A numeric vector with no attributes representing
the data.
size
An integer. The number of observations in the object.
This is equivalent to the number of rows.
shape
An integer vector. The shape corresponds to all of the
dimensions in the object except for the first one (the size).
dim_names
A list. For no names, NULL, in which case a list of
empty characters will be constructed. Otherwise the list must
be the same length as the total number of dimensions
(i.e length(c(size, shape))). Each element of the list much be either
a character vector the same size as the corresponding dimension, or
character(0) for no names for that dimension.