Field of matrices being returned with class `matrix` instead of `list`

Start by running the supplied RcppArmadillo reproducer and compare class(), typeof(), dim(), and str() for the arma::fieldarma::cube result. Trace the exporter path for arma::field and determine why the returned list receives matrix dimensions; done means the field is a list without dimensions while the contained cubes retain their array structure.

Stumbled across an interesting hiccup in the exporter for arma::field<T> (the generic vector). In particular, the attributes associated with an arma::field<arma::mat> shows as matrix under a class(x) call instead of list. The only way to obtain the list type is to use a typeof() call.

#include<RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
Rcpp::List test_case() {
  
  // Random dimensions
  int N = 5,
      P = 8,
      J = 10,
      K = 3,
      S = 2;

  // Create a 2d matrix
  arma::mat standard_2d_matrix(P, N);

  // Create a 3rd array
  arma::cube standard_3d_array(J, S, N);
  
  // Create a 4D Array
  arma::field<arma::cube> nonstandard_4d_array(N);
  
  // Fill it with cubes
  nonstandard_4d_array.fill(arma::ones<arma::cube>(J, K, S));
  
  return Rcpp::List::create(
    Rcpp::Named("d2", standard_2d_matrix),
    Rcpp::Named("d3", standard_3d_array),
    Rcpp::Named("d4", nonstandard_4d_array)
  );

}

The class issue is shown under d4 being matrix instead of list.

exporter_check = test_case() 

sapply(exporter_check, FUN = class)
#       d2       d3       d4 
# "matrix"  "array" "matrix" 
sapply(exporter_check, FUN = typeof)
#      d2       d3       d4 
# "double" "double"   "list" 

Moreover, the list is registering as a matrix with dim(x) of N x 1 instead of a list without dimensions.

sapply(exporter_check, FUN = dim)
# $d2
# [1] 8 5
# $d3
# [1] 10  2  5
# $d4
# [1] 5 1

And the object structure:

str(exporter_check)
# List of 3
# $ d2: num [1:8, 1:5] 1.49e-154 1.49e-154 2.57e-322 6.95e-310 6.95e-310 ...
# $ d3: num [1:10, 1:2, 1:5] 1.49e-154 1.49e-154 9.96e-320 1.63e-322 1.43e-322 ...
# $ d4:List of 5
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..$ : num [1:10, 1:3, 1:2] 1 1 1 1 1 1 1 1 1 1 ...
#  ..- attr(*, "dim")= int [1:2] 5 1