[R] S4 pass-by-value work-around?

[Henrik Bengtsson]

Quick comment.  Use an environment to hold your "fields" and then pass
around the environment variable.  Any modification to variables inside
the environment done from anywhere (inside/outside functions) will be
persistent.  Then wrap this up in a class structure, overload, say,
'$', '[[', '$<-', '[[<-' to get() and assign() to your object and you
have the first seed of what is done in R.oo.

Note that '$' are '$<-' indeed available for environments.  However,
it is not recommended to extend the environment class itself.  Instead
you should wrap the environment up in a list and work from that.
Search the R mailing list archives for issues you would/will face if
you inherit from environment.

My $.02

Henrik
(R.oo author)

On Wed, Jun 18, 2008 at 10:26 PM, Jeffrey Spies <jspies2008 at gmail.com> wrote:
>
> Howdy all,
>
> I have a problem that I'd like some advice/help in solving---it has to do
> with R's pass-by-value system.  I understand the issue, but am wondering if
> anyone has found a working solution in dealing with it for cases when one
> wants to modify an object inside of a method, specifically when working with
> S4.  I'm aware that R.oo is able to deal with this using S3, but I'd really
> rather stick to S4.
>
> The basics of what I would like to do are coded below:
>
> setClass("MyMatrix",
>        representation(
>                parameters="matrix",
>                uniqueCount="numeric"
>        ),
>        prototype(
>                parameters=matrix(numeric(0),0,0),
>                uniqueCount=1
>        )
> )
>
> setGeneric("createUniqueName", function(object)
> standardGeneric("createUniqueName"))
>
> setMethod("createUniqueName", "MyMatrix", function(object){
>        retval <- paste("unique_", object at uniqueCount, sep="")
>        object at uniqueCount <- object at uniqueCount + 1
>        return(retval)
> })
>
> x <- new("MyMatrix", parameters=matrix(0, 2, 2))
> createUniqueName(x) # returns "unique_1"
> x # x at uniqueCount is still 1
>
> I understand why this is happening, but am wondering how people in the
> community have dealt with it, specifically when using S4.  Any advice would
> be appreciated.  Also, I am aware that this is somewhat of a silly example,
> but it should allow you to see what I'm trying to accomplish.
>
> Thank you,
>
> Jeff.
>
>
> --
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>
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