Bug in copying of S4 objects (PR#8112)

I've poked in the source a bit.  Here are some notes in case someone
has time to look into this.

The main internal difference between <<- and <- for complex
assignments is that <- calls EnsureLocal which calls duplicate on the
value of the left hand side value if NAMED == 2.  In principle I don't
tink this should be necessary, but it means that assignment functions
called for <- assignments will not see NAMED == 2.  This protects
agains internal assignment functions that destructively modify without
looking at NAMED, which is what @<- and slot<- do.  But <<- does not
do this defensive duplicating, hence the problem with <<-.  Similar
problems occur if structures contain environments used to implement
reference behaviour since the copying stops at the environment.

The appropriate fix is to insure that @<- and slot<- respect NAMED and
duplicate when NAMED == 2, as attr<- does.  This will require either
making @<- and slot<- into SPECIALSXP's or some sort of
underhandedness to allow them to remain closures.  One possibility
might be to define a SPECIALSXP that looks at the NAMED value of the
object argument (by inspecting the promise before it is forced).  This
can then be passed into the internal R_set_slot function via .Call.

luke

Another variant of what is probably the same issue:

   > setClass("foo", representation(a = "numeric"))

   [1] "foo"

   > f <- function() x at a <<- 2
   > x <- new("foo",a=1)
   > y <- x
   > f()
   > x

   An object of class ???foo???
   Slot "a":
   [1] 2

   > y

   An object of class ???foo???
   Slot "a":
   [1] 2

luke

On Wed, 31 Aug 2005, murdoch at stats.uwo.ca wrote:

If I have an S4 object, and I make a copy, changes to the original
aren't reflected in the copy:

setClass("foo", representation(slot="numeric"))
x <- new("foo", slot=1)
y <- x
x at slot <- 2
y

An object of class "foo"
Slot "slot":
[1] 1

This is as it should be.  However, if I call the slot assignment
function in a funny way, y *does* receive the changes:

x <- new("foo", slot=1)
y <- x
assign("x", "@<-"(x, "slot", 2))
y

An object of class "foo"
Slot "slot":
[1] 2

This happens in the current R-devel in Windows, and R-patched too.

version

         _
platform i386-pc-mingw32
arch     i386
os       mingw32
system   i386, mingw32
status   Under development (unstable)
major    2
minor    2.0
year     2005
month    08
day      31
svn rev  35467
language R

Duncan Murdoch

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Luke Tierney
Chair, Statistics and Actuarial Science
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu