Re: Does anybody know why boost::array does not support multiple dimensions?

From: Seth (bugs_at_[hidden])
Date: 2022-01-02 14:13:47

On Sun, Jan 2, 2022, at 9:40 AM, Ivan Matek via Boost wrote:
> Answering multiple > @Robert
> I think multi_array is still heap allocating, but unlike vector<vector> it
> does one heap allocation so it is more efficient.

Taking the liberty to appropriate that as a response to my post as well: https://godbolt.org/z/o4x6vGhW5

#include <boost/container/static_vector.hpp>
#include <boost/container/small_vector.hpp>
#include <boost/multi_array.hpp>
#include <fmt/ranges.h>

namespace bc = boost::container;
int main() {
    constexpr auto K=2, L=3, M=5, NUM=K*L*M;
    using boost::extents;
    using boost::indices;

    bc::static_vector<float, K*L*M> backing(NUM); // or small_vector that *can* optionally allocate
    std::iota(std::begin(backing), std::end(backing), 0);

    boost::multi_array_ref<float, 3> arr(backing.data(), extents[K][L][M]);
    fmt::print("K×L×M: {}\n", arr);

    arr.reshape(std::array{L,M,K});
    fmt::print("L×M×K: {}\n", arr);

    using range = boost::multi_array_types::index_range;
    fmt::print("3d slice: {}\n", arr[indices[range(2, 3)][range(0, M, 2)][range()]]);
    fmt::print("2d slice: {}\n", arr[indices[range(2, 3)][range(0, M, 2)][1]]);
}

This demos some of the Multi-Array interface with zero dynamic allocation. The library has more expressive indexing/layout options (see "general storage ordering" https://www.boost.org/doc/libs/1_78_0/libs/multi_array/doc/user.html#sec_storage).