oop - 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