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
#include
#include
#include
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 backing(NUM); // or small_vector that *can* optionally allocate
std::iota(std::begin(backing), std::end(backing), 0);
boost::multi_array_ref 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_sto...).
