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

//
//  indexed_iterator.hpp
//  iterator
//
//  Created by Sam Jaffe on 3/5/17.
//

#pragma once

#include <iterator>

namespace iterator {
  template <typename Iterator>
  class indexed_iterator {
  private:
    using base_value_type = typename std::iterator_traits<Iterator>::value_type;
    using base_reference = typename std::iterator_traits<Iterator>::reference;
  public:
    using index_type = std::size_t;
    using value_type = std::pair<index_type, base_value_type>;
    using reference = std::pair<index_type, base_reference>;
    using pointer = void;
    using difference_type = typename std::iterator_traits<Iterator>::difference_type;
    using iterator_category = typename std::iterator_traits<Iterator>::iterator_category;
    
    indexed_iterator() : _base(), _index(0) {}
    indexed_iterator(Iterator base, index_type idx = 0) : _base(base), _index(idx) {}
    
    template <typename OtherIterator>
    indexed_iterator(indexed_iterator<OtherIterator> const & oiter) :
    _base(oiter._base), _index(oiter._index) {
      
    }
    
    reference operator*() const { return reference{ _index, *_base }; }
    
    indexed_iterator & operator++() { ++_base; ++_index; return *this; }
    indexed_iterator operator++(int) { indexed_iterator tmp{*this}; operator++(); return tmp; }
    bool operator==(indexed_iterator const & other) const { return _base == other._base; }
    bool operator!=(indexed_iterator const & other) const { return _base != other._base; }
    
    // Requires: iterator_category = bidirectional_iterator_tag
    indexed_iterator & operator--() { --_base; --_index; return *this; }
    indexed_iterator operator--(int) { indexed_iterator tmp{*this}; operator--(); return tmp; }
    
    // Requires: iterator_category = random_access_iterator_tag
    indexed_iterator operator+(difference_type n) const { return indexed_iterator{*this} += n; }
    indexed_iterator & operator+=(difference_type n) { _index += n; _base += n; return *this; }
    indexed_iterator operator-(difference_type n) const { return indexed_iterator{*this} -= n; }
    indexed_iterator & operator-=(difference_type n) { _index -= n; _base -= n;return *this; }
    bool operator<=(indexed_iterator const & other) const { return _base <= other._base; }
    bool operator< (indexed_iterator const & other) const { return _base <  other._base; }
    bool operator>=(indexed_iterator const & other) const { return _base >= other._base; }
    bool operator> (indexed_iterator const & other) const { return _base >  other._base; }
  private:
    template <typename It> friend class ::iterator::indexed_iterator;
    Iterator _base;
    index_type _index;
  };
}