Avoiding deep copies

Is it correct that there is no way of avoiding deep copying
of data structures? Or asked from a different perspective,
is it true that there are no pointers? :)
(not that I am a fan of pointer, they just let me
decide when to do deep copy on my own when the
memory manager doesnt do it for me :) )

I was considering writing code in R that would need
the internal representation of complex graph structures
where several nodes in the graph point to the same
huge array of data.
Now, obviously one cannot just do something like
attr(thenode,"mydatavec")<-thedatavec or
thenode$mydatavec<-thedatavec
since all the data in thedatavec will be copied
with this assignment.
Are there commonly accepted/used workarounds?

Johann

There are two (useful) types of object that are passed by reference in
R: environments and external pointers.  External pointers are useful
for managing data allocated at the C level.  Environments are useful
for doing this sort of thing at the R level.  For example, if you
define, say,

new.ref <- function(value = NULL) {
    ref <- new.env()
    assign("value", value, env = ref)
    ref
}

ref.value <- function(ref) get("value", env = ref)

"ref.value<-" <- function(ref, value) {
    assign("value", value, env = ref)
    ref
}

then

r <- new.ref(<data that should not be copied>)

produces a reference object that will not be deep copied, and

s <- r

makes s another name for that reference, so that ref.value(s) <- <new value>
changes the value returned by ref.value(r):

r <- new.ref(1)
ref.value(r)

[1] 1

s<-r
ref.value(s) <- 2
ref.value(s)     

[1] 2

ref.value(r)

[1] 2

A slightly fancier version would wrap the environment in a list and
attach a class to it, something like

new.ref <- function(value = NULL) {
    ref <- list(env = new.env())
    class(ref) <- "refObject"
    assign("value", value, env = ref$env)
    ref
}

is.refObject <- function(r) inherits(r, "refObject")

print.refObject <- function(r) cat("<reference>\n")

ref.value <- function(ref) {
    if (! is.refObject(ref)) stop("not a refObject")
    get("value", env = ref$env)
}

"ref.value<-" <- function(ref, value) {
    if (! is.refObject(ref)) stop("not a refObject")
    assign("value", value, env = ref$env)
    ref
}

Then

r <- new.ref(1)
r

<reference>

ref.value(3)

Error in ref.value(3) : not a refObject


We've talked about adding something like this off and on but have not
gotten around to it yet.

luke

Luke Tierney
University of Minnesota                      Phone:           612-625-7843
School of Statistics                         Fax:             612-624-8868
313 Ford Hall, 224 Church St. S.E.           email:      luke at stat.umn.edu
Minneapolis, MN 55455 USA                    WWW:  http://www.stat.umn.edu
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