CODEWITHSUNDEEP
c++constexprstdarray
NoSenseEtAl
NoSenseEtAl

Reputation: 30058

Is it possible to initialize constexpr std::array member in a programtic way

Lets assume I want to write struct that has a member constexpr std::array that contains first N fibs, where N is a template argument.

Something like this but with vals being avaliable at compile time:

template <int N>
struct first_n_fibs {
    static_assert(N>0);
    static const std::array<int, N> vals;
    static std::array<int, N> init_fibs(){
        std::array<int,N> result;
        if (N==1) {
            return std::array<int,N>{1};
        } else {
            result[0]=1;
            result[1]=1;
            for(int i =2; i<N;++i) {
                result[i]=result[i-2]+result[i-1];
            }
        }
        return result;
    }
};

template<int N>
const std::array<int, N> first_n_fibs<N>::vals=init_fibs();


int main(){
    std::cout << first_n_fibs<2>::vals.back() << std::endl;
    std::cout << first_n_fibs<5>::vals.back() << std::endl;
    std::cout << first_n_fibs<6>::vals.back() << std::endl;
}

I suspect there is no workaround since std::array constructors are not constexpr, so if somebody knows any workarounds involving C arrays or boost I would be happy with that.

Upvotes: 3

Views: 6588

Answers (2)

Guillaume Racicot
Guillaume Racicot

Reputation: 41770

You can use immediately called lambda:

struct foo {
    static constexpr auto arr = []{
        std::array<int, 6> a{};

        for (auto& e : a) {
            // stuff
        }

        return a;
    }();
};

But as HolyBlackCat said, std::array is constexpr constructible, since it has no defined constructor, only compiler defined ones.

Here's live example at coliru

Upvotes: 2

HolyBlackCat
HolyBlackCat

Reputation: 96334

You don't need anything special, constexpr function requirements are very relaxed these days:

#include <iostream>
#include <array>

template <int N> constexpr std::array<int, N> first_n_fibs()
{
    std::array<int, N> ret{};
    ret[0] = 0;
    if (N == 1) return ret;
    ret[1] = 1;
    for (int i = 2; i < N; i++)
        ret[i] = ret[i-2] + ret[i-1];
    return ret;
}

int main()
{
    constexpr auto a = first_n_fibs<3>();
}

(try it live)

std::array constructors are not constexpr

Apparently it has no user-defined constructors at all, so nothing stops its construction from being constexpr.

Upvotes: 7

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