iterator.h

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/*
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  it under the terms of the GNU General Public License, version 2.0,
  as published by the Free Software Foundation.
 
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  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
 
  You should have received a copy of the GNU General Public License
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*/
 
#ifndef MYSQL_HARNESS_STDX_ITERATOR_H_
#define MYSQL_HARNESS_STDX_ITERATOR_H_
 
#include <type_traits>
 
// from C++20
 
namespace stdx {
namespace impl {
template <class T, bool = std::is_array_v<T>>
struct indirectly_readable_traits_array;
 
template <class T>
struct indirectly_readable_traits_array<T, true> {
  using value_type = std::remove_cv_t<std::remove_extent_t<T>>;
};
 
template <class T, bool = std::is_object_v<T>>
struct indirectly_readable_traits_pointer {};
 
template <class T>
struct indirectly_readable_traits_pointer<T, true> {
  using value_type = std::remove_cv_t<T>;
};
 
template <class T, typename = std::void_t<>>
struct has_value_type : std::false_type {};
 
template <class T>
struct has_value_type<T,
                      std::void_t<typename std::remove_cvref_t<T>::value_type>>
    : std::true_type {};
 
template <class T, bool = has_value_type<T>::value>
struct indirectly_readable_traits_member_value_type;
 
template <class T>
struct indirectly_readable_traits_member_value_type<T, true> {
  using value_type = typename T::value_type;
};
 
template <class T, typename = std::void_t<>>
struct has_element_type : std::false_type {};
 
template <class T>
struct has_element_type<T, std::void_t<typename T::element_type>>
    : std::true_type {};
 
template <class T, bool = has_element_type<T>::value>
struct indirectly_readable_traits_member_element_type;
 
template <class T>
struct indirectly_readable_traits_member_element_type<T, true> {
  using value_type = typename T::element_type;
};
 
template <class T, typename = std::void_t<>>
struct has_reference : std::false_type {};
 
template <class T>
struct has_reference<T, std::void_t<typename T::reference>> : std::true_type {};
 
template <class T, bool = has_reference<T>::value>
struct iter_reference;
 
template <class T>
struct iter_reference<T, false> {
  using reference = decltype(*std::declval<T &>());
};
 
template <class T>
struct iter_reference<T, true> {
  using reference = typename T::reference;
};
 
}  // namespace impl
 
template <class T, class Enable = void>
struct indirectly_readable_traits {};
 
template <class T>
struct indirectly_readable_traits<T *, std::enable_if_t<std::is_object_v<T>>>
    : impl::indirectly_readable_traits_pointer<T> {};
 
template <class T>
struct indirectly_readable_traits<T, std::enable_if_t<std::is_array_v<T>>>
    : impl::indirectly_readable_traits_array<T> {};
 
template <class T>
struct indirectly_readable_traits<
    T, std::enable_if_t<impl::has_value_type<T>::value>>
    : impl::indirectly_readable_traits_member_value_type<
          std::remove_cvref_t<T>> {};
 
template <class T>
struct indirectly_readable_traits<
    T, std::enable_if_t<impl::has_element_type<T>::value>>
    : impl::indirectly_readable_traits_member_element_type<T> {};
 
template <class T>
struct indirectly_readable_traits<const T> : indirectly_readable_traits<T> {};
 
template <class T>
using iter_value_t =
    typename indirectly_readable_traits<std::remove_cvref_t<T>>::value_type;
 
template <class T>
using iter_reference_t =
    typename impl::iter_reference<std::remove_cvref_t<T>>::reference;
 
}  // namespace stdx
 
#endif