是否可以将constexpr std :: array转换为std :: integer_sequence？

Question

也就是说，将其constexpr std::array<int,2>{1,2}传递给将吐出类型的某些函数或帮助器类std::integer_sequence<int, 1, 2>？

It seems easy to jump from the type world to the "constexpr value" world (e.g., to do the reverse conversion), but hard or impossible to do the reverse.

Answer 1

似乎您可以在C ++ 17中做到这一点，但要在调用站点上引入lambda：

template <size_t N, typename F, size_t... indexes>
constexpr auto make_seq_helper(F f, std::index_sequence<indexes...> is) {
    return std::integer_sequence<int, std::get<indexes>(f())...>{};
}

template <typename F>
constexpr auto make_seq(F f) {
    constexpr size_t N = f().size();
    using indexes = std::make_index_sequence<N>;
    return make_seq_helper<N>(f, indexes{});
};

make_seq像这样调用：

    constexpr std::array a{7, 15, 28};
    auto x = make_seq([](){ return a; });

结果为xwith类型std::integer_sequence<int, 7, 15, 28>。我不确定lambda用法是否可以删除。