Hi all! This is likely related to #4783 and #4816.

Consider this example:

library(data.table)

x <- tibble::tibble(a = c(3,2,1))
foo <- function(dt) {
  print(address(dt))
  setDT(dt)
  print(address(dt))
  setkeyv(dt, "a")
  return(dt[])
}

> foo(x)
[1] "0x106cbd5c8"
[1] "0x115082a00"
   a
1: 1
2: 2
3: 3

The above is expected. From what I understood setDT will make a shallow copy of x and then reference another object, hence the change in address from 0x106cbd5c8 to 0x115082a00. However, what is unexpected to me is that the order of the original variable, x, also changed:

> x
   a
1: 1
2: 2
3: 3

Is this expected behavior? I thought that the setDT inside foo would make a shallow copy of the passed object, and would then reference that copy instead of the original, so that setkeyv would only arrange the data inside the function, but not the data outside the function. Just like in the example in this #SO where the data in the original variable is not modified.

Question: Why does setkeyv reorders x, even though we are calling setDT inside foo?