The functions dimnames.xts and dimnames<-.xts are
methods for the base functions dimnames and dimnames<-.
xts objects by design are intended for lightweight management
of time-indexed data.
Rownames are redundant in this design, as well
as quite burdensome with respect to memory consumption and
internal copying costs.
rownames and colnames in R make use of dimnames method
dispatch internally, and thus require only modifications
to dimnames to enforce the xts no rownames requirement.
To prevent accidental setting of rownames, dimnames<-
for xts will simply set the rownames to NULL
when invoked, regardless of attempts to set otherwise.
This is done for internal compatibility reasons, as well as
to provide consistency in performance regardless of object use.
User level interaction with either dimnames or rownames will
produce a character vector of the index, formatted based
on the current specification of indexFormat. This
occurs within the call by converting the results
of calling index(x) to a character string, which itself
first creates the object type specified internally from the
underlying numeric time representation.