cpp-useful-iterators/include/iterator/facade.h

#pragma once

#include <iterator>
#include <type_traits>

#include <iterator/detail/arrow_proxy.h>
#include <iterator/detail/facade_traits.h>
#include <iterator/detail/traits.h>

namespace iterator {
template <typename D, typename T>
using difference_type_arg_t =
    std::enable_if_t<std::is_convertible_v<D, detail::distance_to_t<T>>>;

template <typename self_type> class facade {
public:
  decltype(auto) operator*() const { return self().dereference(); }

  decltype(auto) operator->() const {
    if constexpr (std::is_reference<decltype(**this)>{}) {
      return std::addressof(**this);
    } else {
      return detail::arrow_proxy{**this};
    }
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  decltype(auto) operator[](D off) const {
    return *(self() + off);
  }

  self_type & operator++() {
    if constexpr (detail::is_advanceable_iterator_v<self_type>) {
      self() += 1;
    } else {
      self().increment();
    }
    return self();
  }

  auto operator++(int) {
    if constexpr (detail::is_single_pass_iterator_v<self_type>) {
      ++*this;
    } else {
      auto tmp = self();
      ++*this;
      return tmp;
    }
  }

  self_type & operator--() {
    if constexpr (detail::is_advanceable_iterator_v<self_type>) {
      self() -= 1;
    } else {
      self().decrement();
    }
    return self();
  }

  self_type operator--(int) {
    auto tmp = self();
    --*this;
    return tmp;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  friend self_type & operator+=(self_type & self, D off) {
    static_assert(detail::is_advanceable_iterator_v<self_type>,
                  "must be advancable");
    self.advance(off);
    return self;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  friend self_type & operator-=(self_type & self, D off) {
    static_assert(detail::is_advanceable_iterator_v<self_type>,
                  "must be advancable");
    self.advance(-off);
    return self;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  friend auto operator+(self_type self, D off) {
    static_assert(detail::is_advanceable_iterator_v<self_type>,
                  "must be advancable");
    return self += off;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  friend auto operator+(D off, self_type self) {
    static_assert(detail::is_advanceable_iterator_v<self_type>,
                  "must be advancable");
    return self += off;
  }

  template <typename D, typename = difference_type_arg_t<D, self_type>>
  friend auto operator-(self_type self, D off) {
    static_assert(detail::is_advanceable_iterator_v<self_type>,
                  "must be advancable");
    return self -= off;
  }

  friend auto operator-(self_type const & left, self_type const & right) {
    return right.distance_to(left);
  }

  friend bool operator==(self_type const & left, self_type const & right) {
    if constexpr (detail::has_distance_to_v<self_type>) {
      return (left - right) == 0;
    } else {
      return left.equal_to(right);
    }
  }

  friend bool operator!=(self_type const & left, self_type const & right) {
    return !(left == right);
  }

  friend bool operator<(self_type const & left, self_type const & right) {
    return (left - right) < 0;
  }

  friend bool operator<=(self_type const & left, self_type const & right) {
    return (left - right) <= 0;
  }

  friend bool operator>(self_type const & left, self_type const & right) {
    return (left - right) > 0;
  }

  friend bool operator>=(self_type const & left, self_type const & right) {
    return (left - right) >= 0;
  }

private:
  self_type & self() { return *static_cast<self_type *>(this); }
  self_type const & self() const {
    return *static_cast<self_type const *>(this);
  }
};
}

#include <iterator/detail/macro.h>

// In C++20, a concept/requires could be used to eschew the need for the below
// macros.
template <typename I> struct std::iterator_traits<::iterator::facade<I>> {
  using reference = DEREF_TYPE(I);
  using value_type = std::remove_cv_t<std::remove_reference_t<reference>>;
  using pointer = TYPE(I, operator->());
  using difference_type = ::iterator::detail::distance_to_t<I>;
  using iterator_category = ::iterator::detail::facade_category_t<I>;
};

#define MAKE_ITERATOR_FACADE_TYPEDEFS(type)                                    \
  template <>                                                                  \
  struct std::iterator_traits<type>                                            \
      : std::iterator_traits<::iterator::facade<type>> {}

#define MAKE_ITERATOR_FACADE_TYPEDEFS_T(type)                                  \
  template <typename... T>                                                     \
  struct std::iterator_traits<type<T...>>                                      \
      : std::iterator_traits<::iterator::facade<type<T...>>> {}

#include <iterator/detail/undef.h>