Proposing feature requests to the R Core Team
setRefClass(
    "A",
    fields=list(a="numeric", b="character"),
    methods=list(
        reset=function(fields=NULL, ...){
            temp <- new("A")
            if(is.null(fields)){
                fields <- names(getRefClass("A")$fields())
            }
            sapply(fields, function(x){
                .self$field(name=x, value=temp$field(x))
            })
            return(TRUE)
        }
    )
)

x <- new("A", a=1:10, b=letters[1:10])

x$a
x$b
x$reset(fields="a")

x$a
x$b

x$reset()
x$a
x$b