When does initialize check for object validity? - r

Question

From Chambers' (excellent) Extending R (2016):
A validity method will be called automatically from the default method for initialize(). The recommended form of an initialize method ends with a callNextMethod() call, to ensure that subclass slots can be specified in a call to the generator for the class. If this convention is followed, initialization will end with a call to the default method, and the validity method will be called after all initialization has occurred.
I thought I understood, but the behavior I am getting does not seem to follow this convention.
setClass("A", slots = c(s1 = "numeric"))
setValidity("A", function(object) {
if (length(object#s1) > 5) {
return("s1 longer than 5")
}
TRUE
})
setMethod("initialize", "A", function(.Object, s1, ...) {
if (!missing(s1)) .Object#s1 <- s1 + 4
callNextMethod(.Object, ...)
})
A <- new("A", rep(1.0, 6))
A
# An object of class "A"
# Slot "s1":
# [1] 5 5 5 5 5 5
validObject(A)
# Error in validObject(A) : invalid class “A” object: s1 longer than 5
I expected the validity checking to be done by adding callNextMethod() to the end of the initialize method. Adding an explicit validObject(.Object) before callNextMethod() works, but I am clearly not understanding something here.
Obviously, I can also do all the same checks in the validity method, but ideally all of the validity checking would occur within setValidity so future edits live in one place.
Changing the initialize function slightly gives the desired result -- is there a reason to use one approach over the other? Chambers seems to prefer using .Object#<- whereas I have seen the following method elsewhere (Gentlemman & Hadley).
setMethod("initialize", "A", function(.Object, s1, ...) {
if (!missing(s1)) s1 + 4
else s1 <- numeric()
callNextMethod(.Object, s1 = s1, ...)
})

Answer 1

Perhaps the best guide comes from initialize itself — if you inspect the code for the default method
getMethod("initialize",signature(.Object="ANY"))
then you see that it does indeed contain an explicit call to validObject at the end:
...
validObject(.Object)
}
.Object
}
so if you define your own initialize method, the most similar thing you could do would be to call it at the end of your method, right before you call callNextMethod.
In your case, when you call callNextMethod, that is only checking that the slot you have created is a valid numeric object (which it is), rather than checking the validity of the larger object (which requires the s1 slot to be no longer than 5 elements)