New subject: Implementing python's enumerate with iterator adaptors

17 Apr 2012

      Hi,

I am implementing something similar to python's enumerate using boost iterator adapter, but I am running into two problems I am unable to solve. Both problems use my Enumerate-adaptor defined as:
#include <utility>

#include <boost/iterator/iterator_adaptor.hpp>

namespace Iterator
{
// some simple wrapper to have better names than std::pair
template<typename T, typename X>
struct Enumerated
{
   Enumerated(T i, X x):
      count(i),
      value(x)
   {

   }

   T count;
   X value;
};

template< typename Iterator, typename T = typename Iterator::difference_type>
class Enumerator:
   public boost::iterator_adaptor<Enumerator<Iterator, T>,
                                  Iterator,
                                  Enumerated<T, typename Iterator::value_type>,
                                  boost::use_default,
                                  Enumerated<T, typename Iterator::reference> >
{
public:
   Enumerator():
      mCount(),
      mPos(),
      mStep()
   {

   }

   template<typename It>
   explicit Enumerator(It i, T start=T(), T step=T(1) ):
      mCount(start),
      mPos(i),
      mStep(step)
   {

   }

private:
   friend class boost::iterator_core_access;

   Enumerated<T, typename Iterator::reference >  dereference() const
   {
      return Enumerated<T, typename Iterator::reference >(mCount, *mPos);
   }

   Enumerated<T, typename Iterator::reference >  dereference()
   {
      return Enumerated<T, typename Iterator::reference >(mCount, mPos);
   }

   bool equal(Enumerator<Iterator, T> const& other) const
   {
      return mPos == mPos;
   }

   void increment()
   {
      mCount += mStep;
      mPos += 1;
   }

   void decrement()
   {
      mCount -= mStep;
      mPos -= 1;
   }

   void advance(typename Iterator::difference_type step)
   {
      mCount += step * mStep;
      mPos += step;
   }

   typename Iterator::difference_type distance_to(Enumerator<Iterator, T> const& other) const
   {
      return std::distance(mPos, other.mPos);
   }

   T mCount;
   Iterator mPos;
   typename Iterator::difference_type mStep;
};

template<typename Iterator, typename T>
Enumerator<Iterator, T> enumerate(Iterator i, T start=0, T step=1)
{
   return Enumerator<Iterator, T>(i, start, step);
}

When I compile the following sample program, msvc (VS2010) prints an error, but g++ does not have any problems.
#include <vector>
#include <iostream>
#include <algorithm>

void foo()
{
   std::vector<int> v;

   auto i = Iterator::enumerate(v.begin(), 0, 1);
   auto j = Iterator::enumerate(v.end(), 0, 1);

   std::for_each( i, j, [](decltype(*i) e)
   {
      std::cout << e.count << ", " << e.value << std::endl;
   });
}
Error      1             error C2665: 'std::_Debug_range2' : none of the 2 overloads could convert all the argument types                d:\programme\microsoft visual studio 10.0\vc\include\xutility                728

My second problem can be reproduced with the following sample program on both msvc and g++:

#include <vector>
#include <iostream>
#include <algorithm>

void foo()
{
   std::vector<int> v;

   auto i = Iterator::enumerate(v.begin(), 0, 1);

   auto y = i->count;
   auto z = i->value;
}
MSVC:
Error      3             error C2664: 'boost::implicit_cast' : cannot convert parameter 1 from 'Iterator::Enumerated<T,X> *' to 'boost::detail::operator_arrow_proxy<T>'         c:\software development\externals\include\boost\iterator\iterator_facade.hpp  326

G++:
In file included from /usr/include/boost/iterator/iterator_adaptor.hpp:15:0,
                 from Enumerate.h:18,
                 from Enumerate.cpp:1:
/usr/include/boost/iterator/iterator_facade.hpp: In static member function 'static boost::detail::operator_arrow_result<ValueType, Reference, Pointer>::type boost::detail::operator_arrow_result<ValueType, Reference, Pointer>::make(Reference) [with ValueType = Iterator::Enumerated<int, int>, Reference = Iterator::Enumerated<int, int&>, Pointer = Iterator::Enumerated<int, int>*, boost::detail::operator_arrow_result<ValueType, Reference, Pointer>::type = boost::detail::operator_arrow_proxy<Iterator::Enumerated<int, int> >]':
/usr/include/boost/iterator/iterator_facade.hpp:648:49:   instantiated from 'boost::iterator_facade<I, V, TC, R, D>::pointer boost::iterator_facade<I, V, TC, R, D>::operator->() const [with Derived = Iterator::Enumerator<__gnu_cxx::__normal_iterator<int*, std::vector<int> >, int>, Value = Iterator::Enumerated<int, int>, CategoryOrTraversal = boost::random_access_traversal_tag, Reference = Iterator::Enumerated<int, int&>, Difference = int, boost::iterator_facade<I, V, TC, R, D>::pointer = boost::detail::operator_arrow_proxy<Iterator::Enumerated<int, int> >]'
Enumerate.cpp:27:14:   instantiated from here
/usr/include/boost/iterator/iterator_facade.hpp:327:49: error: no matching function for call to 'implicit_cast(Iterator::Enumerated<int, int&>*)'

Any help would be greatly appreciated,
Bets regards,
  Jens

Implementing python's enumerate with iterator adaptors

Jens Auer

Mateusz Loskot

Jens Auer

Jeffrey Lee Hellrung, Jr.

Jens Auer

Jeffrey Lee Hellrung, Jr.

Jens Auer

Jeffrey Lee Hellrung, Jr.

tags

participants (3)