How to create a multi dimensional std::array using a template?

Question

How can I create a multi dimensional std::array of type Type with (of course) known initial constexpr dimensions with some smart variadic template "MDA".

The number of dimensions shall be variable.

In the end I want to be able to write:

MDA<int,3,4,5,6> a4d{};

and the result should be equal to

std::array < std::array < std::array < std::array<int, 6> , 5 > , 4 > , 3 > a4d{};

And save a lot of (complicated, or even error prone) typing work . . .

Edit:

I am not sure if this possible at all. But what I am looking for, is some "typing saver", maybe in conjunction with a using statement.

I can't help but notice, that every other "How to..." question of yours has an attempt of implementation clearly displayed, while this one is indeed seems like more of a request than a question on some concrete obstacle. Hence, the vote to close I guess. — rawrex
– rawrex, Commented Aug 7, 2021 at 10:59
Hm, since I am not a native english speaker, I may misunderstand your comment. I asked my question in good faith. I tried many embarrasing things, but I cannot find a solution. So, I am asking the community for help. I hope that this is allowed here on SO. If there is a misbehaviour on my end, I will delete the question. Although I am really interested in the solution . . . And obviously a "how to" question is obviously not good. I do not understand the subtelty of this, but will try to avoid that in the furture — A M
– A M, Commented Aug 7, 2021 at 11:14

Quimby · Accepted Answer · 2021-08-07 13:54:45Z

5

Certainly possible with use of helper class templates, C++11 (except the static assert part)

#include <array>

template<typename T, std::size_t Head, std::size_t... Tail>
struct MDA_impl{
    using type = std::array<typename MDA_impl<T, Tail...>::type, Head>;
};
// Base specialization
template<typename T, std::size_t N>
struct MDA_impl<T,N>{
    using type = std::array<T, N>;
};

template<typename T, std::size_t... Ns>
using MDA = typename MDA_impl<T,Ns...>::type;

int main(){
    static_assert(std::is_same_v<MDA<int,3,4,5,6>,
                  std::array<std::array<std::array<std::array<int, 6>,5>,4>,3>>);
}

edited Aug 7, 2021 at 13:54

answered Aug 7, 2021 at 11:35

Quimby

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Comments

G. Sliepen · Accepted Answer · 2021-08-07 12:15:46Z

2

You can use the so-called meta-functions.

template<class T, int N, int... M>
struct MDA_struct
{
    using type = std::array<typename MDA_struct<T, M...>::type, N>;
};

template<class T, int N>
struct MDA_struct<T, N>
{
    using type = std::array<T, N>;
};

template<class T, int... N>
using MDA = typename MDA_struct<T, N...>::type;

edited Aug 7, 2021 at 12:15

G. Sliepen

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answered Aug 7, 2021 at 11:01

Karen Baghdasaryan

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2 Comments

Quimby Over a year ago

You cannot overload using statements. But your idea is correct, just use structs and partial specialization for the implementation details.

G. Sliepen Over a year ago

The code should now work correctly, see godbolt.org/z/ff6TnfEz7

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