Private Members in R Reference Class

Question

Is it possible to have private member fields inside of an R reference class. Playing with some of the online examples I have:

> Account <- setRefClass(    "ref_Account"
>      , fields = list(
>       number = "character"
>       , balance ="numeric")
>      , methods = list( 
>     deposit <- function(amount) {
>       if(amount < 0)   {
>         stop("deposits must be positive")
>       }
>       balance <<- balance + amount
>     }
>     , withdraw <- function(amount) {
>       if(amount < 0)   {
>         stop("withdrawls must be positive")
>       }
>       balance <<- balance - amount
>     }       
>   ) )
> 
> 
> tb <- Account$new(balance=50.75, number="baml-029873") tb$balance
> tb$balance <- 12 
> tb$balance

I hate the fact I can update the balance directly. Perhaps that the old pure OO in me, I really would like to be able make the balance private, at least non-settable from outside the class.

Thoughts

Answer 1

To solve the issue of privacy I create an own class, "Private", which has new methods to access the object, i.e. $ and [[. These methods will throw an error if the client tries to access 'private' member. Private member are identified by the name (leading period). As reference Objects are environments in R one can work around this, but it is my solution at this time and I think more convenient to use get/set methods provided by the class. So this is more the 'hard-to-set-from-outside-the-class' solution to the question.

I have organised this inside a R-package so the following code makes use of that package and modifies the above example such that an assignment to tb$.balance produces an error. I also use the function Class which is just a wrapper around setRefClass so this is still in the scope of R's reference classes provided by the methods package and used in the question.

devtools::install_github("wahani/aoos")
library("aoos")

Account <- defineRefClass({
    Class <- "Account"
    contains <- "Private"

    number <- "character"
    .balance <- "numeric"

    deposit <- function(amount) {
        if(amount < 0) stop("deposits must be positive")
        .balance <<- .balance + amount
    }

    withdraw <- function(amount) {
        if(amount < 0) stop("withdrawls must be positive")
        .balance <<- .balance - amount
    }
})

tb <- Account(.balance = 50.75, number = "baml-029873") 
tb$.balance # error
tb$.balance <- 12 # error

Answer 2

This answer doesn't work with R > 3.00, so don't use it!

As has been mentioned, you can't have private member fields. However, if you use the initialize method, then the balance isn't displayed as a field. For example,

Account = setRefClass("ref_Account", 
                       fields = list(number = "character"),
                       methods = list(
                           initialize = function(balance, number) {
                               .self$number = number
                               .self$balance = balance
                           })

As before, we'll create an instance:

tb <- Account$new(balance=50.75, number="baml-0029873")
##No balance
tb

Reference class object of class "ref_Account"
Field "number":
[1] "baml-0029873"

As I mentioned, it isn't truly private, since you can still do:

R> tb$balance
[1] 50.75
R> tb$balance = 12 
R> tb$balance
[1] 12