iteratorfacades.hh

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// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
// vi: set et ts=4 sw=2 sts=2:
// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
#ifndef DUNE_ITERATORFACADES_HH
#define DUNE_ITERATORFACADES_HH

#include <iterator>
#include <type_traits>

#include <dune/common/typetraits.hh>
#include <dune/common/concept.hh>

namespace Dune
{
  template<class T, class V, class R = V&, class D = std::ptrdiff_t>
  class ForwardIteratorFacade
  {

  public:
    /* type aliases required by C++ for iterators */
    using iterator_category = std::forward_iterator_tag;
    using value_type = typename std::remove_const<V>::type;
    using difference_type = D;
    using pointer = V*;
    using reference = R;

    typedef T DerivedType;

    typedef V Value;

    typedef V* Pointer;

    typedef D DifferenceType;

    typedef R Reference;

    constexpr Reference operator*() const
    {
      return static_cast<DerivedType const*>(this)->dereference();
    }

    constexpr Pointer operator->() const
    {
      return &(static_cast<const DerivedType *>(this)->dereference());
    }

    constexpr DerivedType& operator++()
    {
      static_cast<DerivedType *>(this)->increment();
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator++(int)
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      this->operator++();
      return tmp;
    }
  };

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator==(const ForwardIteratorFacade<T1,V1,R1,D>& lhs,
             const ForwardIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).equals(static_cast<const T2&>(rhs));
    else
      return static_cast<const T2&>(rhs).equals(static_cast<const T1&>(lhs));
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator!=(const ForwardIteratorFacade<T1,V1,R1,D>& lhs,
             const ForwardIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return !static_cast<const T1&>(lhs).equals(static_cast<const T2&>(rhs));
    else
      return !static_cast<const T2&>(rhs).equals(static_cast<const T1&>(lhs));
  }

  template<class T, class V, class R = V&, class D = std::ptrdiff_t>
  class BidirectionalIteratorFacade
  {

  public:
    /* type aliases required by C++ for iterators */
    using iterator_category = std::bidirectional_iterator_tag;
    using value_type = typename std::remove_const<V>::type;
    using difference_type = D;
    using pointer = V*;
    using reference = R;

    typedef T DerivedType;

    typedef V Value;

    typedef V* Pointer;

    typedef D DifferenceType;

    typedef R Reference;

    constexpr Reference operator*() const
    {
      return static_cast<DerivedType const*>(this)->dereference();
    }

    constexpr Pointer operator->() const
    {
      return &(static_cast<const DerivedType *>(this)->dereference());
    }

    constexpr DerivedType& operator++()
    {
      static_cast<DerivedType *>(this)->increment();
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator++(int)
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      this->operator++();
      return tmp;
    }


    constexpr DerivedType& operator--()
    {
      static_cast<DerivedType *>(this)->decrement();
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator--(int)
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      this->operator--();
      return tmp;
    }
  };

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename std::enable_if<std::is_convertible<T2,T1>::value,bool>::type
  operator==(const BidirectionalIteratorFacade<T1,V1,R1,D>& lhs,
             const BidirectionalIteratorFacade<T2,V2,R2,D>& rhs)
  {
    return static_cast<const T1&>(lhs).equals(static_cast<const T2&>(rhs));
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr
  typename std::enable_if<std::is_convertible<T1,T2>::value && !std::is_convertible<T2,T1>::value,
      bool>::type
  operator==(const BidirectionalIteratorFacade<T1,V1,R1,D>& lhs,
             const BidirectionalIteratorFacade<T2,V2,R2,D>& rhs)
  {
    return static_cast<const T2&>(rhs).equals(static_cast<const T1&>(lhs));
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator!=(const BidirectionalIteratorFacade<T1,V1,R1,D>& lhs,
             const BidirectionalIteratorFacade<T2,V2,R2,D>& rhs)
  {
    return !(lhs == rhs);
  }

  template<class T, class V, class R = V&, class D = std::ptrdiff_t>
  class RandomAccessIteratorFacade
  {

  public:
    /* type aliases required by C++ for iterators */
    using iterator_category = std::random_access_iterator_tag;
    using value_type = typename std::remove_const<V>::type;
    using difference_type = D;
    using pointer = V*;
    using reference = R;

