cpp-useful-iterators/include/iterator/detail/recursive_iterator_traits.hpp

#pragma once

#include <iterator>
#include <string>
#include <tuple>
#include <utility>

namespace iterator { namespace detail {
  struct terminal_layer_tag_t;
  struct continue_layer_tag_t;

  template <typename> struct void_t { using type = void; };

  template <typename IterType>
  using value_iterator = decltype(std::begin(*std::declval<IterType>()));

  template <typename IterType>
  using mapped_iterator =
      decltype(std::begin(std::declval<IterType>()->second));

  /**
   * @class next_layer_type
   * @breif A template metaprogramming type for unifying associative and
   * non-associative containers.
   */
  template <typename V, typename Tag> struct next_layer_type {
    using type = std::tuple<V>;
  };
  template <typename V> struct next_layer_type<V, continue_layer_tag_t> {
    using type = V;
  };

  template <typename T, typename = void>
  struct is_associative : std::false_type {};
  template <typename T>
  struct is_associative<T, std::enable_if_t<std::is_const<
                               typename T::value_type::first_type>::value>>
      : std::true_type {};

  template <typename T>
  using is_associative_t = std::enable_if_t<is_associative<T>::value>;

  /**
   * Type deduction guides for constructing recursive iterators.
   */
  enum class typeclass { TERMINAL, CONTAINER, ASSOCIATIVE_CONTAINER };

  template <typename T, typename = void> struct typeclass_t {
    static constexpr typeclass const value{typeclass::TERMINAL};
  };

  template <>
  struct typeclass_t<typename std::string::iterator> : typeclass_t<void> {};
  template <>
  struct typeclass_t<typename std::string::const_iterator> : typeclass_t<void> {
  };

  template <typename T>
  struct typeclass_t<T, typename void_t<value_iterator<T>>::type> {
    static constexpr typeclass const value{typeclass::CONTAINER};
  };

  template <typename T>
  struct typeclass_t<T, typename void_t<mapped_iterator<T>>::type> {
    static constexpr typeclass const value{typeclass::ASSOCIATIVE_CONTAINER};
  };

}}