Access object by address / pointer

Question

Can I access data.table object created in current R session by its memory address or pointer?

library(data.table)

DT <- data.table(a = 1:10, b = letters[1:10])
address(DT)
# [1] "0x6bf9b90"
attr(DT,".internal.selfref",TRUE)
# <pointer: 0x2655cc8>

Answer 1

This is somewhat of a silly way of doing it (as compared to how you can cast pointers in e.g. C++), but you could do:

# recursively iterate over environments
find.by.address = function(addr, env = .GlobalEnv) {
  idx = which(sapply(ls(env), function(x) address(get(x, env = env))) == addr)
  if (length(idx) != 0)
    return (get(ls(env)[idx], env = env))

  # didn't find it, let's iterate over the other environments
  idx = which(sapply(ls(env), function(x) is.environment(get(x, env = env))))
  for (i in idx) {
    res = find.by.address(addr, get(ls(env)[i], env = env))
    if (res != "couldn't find it") return (res)
  }

  return ("couldn't find it")
}

DT = data.table(a = 1)
e = new.env()
e$DT = data.table(b = 2)
e$f = new.env()
e$f$DT = data.table(c = 2)

find.by.address(address(DT))
#   a
#1: 1
find.by.address(address(e$DT))
#   b
#1: 2
find.by.address(address(e$f$DT))
#   c
#1: 2