Tersely copy keys from a map

element_get.h

#include <cstddef>
#include <tuple>
#include <utility>

template <size_t I, typename T>
struct element_get_helper;

template <size_t I, typename... Ts>
struct element_get_helper<I, std::tuple<Ts...>>
{
  using T = std::tuple<Ts...>;
  constexpr decltype(auto) operator()(const T& t) const {
    return std::get<I>(t);
  }
  constexpr decltype(auto) operator()(T& t) const {
    return std::get<I>(t);
  }
  constexpr decltype(auto) operator()(T&& t) const {
    return std::get<I>(std::move(t));
  }
};

template <size_t I, typename T, typename U>
struct element_get_helper<I, std::pair<T, U>>
{
  using P = std::pair<T, U>;
  constexpr decltype(auto) operator()(const P& p) const {
    return std::get<I>(p);
  }
  constexpr decltype(auto) operator()(P& p) const {
    return std::get<I>(p);
  }
  constexpr decltype(auto) operator()(P&& p) const {
    return std::get<I>(std::move(p));
  }
};

template <size_t I, typename T, size_t N>
struct element_get_helper<I, std::array<T, N>>
{
  using A = std::array<T, N>;
  constexpr decltype(auto) operator()(const A& a) const {
    return std::get<I>(a);
  }
  constexpr decltype(auto) operator()(A& a) const {
    return std::get<I>(a);
  }
  constexpr decltype(auto) operator()(A&& a) const {
    return std::get<I>(std::move(a));
  }
};

template <size_t N, typename C>
constexpr inline auto eget(const C&)
{
  return element_get_helper<N, typename C::value_type>{};
};

template <typename C>
constexpr inline auto efirst(const C&)
{
  return element_get_helper<0, typename C::value_type>{};
}

template <typename C>
constexpr inline auto esecond(const C&)
{
  return element_get_helper<1, typename C::value_type>{};
}

// To copy keys out of a map:
//
// map<string, string> m;
// ...
// vector<string> v;
// transform(m.cbegin(), m.cend(), back_inserter(v), efirst(m));

// or to move values:
//
// vector<string> v;
// transform(make_move_iterator(m.begin()),
//           make_move_iterator(m.cend()),
//           back_inserter(v), esecond(m));

eget a tuple compile error, tuple has not value_type.