[C++-sig] const_iterator type

March 27, 2013

      Hi,

I'm trying to wrap an iterable class, which returns a custom iterator  
typedef'd as const_iterator, from its begin and end methods.

When using boost::python::(iterator|iterators|range) to create an iterator  
in the exposed __iter__ function in the exposed class_ def, I get a load  
of compile errors:-

/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 mylib::MyConstIterator'
/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 mylib::MyConstIterator'
/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 mylib::MyConstIterator'
/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 mylib::MyConstIterator'
/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 mylib::MyConstIterator'

Here's a simple example demonstrating the problem:-

// const_iterator.cpp
#include <boost/python/class.hpp>
#include <boost/python/copy_const_reference.hpp>
#include <boost/python/iterator.hpp>
#include <boost/python/module.hpp>

// External lib declaration
//

namespace notmylib {

     // forward declaration
     class MyConstIterator;

     class MyData
     {
         public:
             MyData(void);
             virtual ~MyData(void);
             // ...

             typedef MyConstIterator const_iterator;
             const_iterator begin(void) const;
             const_iterator end(void) const;
     };

     class MyConstIterator
     {
         public:
             typedef std::pair<int, int> TRange;

             // constructors
             MyConstIterator(void);
             MyConstIterator(const MyData);

             // dtor
             ~MyConstIterator(void) {}

             // copy constructor
             MyConstIterator(const MyConstIterator& iter);

             // assignment operator
             MyConstIterator& operator= (const MyConstIterator& iter);

             // Comparison operators
             bool operator==(const MyConstIterator& iter) const;
             bool operator!=(const MyConstIterator& iter) const;

             // Get Data Range
            TRange GetRange(void) const;

             // Go backwards one step
             void Rewind(void);
         private:
             MyData m_data;
             size_t m_index;
     };
}

BOOST_PYTHON_MODULE(const_iterator)
{
     boost::python::class_<notmylib::MyData,
                           boost::noncopyable>("Data")
         .def("__iter__",
             boost::python::iterator<
                 const notmylib::MyData,
                 boost::python::return_value_policy<
                     boost::python::copy_const_reference> >() )
         // ...
         ;
}

Although this doesn't have the member function definitions, so won't work  
as a complete example from Python, it completely replicates the compile  
error I get with the actual class I'm trying to wrap. Command to compile,  
using GCC 4.7.2:-

g++ -c -L/usr/lib -g -O1  -fPIC -I/usr/include/python2.7  
const_iterator.cpp -o const_iterator.o -lboost_python

I couldn't find any examples using const_iterator in the test directory,  
or elsewhere, so can't find anything to go by. I thought to manually add  
those typedef's the compilers complaining about, to a Python wrapper class  
derived from MyIterator. Is there another class I can inherit from, in  
boost or the STL, to do this for me?

Any alternative suggestions or recommendations would be appreciated!

Kind regards,
Alex

--

[C++-sig] const_iterator type

Alex Leach