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
On Thu, 1 Sep 2005, Luke Tierney wrote:
Another variant of what is probably the same issue:
> setClass("foo", representation(a = "numeric"))
[1] "foo"
> f <- function() [EMAIL PROTECTED] <<- 2
> x <- new("foo",a=1)
> y <- x
> f()
> x
An object of class âfooâ
Slot "a":
[1] 2
> y
An object of class âfooâ
Slot "a":
[1] 2
luke
On Wed, 31 Aug 2005, [EMAIL PROTECTED] 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
> [EMAIL PROTECTED] <- 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: [EMAIL PROTECTED]
Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
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