    typedef T DerivedType;

    typedef V Value;

    typedef V* Pointer;

    typedef D DifferenceType;

    typedef R Reference;

    constexpr Reference operator*() const
    {
      return static_cast<DerivedType const*>(this)->dereference();
    }

    constexpr Pointer operator->() const
    {
      return &(static_cast<const DerivedType *>(this)->dereference());
    }

    constexpr Reference operator[](DifferenceType n) const
    {
      return static_cast<const DerivedType *>(this)->elementAt(n);
    }

    constexpr DerivedType& operator++()
    {
      static_cast<DerivedType *>(this)->increment();
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator++(int)
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      this->operator++();
      return tmp;
    }

    constexpr DerivedType& operator+=(DifferenceType n)
    {
      static_cast<DerivedType *>(this)->advance(n);
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator+(DifferenceType n) const
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      tmp.advance(n);
      return tmp;
    }


    constexpr DerivedType& operator--()
    {
      static_cast<DerivedType *>(this)->decrement();
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator--(int)
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      this->operator--();
      return tmp;
    }

    constexpr DerivedType& operator-=(DifferenceType n)
    {
      static_cast<DerivedType *>(this)->advance(-n);
      return *static_cast<DerivedType *>(this);
    }

    constexpr DerivedType operator-(DifferenceType n) const
    {
      DerivedType tmp(static_cast<DerivedType const&>(*this));
      tmp.advance(-n);
      return tmp;
    }


  };

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator==(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
             const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).equals(static_cast<const T2&>(rhs));
    else
      return static_cast<const T2&>(rhs).equals(static_cast<const T1&>(lhs));
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator!=(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
             const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return !static_cast<const T1&>(lhs).equals(static_cast<const T2&>(rhs));
    else
      return !static_cast<const T2&>(rhs).equals(static_cast<const T1&>(lhs));
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator<(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
            const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).distanceTo(static_cast<const T2&>(rhs))>0;
    else
      return static_cast<const T2&>(rhs).distanceTo(static_cast<const T1&>(lhs))<0;
  }


  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator<=(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
             const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).distanceTo(static_cast<const T2&>(rhs))>=0;
    else
      return static_cast<const T2&>(rhs).distanceTo(static_cast<const T1&>(lhs))<=0;
  }


  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator>(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
            const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).distanceTo(static_cast<const T2&>(rhs))<0;
    else
      return static_cast<const T2&>(rhs).distanceTo(static_cast<const T1&>(lhs))>0;
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,bool>::type
  operator>=(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
             const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return static_cast<const T1&>(lhs).distanceTo(static_cast<const T2&>(rhs))<=0;
    else
      return static_cast<const T2&>(rhs).distanceTo(static_cast<const T1&>(lhs))>=0;
  }

  template<class T1, class V1, class R1, class D,
      class T2, class V2, class R2>
  constexpr typename EnableIfInterOperable<T1,T2,D>::type
  operator-(const RandomAccessIteratorFacade<T1,V1,R1,D>& lhs,
            const RandomAccessIteratorFacade<T2,V2,R2,D>& rhs)
  {
    if(std::is_convertible<T2,T1>::value)
      return -static_cast<const T1&>(lhs).distanceTo(static_cast<const T2&>(rhs));
    else
      return static_cast<const T2&>(rhs).distanceTo(static_cast<const T1&>(lhs));
  }


  template<class ProxyType>
  class ProxyArrowResult
  {
  public:
    constexpr ProxyArrowResult(ProxyType&& p)
      noexcept(std::is_nothrow_constructible_v<ProxyType, ProxyType&&>)
      : p_(std::move(p))
    {}

    constexpr ProxyArrowResult(const ProxyType& p)
      noexcept(std::is_nothrow_constructible_v<ProxyType, const ProxyType&>)
      : p_(p)
    {}

    constexpr const ProxyType* operator->() const noexcept
    {
      return &p_;
    }

    constexpr ProxyType* operator->() noexcept
    {
      return &p_;
    }

  private:
    ProxyType p_;
  };


  struct IteratorFacadeAccess
  {

    template<class It>
    static constexpr auto baseIterator(It&& it) noexcept
      -> decltype(it.baseIterator())
    {
      return it.baseIterator();
    }

    template<class It>
    static constexpr auto derived(It&& it) noexcept
      -> decltype(it.derived())
    {
      return it.derived();
    }

