Can an object in R have more than one class?

Question

Suppose I have a function like this:

myf = function(x) {
res = dostuff(x)
res # this is a data.frame
}

I want to do something special about res, for example, I want to make some generic functions like print.myf, summary.myf, ... , so I can go ahead and give it a class:

myf = function(x) {
res = dostuff(x)
class(res) = 'myf'
res
}

But this way I cannot use it as a data.frame any more.

Answer 1

Yes, my standard (simple) example is

R> now <- Sys.time()
R> class(now)
[1] "POSIXct" "POSIXt" 
R> class(as.POSIXlt(now))
[1] "POSIXlt" "POSIXt" 
R> 

It is also the reason behind the pro tip of testing with inherits("someClass") rather than testing the result of class(obj)=="someClass".