Using sd as a generic function in R

Question

If I a have a class called foo, then it is straightforward to overload the summary function

summary.foo = function(x, ...) print("bar")

However this technique does not work with the sd function, that is

> bar = createFooClass()
> sd.foo = function(x, ...) print("Hi")
> sd(bar)
  error: is.atomic(x) is not TRUE

What is the correct way of overloading this function?

Answer 1

You can hijack any non-generic function, make it (S3) generic and set the original version to be the default version. For example:

## make an S3 generic for sd
sd <- function(x, ...) UseMethod("sd")
## take the usual definition of sd,
## and set it to be the default method
sd.default <- stats::sd
## create a method for our class "foo"
sd.foo = function(x, ...) print("Hi")

A final step, if this is in a package, is to add a ... argument to sd.default to allow passing of package checks:

formals(sd.default) <- c(formals(sd.default), alist(... = ))

giving:

> args(sd.default)
function (x, na.rm = FALSE, ...) 
NULL
> args(stats::sd)
function (x, na.rm = FALSE) 
NULL

This then gives the desired behaviour:

> bar <- 1:10
> sd(bar)
[1] 3.027650
> class(bar) <- "foo"
> sd(bar)
[1] "Hi"

This is documented in section 7.1 of the Writing R Extensions manual

Answer 2

You need to define a new generic for sd.

The easiest way is to use S4, because it handles the default "sd" method automatically:

setClass("foo", list(a = "numeric", names = "character"))

setGeneric("sd")

setMethod("sd", "foo", 
          function(x,  na.rm = FALSE){
              print("This is a foo object!")
              callNextMethod(x@a)
              })

tf <- new("foo", a = 1:10)
sd(tf)
#[1] "This is a foo object!"
#[1] 3.027650

Answer 3

Look at the code of sd()---it effectively dispatches internally. In other words, it is not a generic function but a plain old regular function.

The easiest may just be to modify sd() to branch on class foo.