[R] Testing for S4 objects

[John Chambers]

Let me suggest a different test, because slotNames was written to work
differently when given a string or a class definition.  With your
definition,

R> x <- "classRepresentation"
R> isS4object(x)
[1] TRUE

which I assume is not what you wanted.  (Given a single string, 
slotNames() tries to look up the class definition of that name.)

How about the following?  The logic is that an S4 object must have an
actual class attribute of length 1 (that rules out basic data types,
where class(x) is a string but there is no actual attribute, and also
rules out some S3 objects).  Then if that's true, try to look up the
class definition.  If it is non-null, seems like an S4 object.

R> isS4object <- function(object)(length(attr(object, "class"))==1 &&
+     !is.null(getClass(class(object))))
R> isS4object(x)
[1] FALSE
R> isS4object(getClass(class(x)))
[1] TRUE

This definition seems to work, at least on the examples I could think
of right away.  Notice though, that some classes, such as "ts", that
have been around for a long while are nevertheless legitimate S4
classes, so:

R> t1 = ts(1:12)
R> isS4object(t1)
[1] TRUE

(this applies to either version of isS4object).

There are a couple of details, more appropriate for the r-devel list. 
Seems  a good candidate for a function to add to R.


On Sat, 27 Nov 2004 17:48:30 -0500, John Fox <jfox at mcmaster.ca> wrote:
> 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
> >
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
>