  };



  namespace Impl::Concepts {

    using namespace Dune::Concept;

    template<class R>
    struct BaseIterDereferenceOp
    {
      template<class It>
      auto require(const It& it) -> decltype(
        *(IteratorFacadeAccess::baseIterator(it)),
        requireConvertible<decltype(*(IteratorFacadeAccess::baseIterator(it))), R>()
      );
    };

    struct IterEqualsOp
    {
      template<class It1, class It2>
      auto require(const It1& it1, const It2& it2) -> decltype(
        requireConvertible<bool>(it1 == it2)
      );
    };

    struct BaseIterEqualsOp
    {
      template<class It1, class It2>
      auto require(const It1& it1, const It2& it2) -> decltype(
        Dune::Concept::requireConvertible<bool>(IteratorFacadeAccess::baseIterator(it1) == IteratorFacadeAccess::baseIterator(it2))
      );
    };

    struct BaseIterIncrementOp
    {
      template<class It>
      auto require(It it) -> decltype(
        ++(IteratorFacadeAccess::baseIterator(it))
      );
    };

    struct BaseIterDecrementOp
    {
      template<class It>
      auto require(It it) -> decltype(
        --(IteratorFacadeAccess::baseIterator(it))
      );
    };

    template<class D>
    struct IterAdvanceOp
    {
      template<class It>
      auto require(It it) -> decltype(
        it += std::declval<D>()
      );
    };

    template<class D>
    struct BaseIterAdvanceOp
    {
      template<class It>
      auto require(It it) -> decltype(
        IteratorFacadeAccess::baseIterator(it) += std::declval<D>()
      );
    };

    template<class D>
    struct IterDistanceOp
    {
      template<class It1, class It2>
      auto require(const It1& it1, const It2& it2) -> decltype(
        Dune::Concept::requireConvertible<D>(it1 - it2)
      );
    };

    template<class D>
    struct BaseIterDistanceOp
    {
      template<class It1, class It2>
      auto require(const It1& it1, const It2& it2) -> decltype(
        Dune::Concept::requireConvertible<D>(IteratorFacadeAccess::baseIterator(it1) - IteratorFacadeAccess::baseIterator(it2))
      );
    };

  } // namespace Impl::Concept



  template<class It, class C, class V, class R = V&, class P = V*, class D = std::ptrdiff_t>
  class IteratorFacade
  {
    static constexpr bool isBidirectional = std::is_convertible_v<C, std::bidirectional_iterator_tag>;
    static constexpr bool isRandomAccess = std::is_convertible_v<C, std::random_access_iterator_tag>;

    // We make IteratorFacadeAccess a friend to allow forwarding of the derived()
    // methods to the free operators instead of havin to do raw casts there.
    // This allows to encapsulate all casts within IteratorFacade itself.
    friend IteratorFacadeAccess;

  protected:

    using DerivedIterator = It;

    constexpr const DerivedIterator& derived() const
    {
      return static_cast<const DerivedIterator&>(*this);
    }

    constexpr DerivedIterator& derived()
    {
      return static_cast<DerivedIterator&>(*this);
    }

  public:

    // Standard types of of C++ iterators
    using iterator_category = C;
    using value_type = typename std::remove_const<V>::type;
    using reference = R;
    using pointer = P;
    using difference_type = D;

    // Corresponding Dune typedefs
    using Value = value_type;
    using Reference = reference;
    using Pointer = pointer;
    using DifferenceType = difference_type;

