With R S4 classes, is it possible to have non-optional constructor parameters

Question

Suppose I have an S4 class Test that has a single slot name. A valid name must be at least one character long, so Test(name = "Bob") should work but Test(name = "") should throw an error. An undefined name should also give an error: Test().

My class is defined as:

Test <- setClass(
    "Test", 
    slots = c(name = "character"), 
    validity = function(object) {

        if (nchar(object@name) == 0) {

            return("name must at least one character long")
        }

        T
    })

Testing the class in the console, I find that my validity function is not executed for the unassigned case:

> Test(name = "Bob")
An object of class "Test"
Slot "name":
[1] "Bob"

> Test(name = "")
Error in validObject(.Object) : 
  invalid class “Test” object: name must at least one character long

> Test()
An object of class "Test"
Slot "name":
character(0)

How can I ensure that an error is always thrown when an invalid object is created?

Answer 1

One way to ensure the validity of an S4 is the use of prototype to initialise the slots like this

Test <- setClass(
    "Test",
    slots = c(name = "character"),
    prototype = prototype(name = 'name_default'),
    validity = function(object) {
        if (nchar(object@name) == 0) {
            return("name must at least one character long")
        }
    })
Test(name = "Bob")
## An object of class "Test"
## Slot "name":
## [1] "Bob"
Test(name = '')
## Error in validObject(.Object) : 
##   invalid class "Test" object: name must at least one character long
Test()
## An object of class "Test"
## Slot "name":
## [1] "name_default"

another way would be to create a constructor to test the presence of name:

consTest <- function(name) {
    if (missing(name)) {
        stop("name is missing")
    } else {
        new(Class = "Test", name = name)
    }
}
consTest(name = "Bob") # similar to Test(...)
consTest(name = '') # similar to Test(...)
consTest()
## Error in consTest() (from Retest.R@13#3) : name is missing