R's setnames: how does it detect linked objects?

Question

The setnames function in R (data.table package) is unusual for R in that it changes input by reference, meaning that it does not copy the table on which it operates. (Not to be confused with setNames from the stats package).

This has a surprising (and inconvenient) consequence: it will change any object to which the names attribute was previously saved. Viz:

require("data.table")

dt <- data.table(x = 1, y = 2)

dtnms <- function(dt){
    nms <- names(dt)
    print(nms)
    setnames(dt, c("a", "b"))
    nms
}

What would you dtnms(dt) to return? "x" "y" of course. Except it doesn't - setnames also modifies nms within the function, so that while "x" "y" is printed before setnames, "a" "b" is returned. If you put a stop() before the setnames, you can see that nms is just a character vector, with no special class or other indication of its provenance. So we would expect setnames to have no idea where nms had come from, yet somehow the link is there for setnames to "see". How is this possible? (This works the same with simple data frames).

A few further observations (remember to reset dt each time):

1. The link is propagated by further assignments

   dtnms2 <- function(dt){
   nms <- names(dt)
       print(nms)
       nms2 <- nms
       setnames(dt, c("a", "b"))
       nms2
   }
   

   dtnms2(dt) gives "a" "b"

2. The link is not just a question of similarity

   dtnms3 <- function(dt){
       chv <- c("x", "y")
       setnames(dt, c("a", "b"))
       chv
   }
   

   dtnms3(dt) gives "x" "y"

3. The link is not detectable by identical

   dtnms4 <- function(dt){
       chv <- c("x", "y")
       nms <- names(dt)
       identical(chv, nms)
   }
   

   dtnms4(dt) gives TRUE

4. The link can be broken (which is probably the best way round this)

   dtnms5 <- function(dt){
       nms <- names(dt)
       nms <- paste(nms)
       setnames(dt, c("a", "b"))
       nms
   }
   

   dtnms5(dt) gives "x" "y", back to the expected value. paste has broken the link, whatever it was.

So my question is, what is the link? Why does setnames change the nms object, which is just a plain old character vector bearing no sign of where it came from?