    // Only defined to do static assertions.
    IteratorFacade()
    {
      static_assert(std::is_signed_v<difference_type>,
        "Type used as difference_type must be signed");
      const DerivedIterator& constDerived = derived();
      static_assert(std::is_convertible_v<decltype(*constDerived), reference>,
        "Derived class does not implement `*it` or `*(it.baseIterator())` for const `it` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
      static_assert(std::is_convertible_v<decltype(++derived()), DerivedIterator&>,
        "Derived class does not implement `++it`, `++(it.baseIterator())`, or `it+=1` for mutable `it` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
      static_assert(std::is_convertible_v<decltype(constDerived==constDerived), bool>,
        "Derived class does not implement `it1==it2` or `it1.baseIterator()==it2.baseIterator()` for const `it1` and `it2` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
      if constexpr (isBidirectional)
        static_assert(std::is_convertible_v<decltype(--derived()), DerivedIterator&>,
          "Derived class does not implement `--it`, `--(it.baseIterator())`, or `it-=1` for mutable `it` required by IteratorFacade<..., std::bidirectional_iterator_tag, ...>.");
      if constexpr (isRandomAccess)
      {
        static_assert(std::is_convertible_v<decltype(derived()+=std::declval<difference_type>()), DerivedIterator&>,
          "Derived class does not implement `it+=` or `it.baseIterator()+=` for mutable `it` required by IteratorFacade<..., std::random_access_iterator_tag, ...>.");
        static_assert(std::is_convertible_v<decltype(constDerived-constDerived), difference_type>,
          "Derived class does not implement `it1-it2` or `it1.baseIterator()-it2.baseIterator()` for const `it1` and `it2` required by IteratorFacade<..., std::random_access_iterator_tag, ...>.");
      }
    }

    constexpr decltype(auto) operator*() const
    {
      if constexpr (Dune::models<Impl::Concepts::BaseIterDereferenceOp<reference>, DerivedIterator>())
        return *(IteratorFacadeAccess::baseIterator(derived()));
      else
        static_assert(AlwaysFalse<It>::value,
          "Derived class does not implement `*it` or `*(it.baseIterator())` for const `it` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
    }

    constexpr pointer operator->() const
    {
      if constexpr (std::is_pointer_v<pointer>)
        return std::addressof(*derived());
      else
        return pointer(*derived());
    }

    constexpr decltype(auto) operator++()
    {
      if constexpr (Dune::models<Impl::Concepts::BaseIterIncrementOp, DerivedIterator>())
      {
        ++(IteratorFacadeAccess::baseIterator(derived()));
        return derived();
      }
      else if constexpr (Dune::models<Impl::Concepts::IterAdvanceOp<difference_type>, DerivedIterator>())
      {
        derived() += 1;
        return derived();
      }
      else
        static_assert(AlwaysFalse<It>::value,
          "Derived class does not implement `++it`, `++(it.baseIterator())`, or `it+=1` for mutable `it` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
    }

    constexpr DerivedIterator operator++(int)
    {
      DerivedIterator tmp(derived());
      ++derived();
      return tmp;
    }

    template<bool dummy=true, std::enable_if_t<isBidirectional and dummy, int> =0>
    constexpr decltype(auto) operator--()
    {
      if constexpr (Dune::models<Impl::Concepts::BaseIterDecrementOp, DerivedIterator>())
      {
        --(IteratorFacadeAccess::baseIterator(derived()));
        return derived();
      }
      else if constexpr (Dune::models<Impl::Concepts::IterAdvanceOp<difference_type>, DerivedIterator>())
      {
        derived() -= 1;
        return derived();
      }
      else
        static_assert(AlwaysFalse<It>::value,
          "Derived class does not implement `--it`, `--(it.baseIterator())`, or `it-=1` for mutable `it` required by IteratorFacade<..., std::bidirectional_iterator_tag, ...>.");
    }

    template<bool dummy=true, std::enable_if_t<isBidirectional and dummy, int> =0>
    constexpr DerivedIterator operator--(int)
    {
      DerivedIterator tmp(derived());
      --derived();
      return tmp;
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    constexpr reference operator[](difference_type n) const
    {
      return *(derived()+n);
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    constexpr decltype(auto) operator+=(difference_type n)
    {
      if constexpr (Dune::models<Impl::Concepts::BaseIterAdvanceOp<difference_type>, DerivedIterator>())
      {
        IteratorFacadeAccess::baseIterator(derived()) += n;
        return derived();
      }
      else
        static_assert(AlwaysFalse<It>::value,
          "Derived class does not implement `it+=` or `it.baseIterator()+=` for mutable `it` required by IteratorFacade<..., std::random_access_iterator_tag, ...>.");
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    constexpr DerivedIterator operator+(difference_type n) const
    {
      DerivedIterator tmp(derived());
      tmp += n;
      return tmp;
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    friend constexpr DerivedIterator operator+(difference_type n, const IteratorFacade& it)
    {
      DerivedIterator tmp(it.derived());
      tmp += n;
      return tmp;
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    constexpr DerivedIterator& operator-=(difference_type n)
    {
      derived() += (-n);
      return derived();
    }

    template<bool dummy=true, std::enable_if_t<isRandomAccess and dummy, int> =0>
    constexpr DerivedIterator operator-(difference_type n) const
    {
      DerivedIterator tmp(derived());
      tmp -= n;
      return tmp;
    }

