cpp-useful-iterators/include/iterator/unkeyed_iterator.hpp

//
//  unkeyed_iterator.hpp
//  iterator
//
//  Created by Sam Jaffe on 2/20/17.
//

#pragma once

#include "facade.h"
#include <iterator>

namespace iterator {
  /**
   * At first glance, this class seems like it acts as an iterator that would be
   * used in the C++ equivalent of Java's Map::values() function. However,
   * notices that we do not assume that the input iterator is a map iterator.
   * The useful feature of this iterator - and the source of its name - is its
   * ability to be composed with recursive_iterator. This means that you can
   * have something crazy like a four-level map and then do:
   * \code
   * std::map<int, std::map<std::string, std::map<int, BigType>>> object;
   * auto const rit = make_recursive_iterator(object);
   * for (unkeyed_iterator<decltype(rit)> it = rit, end = {}; it != end; ++it) {
   *   // Process only BigType, discarding all of the keys that we need to walk
   * }
   * \endcode
   */
  template <typename Iterator>
  class unkeyed_iterator : public facade<unkeyed_iterator<Iterator>> {
  private:
    static constexpr std::size_t const value_index =
        std::tuple_size<typename Iterator::value_type>::value - 1;

  public:
    unkeyed_iterator() = default;
    unkeyed_iterator(Iterator it) : base(it) {}

    decltype(auto) dereference() const { return std::get<value_index>(*base); }
    void increment() { ++base; }
    void decrement() { --base; }
    bool equal_to(unkeyed_iterator const & other) const {
      return base == other.base;
    }

  private:
    Iterator base;
  };
}

MAKE_ITERATOR_FACADE_TYPEDEFS_T(::iterator::unkeyed_iterator)