Quick Q: Array Initialisation Compile Time - Constexpr Sequence

Save to:
Instapaper Pocket Readability

Quick A: Use integer_sequence with a helper function.

Recently on SO:

Array Initialisation Compile Time - Constexpr Sequence

1) How to implement that kind of integer_sequence?

template <std::size_t... Is>
constexpr auto make_sequence_impl(std::index_sequence<Is...>)
    return std::index_sequence<generate_ith_number(Is)...>{};

template <std::size_t N>
constexpr auto make_sequence()
    return make_sequence_impl(std::make_index_sequence<N>{});

2) Is it possible to build an std::array from that integer_sequence at compile time?

template <std::size_t... Is>
constexpr auto make_array_from_sequence_impl(std::index_sequence<Is...>)
    return std::array<std::size_t, sizeof...(Is)>{Is...};

template <typename Seq>
constexpr auto make_array_from_sequence(Seq)
    return make_array_from_sequence_impl(Seq{});


int main()
    constexpr auto arr = make_array_from_sequence(make_sequence<6>());
    static_assert(arr[0] == 0);
    static_assert(arr[1] == 1);
    static_assert(arr[2] == 2);
    static_assert(arr[3] == 4);
    static_assert(arr[4] == 5);
    static_assert(arr[5] == 7);

Add a Comment

You must sign in or register to add a comment.

Comments (0)

There are currently no comments on this entry.