The solution below introduces a dependency on data.table, but otherwise
it does what you need:
---
# special method for Foo objects
length.Foo <- function(x) {
length(unlist(x, recursive = TRUE, use.names = FALSE))
}
# an instance of a Foo object
x <- structure(list(a = 1, b = list(b1 = 1, b2 = 2)), class = "Foo")
# its length
stopifnot(length(x) == 3L)
# get its length as if it were a standard list
.length <- function(x) {
cls <- class(x)
# setattr() does not make a copy, but modifies by reference
data.table::setattr(x, "class", NULL)
# get the length
len <- base::length(x)
# re-set original classes
data.table::setattr(x, "class", cls)
# return the unclassed length
len
}
# to check that we do not make unwanted changes
orig_class <- class(x)
# check that the address in RAM does not change
a1 <- data.table::address(x)
# 'unclassed' length
stopifnot(.length(x) == 2L)
# check that address is the same
stopifnot(a1 == data.table::address(x))
# check against original class
stopifnot(identical(orig_class, class(x)))
---
On 08/24/2018 07:55 PM, Henrik Bengtsson wrote:
Is there a low-level function that returns the length of an object 'x'
- the length that for instance .subset(x) and .subset2(x) see? An
obvious candidate would be to use:
.length <- function(x) length(unclass(x))
However, I'm concerned that calling unclass(x) may trigger an
expensive copy internally in some cases. Is that concern unfounded?
Thxs,
Henrik
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