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

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

#pragma once

#include <iterator/forwards.h>
#include <iterator/proxy.h>

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 = RecursiveIterator(object);
 * for (ValuesIterator<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 ValuesIterator : public Proxy<Iterator, ValuesIterator<Iterator>> {
public:
  using Proxy<Iterator, ValuesIterator<Iterator>>::Proxy;
  decltype(auto) dereference() const {
    using value_type = typename Iterator::value_type;
    constexpr auto index = std::tuple_size<value_type>::value - 1;
    return std::get<index>(*this->impl());
  }
};

template <typename It> ValuesIterator(It) -> ValuesIterator<It>;
}