Using my custom iterator with STL algorithms

Question

I'm trying to create my own iterator, and I've got it working as expected with the std::generate algorithm. However, when I try std::max_element of std::find, I get some cryptic errors.

Here is the interface for my iterator:

template <typename GridT,
          typename GridPtr,
          typename GridRef,
          template <typename> class ShapeT>
class GridIterator
{
public:
    typedef GridIterator<GridT, GridPtr, GridRef, ShapeT> Iterator;

    // Iterator traits - typedefs and types required to be STL compliant
    typedef std::ptrdiff_t           difference_type;
    typedef typename GridT::Element  value_type;
    typedef typename GridT::Element* pointer;
    typedef typename GridT::Element& reference;
    typedef size_t                      size_type;
    std::forward_iterator_tag          iterator_category;


    GridIterator(GridT& grid,
                 ShapeT<typename GridT::Resolution> shape,
                 Index iterStartIndex);

    ~GridIterator();

    Iterator& operator++();
    Iterator  operator++(int);

    typename GridT::Element& operator*();
    typename GridT::Element* operator->();

    bool operator!=(const GridIterator& rhs) const;
    bool operator==(const GridIterator& rhs) const;
    ....

}

Using std::find, I get this error:

In file included from /usr/include/c++/4.6/algorithm:63:0, from ./grid/Map_Grid.h:11, from main.cpp:4: /usr/include/c++/4.6/bits/stl_algo.h: In function ‘_IIter std::find(_IIter, _IIter, const _Tp&) [with _IIter = Map::GridIterator<Map::Grid<double, int>, Map::Grid<double, int>, Map::Grid<double, int>&, Map::Rectangle>, _Tp = int]’: main.cpp:103:50: instantiated from here /usr/include/c++/4.6/bits/stl_algo.h:4404:45: error: no matching function for call to ‘__iterator_category(Map::GridIterator<Map::Grid<double, int>, Map::Grid<double, int>, Map::Grid<double, int>&, Map::Rectangle>&)’ /usr/include/c++/4.6/bits/stl_algo.h:4404:45: note: candidate is: /usr/include/c++/4.6/bits/stl_iterator_base_types.h:202:5: note: template typename std::iterator_traits::iterator_category std::__iterator_category(const _Iter&)

With std::max_element :

In file included from /usr/include/c++/4.6/bits/char_traits.h:41:0, from /usr/include/c++/4.6/ios:41, from /usr/include/c++/4.6/ostream:40, from /usr/include/c++/4.6/iostream:40, from ./grid/Map_GridIterator.h:7, from ./grid/Map_Grid.h:8, from main.cpp:4: /usr/include/c++/4.6/bits/stl_algobase.h: In function ‘const _Tp& std::max(const _Tp&, const _Tp&) [with _Tp = Map::GridIterator<Map::Grid<double, int>, Map::Grid<double, int>, Map::Grid<double, int>&, Map::Rectangle>]’: main.cpp:102:60: instantiated from here /usr/include/c++/4.6/bits/stl_algobase.h:215:7: error: no match for ‘operator<’ in ‘__a < __b’ /usr/include/c++/4.6/bits/stl_algobase.h:215:7: note: candidates are: /usr/include/c++/4.6/bits/stl_pair.h:207:5: note: template<class _T1, class _T2> constexpr bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) /usr/include/c++/4.6/bits/stl_iterator.h:291:5: note: template bool std::operator<(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&) /usr/include/c++/4.6/bits/stl_iterator.h:341:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::reverse_iterator<_IteratorL>&, const std::reverse_iterator<_IteratorR>&) /usr/include/c++/4.6/bits/stl_iterator.h:1049:5: note: template<class _IteratorL, class _IteratorR> bool std::operator<(const std::move_iterator<_IteratorL>&, const std::move_iterator<_IteratorR>&) /usr/include/c++/4.6/bits/stl_iterator.h:1055:5: note: template bool std::operator<(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&) /usr/include/c++/4.6/bits/basic_string.h:2510:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const std::basic_string<_CharT, _Traits, _Alloc>&) /usr/include/c++/4.6/bits/basic_string.h:2522:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const std::basic_string<_CharT, _Traits, _Alloc>&, const _CharT) /usr/include/c++/4.6/bits/basic_string.h:2534:5: note: template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const std::basic_string<_CharT, _Traits, _Alloc>&) /usr/include/c++/4.6/bits/stl_vector.h:1290:5: note: template<class _Tp, class _Alloc> bool std::operator<(const std::vector<_Tp, _Alloc>&, const std::vector<_Tp, _Alloc>&) /usr/include/c++/4.6/tuple:586:5: note: template<class ... _TElements, class ... _UElements> bool std::operator<(const std::tuple<_TElements ...>&, const std::tuple<_Elements ...>&)

Answer 1

You are missing a typedef keyword for declaring an alias indicating the iterator category:

// Iterator traits - typedefs and types required to be STL compliant
//...
typedef std::forward_iterator_tag iterator_category;
~~~~~~^

Without the typedef, you are actually declaring a data member.

To avoid such mistakes, you can utilize the std::iterator class template as a base class, instead of defining those aliases on your own:

class GridIterator : public std::iterator<std::forward_iterator_tag
                                        , typename GridT::Element>