Question

Looking through Hadley Wickham’s S4 wiki:
https://github.com/hadley/devtools/wiki/S4

setClass("Person", representation(name = "character", age = "numeric"), 
  prototype(name = NA_character_, age = NA_real_))
hadley <- new("Person", name = "Hadley")

How can we design a constructor for Person (like this)

Person<-function(name=NA,age=NA){
 new("Person",name=name,age=age)
}

that doesn’t do this:

> Person()
Error in validObject(.Object) : 
  invalid class "Person" object: 1: invalid object for slot "name" in class "Person": got class "logical", should be or extend class "character"
invalid class "Person" object: 2: invalid object for slot "age" in class "Person": got class "logical", should be or extend class "numeric"

Answer 1

It looks like the answer is right there in your example:

Person<-function(name=NA_character_,age=NA_real_){
 new("Person",name=name,age=age)
}

yields

> Person()
An object of class "Person"
Slot "name":
[1] NA

Slot "age":
[1] NA

> Person("Moi")
An object of class "Person"
Slot "name":
[1] "Moi"

Slot "age":
[1] NA

> Person("Moi", 42)
An object of class "Person"
Slot "name":
[1] "Moi"

Slot "age":
[1] 42

However, that is fairly un-S4 and duplicates the default values already assigned in the class definition. Maybe you’d prefer to do

Person <- function(...) new("Person",...)

and sacrifice the ability to call without named arguments?