Initialize a constexpr std::array with the size of an n-dimensional std::array

Question

I wrote a function which takes an N-dimensional std::array and a parameter pack (coordinates) equal to the number of dimensions of the input std::array. I already can estimate the size of each dimension of the std::array with a meta function and I wrote a functions counting the parameters in the pack.

I want to 1) generate a new constexpr std::array with size equal to the number of dimensions of the input std::array. 2) The array shall be initialized with the size of each dimension of the input std::array. Does someone have a tip how to fill the std::array right with C++11 only.

E.g. this code

using array3d = std::array<std::array<std::array<int, 4>, 4>, 4>;
3d_helper<array3d>(array3d(), 0,0,0);

Should generate:

constexpr std::array<int, 3> array = { 4, 4, 4 };

Here is what I have so far:

//! Static estimation of std::array container size
// Declare a generic template (which is called initially)
template <size_t dim, class Array>
struct static_size;

// specialization for std::array and first dimension
// creates a struct with a static member "value = N"
template <class T, size_t N>
struct static_size<0, std::array<T, N>> : std::integral_constant<size_t, N> {};

// specialization for std::array and dimension > 0 -> recurse down in dim
template <size_t dim, class InnerArray, size_t N>
struct static_size<dim, std::array<InnerArray, N>> : static_size<dim - 1, InnerArray> {};

template <class FIRST, class... OTHER>
size_t num_args() {
    return 1 + num_args<OTHER...>();
}

template <class FIRST>
size_t num_args() {
    return 1;
}

template <class ARRAY, class... ARGS>
struct 3d_helper {
    static glm::vec3 at_t(const ARRAY &points, ARGS... args) {
        constexpr size_t nargs = num_args<ARGS...>();
        /*
        constexpr size_t n1 = static_size<0, ARRAY>::value - 1;
        constexpr size_t n2 = static_size<1, ARRAY>::value - 1;
        */
        // ...
        using array_t = std::array<size_t, nargs>;
        // fill it somehow
    }   
};

array<int, 1>, array<int, 2>, array<int, 3>, ... dependent on whether the input is 1d, 2d, 3d, .. — dgrat
– dgrat, Commented Sep 4, 2017 at 15:46
Sorry, I updated, was a typo. I ment to generate from a 3d array a 1d containing just the size. — dgrat
– dgrat, Commented Sep 4, 2017 at 16:00

André Bergner · Accepted Answer · 2017-09-04 16:05:49Z

Based on my solution of this related question here is a way to do it

// Example program
#include <iostream>
#include <string>
#include <array>

// typedefs for certain container classes
template<class T, size_t x>
using array1D = std::array<T, x>;

template<class T, size_t x, size_t y>
using array2D = std::array<std::array<T, y>, x>;

template<class T, size_t x, size_t y, size_t z>
using array3D = std::array<std::array<std::array<T, z>, y>, x>;


template <size_t dim, typename Array>
struct size_of_dim;

// specialization for std array and first dimension
template <typename T, size_t N>
struct size_of_dim<0, std::array<T,N>> : std::integral_constant<size_t, N> {};

// specialization for std array and dimension > 0 → recurse down in dim
template <size_t dim, typename InnerArray, size_t N>
struct size_of_dim<dim, std::array<InnerArray,N>> : size_of_dim<dim-1,InnerArray> {};



template <typename Array>
struct cardinality : std::integral_constant<size_t, 0> {};

template <typename T, size_t N>
struct cardinality<std::array<T,N>> : std::integral_constant<size_t, cardinality<T>::value + 1> {};

template <typename Array>
auto constexpr cardinality_v = cardinality<Array>::value;


template <typename Array, size_t... Ns >
constexpr auto dimensions_impl(std::index_sequence<Ns...>) {
    std::array<size_t, cardinality_v<Array>> result = { size_of_dim<Ns,Array>::value... };
    return result;
}


template <typename Array>
constexpr auto dimensions() {
    return dimensions_impl<Array>(std::make_index_sequence<cardinality_v<Array>>() );
}



int main()
{
    auto test = [](auto arr){
        constexpr auto dims = dimensions<decltype(arr)>();
        for (auto d : dims)
            std::cout << d << ", ";
        std::cout << std::endl;
    };
    test(array1D<float, 1337>());
    test(array2D<float, 7357, 17>());
    test(array3D<float, 3, 4, 5>());
}

DEMO

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