  };



  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2>
  constexpr auto operator==(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    if constexpr (Dune::models<Impl::Concepts::BaseIterEqualsOp, T1, T2>())
      return IteratorFacadeAccess::baseIterator(derivedIt1) == IteratorFacadeAccess::baseIterator(derivedIt2);
    else
      static_assert(AlwaysFalse<T1>::value,
        "Derived class does not implement `it1==it2` or `it1.baseIterator()==it2.baseIterator()` for const `it1` and `it2` required by IteratorFacade<..., std::forward_iterator_tag, ...>.");
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2,
    std::enable_if_t< Dune::models<Impl::Concepts::IterEqualsOp,T1, T2>() , int> =0>
  constexpr bool operator!=(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    return not(derivedIt1 == derivedIt2);
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D>
  constexpr auto operator-(const IteratorFacade<T1,C,V1,R1,P1,D>& it1, const IteratorFacade<T2,C,V2,R2,P2,D>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    if constexpr (Dune::models<Impl::Concepts::BaseIterDistanceOp<D>,T1, T2>())
      return D(IteratorFacadeAccess::baseIterator(derivedIt1) - IteratorFacadeAccess::baseIterator(derivedIt2));
    else
      static_assert(AlwaysFalse<T1>::value,
        "Derived class does not implement `it1-it2` or `it1.baseIterator()-it2.baseIterator()` for const `it1` and `it2` required by IteratorFacade<..., std::random_access_iterator_tag, ...>.");
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2,
    std::enable_if_t< Dune::models<Impl::Concepts::IterDistanceOp<D1>,T1, T2>() , int> =0>
  constexpr bool operator<(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    return (derivedIt1 - derivedIt2) < D1(0);
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2,
    std::enable_if_t< Dune::models<Impl::Concepts::IterDistanceOp<D1>,T1, T2>() , int> =0>
  constexpr bool operator<=(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    return (derivedIt1 - derivedIt2) <= D1(0);
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2,
    std::enable_if_t< Dune::models<Impl::Concepts::IterDistanceOp<D1>,T1, T2>() , int> =0>
  constexpr bool operator>(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    return (derivedIt1 - derivedIt2) > D1(0);
  }

  template<class T1, class T2, class C, class V1, class V2, class R1, class R2, class P1, class P2, class D1, class D2,
    std::enable_if_t< Dune::models<Impl::Concepts::IterDistanceOp<D1>,T1, T2>() , int> =0>
  constexpr bool operator>=(const IteratorFacade<T1,C,V1,R1,P1,D1>& it1, const IteratorFacade<T2,C,V2,R2,P2,D2>& it2)
  {
    const T1& derivedIt1 = IteratorFacadeAccess::derived(it1);
    const T2& derivedIt2 = IteratorFacadeAccess::derived(it2);
    return (derivedIt1 - derivedIt2) >= D1(0);
  }



  template<class IteratorImpl, class IteratorTraits>
  using IteratorFacadeForTraits = Dune::IteratorFacade<
        IteratorImpl,
        typename IteratorTraits::iterator_category,
        typename IteratorTraits::value_type,
        typename IteratorTraits::reference,
        typename IteratorTraits::pointer,
        typename IteratorTraits::difference_type>;


  template<class IteratorCategory, class Reference, class DifferenceType = std::ptrdiff_t>
  struct DefaultIteratorTraits
  {
    using iterator_category = IteratorCategory;
    using reference = Reference;
    using value_type = Dune::AutonomousValue<reference>;
    using pointer = std::conditional_t<std::is_lvalue_reference_v<reference>, value_type*, Dune::ProxyArrowResult<reference>>;
    using difference_type = DifferenceType;
  };



}
#endif