Re: [Rd] Private fields in reference classes (was: Private Variables in R5-Classes possible?)

From: John Chambers <jmc_at_r-project.org>
Date: Wed, 22 Feb 2012 11:55:28 -0800

(Can we please follow the terminology in the ?ReferenceClasses documentation. Creativity is fine but here we need communication)

This has come up (once) before off-list. It could be made part of reference classes via a general rewriting that would improve efficiency as well. Not imminent, but it would be good to know of other applications, including why they are useful.

Meanwhile, it could be implemented for a subclass of reference classes, by adding a special field that lists the private (or public, if you like) fields, and then implementing S4 methods for "$" and "$<-" that check there for disallowed/allowed names. As presumably one wants, this leaves methods in the class free to use the private fields.

John

On 2/21/12 3:07 AM, Claus Jonathan Fritzemeier wrote:
> Hi list,
>
> is there a way to define some kind of private Variable?
>
> I would like to prevent the user from manipulating fields on his own,
> in order to not destroy data structures.
>
> The problem is, that as soon as the variable exists in the environment
> it is accessible via t$secret_value.
>
> test <- setRefClass("test", fields=list(
> secret = function(value){
> cat("the function was called\n")
> if(missing(value)){
> if(exists("secret_value", envir=.self@.xData)){
> return(get("secret_value", envir=.self@.xData, inherits = F))
> }
> else{
> return(NULL)
> }
> }
> assign("secret_value", value=value, , envir=.self@.xData)
> }
> ) )
>
> > t <- test$new()
> > t$secret
> the function was called
> NULL
> > t$secret_value
> Error in envRefInferField(x, what, getClass(class(x)), selfEnv) :
> "secret_value" is not a valid field or method name for reference class
> “test”
> > t$secret <- "Blub"
> the function was called
> > t$secret_value
> [1] "Blub"
>
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>



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