// implement 1
template <typename Tuple, typename Func, size_t Index = 0>
void tuple_for_each(const Tuple& t, const Func& f) {
    constexpr auto n = std::tuple_size_v<Tuple>;
    if constexpr (Index < n) {
        tuple_at<Index>(t, f);
        tuple_for_each<Tuple, Func, Index + 1>(t, f);
    }
}
// implement 2
template <typename Function>
void for_each(const std::tuple<>&, Function) { }

template <typename T, typename... Ts, typename Function>
void for_each(const std::tuple<T, Ts...>& tuple, Function func) {
    func(std::get<0>(tuple));
    for_each(std::tuple<Ts...>(std::get<Ts>(tuple)...), func);
}

第一种用到 size_t Index = 0，惊呼了一下这么玩
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