R6 是我需要的框架.

    # R6 ----------------------------------------------------------------------
student <- R6Class("student", list(
  age = 0,
  initialize = function(age = 20) {
    #stopifnot(is.character(name), length(name) == 1)
    stopifnot(is.numeric(age), length(age) == 1)
    
    #self$name <- name
    self$age <- age
  },
  getolder = function(years = 1) {
    self$age <- self$age + years 
    invisible(self)
  }
  )
)

student$new()
stud1 <- student$new(age = 15)
stud1$getolder(3)
stud1$age #18
stud2 <- student$new(age = 15)

group <- R6Class("group", list(
  s1 = NA,
  s2 = NA,
  initialize = function(s1=NA, s2=NA) {
    if(!all(sapply(list(s1, s2), function(x) inherits(x,"student")))) stop("students not students")
    self$s1 <- s1
    self$s2 <- s2

  },
  getolder = function(stud, years = 1) {
    stud$getolder(years)
    invisible(self)
  }))

gr1 <- group$new(stud1, stud2)
gr1$s1$age #18

gr1$getolder(gr1$s1, years=10)
gr1$s1$age #28

向 hadley 的 高级 R 书 大喊大叫，它预先指定了 R6 对象.

Shout out to hadley's advanced R book which specified upfront the mutable property of R6 objects.