r reference classes - do they have static field members/variables?

Question

I have been playing a little with R's R5 class system to see what it can and can't do. In that process I have stumbled upon what looks like static class field members (which does not appear to be in the documentation - but i could have missed it)

[2014 update]
Warning !!! : The following code does not work with R version >= 3.0
This post, its answers and particularly the comments provide useful insights and reminders about the R5 OO-system and the R language in general. However it is probably a bad idea to cultivate the idiom of using the environment of the R5 class instances directly.
[end 2014 update]

In the following code, the first field is the classic definition of an instance field variable. The second definition appears to create a static class field using an accessor method. I would like to know if this use is kosher (or is my code example simply coincidental). The third field use creates a quasi-private instant field variable using an accessor method.

assertClass <- function(x, className, R5check=FALSE) {
    # simple utility function
    stopifnot(class(x)[1] == className)
    if(R5check) stopifnot(is(x, 'envRefClass'))
}

A <- setRefClass('A',
    fields = list(
        # 1. public, typed, instance field
        myPublicInstanceVar = 'character',

        # 2. this assignment appears static
        #    but if the field me.static.private
        #    was declared in the field list
        #    it would be a local instance var
        myPrivateStaticVar = function(x) {
            if (!missing(x)) {
                assertClass(x, 'character')
                me.static.private <<- x
            }
            me.static.private
        },

        # 3. quasi-private, typed, instance field
        myPrivateInstanceVar = function(x) {
            if (!missing(x)) {
                assertClass(x, 'character')
                .self$me.private <<- x
            }
            .self$me.private
        }
    ),

    methods = list(
        initialize = function (c='default') {
            myPublicInstanceVar <<- c
            myPrivateStaticVar <<- c
            myPrivateInstanceVar <<- c
        }
    )
)

# test instantiation
instance1.of.A <- A$new('first instance')
str(instance1.of.A)

instance2.of.A <- A$new('second instance')
str(instance1.of.A)
str(instance2.of.A)

instance3.of.A <- getRefClass('A')$new('third instance')
instance3.of.A$myPrivateStaticVar <- 'Third instance - changed'
print(instance1.of.A$myPrivateStaticVar)
print(instance2.of.A$myPrivateStaticVar)
print(instance3.of.A$myPrivateStaticVar)
str(instance1.of.A)
str(instance2.of.A)
str(instance3.of.A)

# but not really private ...
instance1.of.A$myPublicInstanceVar # works
instance1.of.A$me.static.private   # DOES NOT WORK - where is this variable stored 
instance1.of.A$me.private          # works

# till death do us part
instance3.of.A <- NULL
gc()
str(instance1.of.A)
str(instance2.of.A)
str(instance3.of.A)

If you run this code - you can see that the second field variable appears to operate as a static class member. What is less clear to me is where the reference class keeps this field (hence my comment in the penultimate line above).

2. doesn't seem at all quirky/strange to me - that's just standard R environment semantics. But I think you misunderstand what's going on - because you've assigned to `me.static.private` in the global environment — hadley, Nov 09 '12 at 13:25
And in 3. I don't think you need `<<-` - you're just assigning to a specified variable `me.private` in a specified environment `.self` — hadley, Nov 09 '12 at 13:26
Hadley - once again, thank you for your insight - on #2 you are right, the `me.static.private` variable is in the global environment. It can be seen there using ls() - on #3, I think `<<-` and `<-` write to the same place (not sure why), but my tests indicate this (and I am left wondering if a different result would occur with a subclass). — Mark Graph, Nov 09 '12 at 21:35
They write to the same place because you've specified the environment (`.self`). I don't think using a subclass would be different because I don't think the environments will be nested. — hadley, Nov 10 '12 at 13:21
See related question: https://stackoverflow.com/questions/8159317/private-members-in-r-reference-class — mjv, Oct 15 '14 at 22:30

score 1 · Accepted Answer · answered Nov 10 '12 at 03:02

1

The short answer - based on Hadley's comments above - is no. R's reference classes do not have static variables.

answered Nov 10 '12 at 03:02

Mark Graph

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r reference classes - do they have static field members/variables?

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