[R] Testing for S4 objects

[John Fox]

Dear Martin,

As it turns out, the test that I proposed (i.e., testing for NULL slotNames)
sometimes fails. For example:

> library(car)
> data(Prestige)
> sum <- summary(lm(prestige ~ income + education, data=Prestige))
> slotNames(sum)
character(0)

The following, however, seems to work (at least as far as I've been able to
ascertain):

isS4object <- function(object) length(slotNames(object)) != 0

I hope that this is a more robust test.

John

--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 

> -----Original Message-----
> From: Martin Maechler [mailto:maechler at stat.math.ethz.ch] 
> Sent: Friday, November 26, 2004 3:18 AM
> To: John Fox
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] Testing for S4 objects
> 
> >>>>> "JohnF" == John Fox <jfox at mcmaster.ca>
> >>>>>     on Thu, 25 Nov 2004 22:28:50 -0500 writes:
> 
>     JohnF> Dear r-help list members, Is there a way to test
>     JohnF> whether an object is an S4 object? The best that I've
>     JohnF> been able to come up with is
> 
>     JohnF> 	isS4object <- function(object) 
> !(is.null(slotNames(object)))
> 
> you can drop one pair of "(..)" to give
> 
>   isS4object <- function(object) !is.null(slotNames(object))
> 
> 
>     JohnF> which assumes that an S4 object has at least one
>     JohnF> slot. I think this is safe, but perhaps I'm missing
>     JohnF> something.
> 
> The question is a very good one -- that I have posed to 
> R-core a while ago myself.
> 
> Inside  utils:::str.default  {which doesn't show the many 
> commments in the *source* of str.default()}, I have wanted a 
> way that even works when the 'methods' package is not 
> attached and use the more obscure 
> 
>     #NOT yet:if(has.class <- !is.null(cl <- class(object)))
>     if(has.class <- !is.null(cl <- attr(object, "class")))# 
> S3 or S4 class
> 	S4 <- !is.null(attr(cl, "package"))## <<<'kludge' FIXME!
> 	##or length(methods::getSlots(cl)) > 0
> 
> For the time being, I'd keep your function, but I don't think 
> we'd guarantee that it will remain the appropriate test in 
> all future.  But till then many things will have happened (if 
> not all of them ;-).
> 
> Martin Maechler, ETH Zurich
>