail::caller_arity&lt;1u&gt;::impl&lt;F, Policies, Sig&gt;::operator()(PyObject*, PyObject*) [with F = boost::python::objects::detail::py_iter_&lt;const mylib::PyData, notmylib::DataIterator, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::python::return_value_policy&lt;boost::python::return_by_value&gt; &gt;; Policies = boost::python::default_call_policies; Sig = boost::mpl::vector2&lt;boost::python::objects::iterator_range&lt;boost::python::return_value_policy&lt;boost::python::return_by_value&gt;, notmylib::DataIterator&gt;, boost::python::back_reference&lt;const mylib::PyData&amp;&gt; &gt;; PyObject = _object]' /usr/include/boost/python/object/py_function.hpp:38:33: required from 'PyObject* boost::python::objects::caller_py_function_impl&lt;Caller&gt;::operator()(PyObject*, PyObject*) [with Caller = boost::python::detail::caller&lt;boost::python::objects::detail::py_iter_&lt;const mylib::PyData, notmylib::DataIterator, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::_bi::protected_bind_t&lt;boost::_bi::bind_t&lt;notmylib::DataIterator, notmylib::DataIterator (*)(const mylib::PyData&amp;), boost::_bi::list1&lt;boost::arg&lt;1&gt; &gt; &gt; &gt;, boost::python::return_value_policy&lt;boost::python::return_by_value&gt; &gt;, boost::python::default_call_policies, boost::mpl::vector2&lt;boost::python::objects::iterator_range&lt;boost::python::return_value_policy&lt;boost::python::return_by_value&gt;, notmylib::DataIterator&gt;, boost::python::back_reference&lt;const mylib::PyData&amp;&gt; &gt; &gt;; PyObject = _object]' pydata.cpp:26:1: required from here /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h:166:53: error: no type named 'iterator_category' in 'class notmylib::DataIterator' /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h:167:53: error: no type named 'value_type' in 'class notmylib::DataIterator' /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h:168:53: error: no type named 'difference_type' in 'class notmylib::DataIterator' /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h:169:53: error: no type named 'pointer' in 'class notmylib::DataIterator' /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/bits/stl_iterator_base_types.h:170:53: error: no type named 'reference' in 'class notmylib::DataIterator' </code></pre> <p>How should such an iterable class be exposed?</p> <p>Cheers, Alex</p>
 

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