tupleの簡易実装試してみた。テンプレートパラメータのintはstd::size_tの方が良いと思うけど面倒臭かったのでint。計算量はO(N)だけどtupleってO(logN)で実装可能なんですかね？

tuple.cpp

template <typename... Args>
class tuple;

template <int Index, typename Tuple>
struct tuple_element;

template <int N, typename... Args>
typename tuple_element<N, tuple<Args...>>::type& get(tuple<Args...>& t);

template <typename Head, typename... Tails>
class tuple<Head, Tails...>
    : public tuple<Tails...>
{
    template <int N, typename... Args>
    friend typename tuple_element<N, tuple<Args...>>::type& get(tuple<Args...>& t);

    Head value;
};

template <>
class tuple<>
{
};

template <int Index, typename Head, typename... Tails>
struct tuple_element<Index, tuple<Head, Tails...>>
    : tuple_element<Index - 1, tuple<Tails...>>
{
};

template <typename Head, typename... Tails>
struct tuple_element<0, tuple<Head, Tails...>>
{
    using type = Head;
    using tuple_type = tuple<Head, Tails...>;
};

template <int N, typename... Args>
typename tuple_element<N, tuple<Args...>>::type& get(tuple<Args...>& t)
{
    return static_cast<tuple_element<N, tuple<Args...>>::tuple_type&>(t).value;
}

int main()
{
    tuple<int, char, int*> t;
    get<0>(t) = 42;
    get<1>(t) = '\n';
    get<2>(t) = nullptr;
}