R: modify any function applied to a S4 class - r

Question

I've been developing a S4 class which is essentially a data.frame with a little bit of extra information. For the purposes of this question, the "extra" features of this class are irrelevant. What matters is that the class contains a data.frame object stored in one of it's slots. (I put the data.frame in a slot, instead of naming it a superclass, because I find that S4 classes which contain data.frames simplify the data.frames to lists for some reason).
Here's a basic example:
setClass('tmp_class', slots = c(df = 'data.frame'))
test_object <- new('tmp_class', df = data.frame(Num = 1:10, Let = letters[1:10]))
Now what I'd like to do is make it so that essentially any function applied to an object of this class is applied to the data.frame in slot #df. It's easy to write methods for specific functions to do this, like:
setMethod('dim', signature = c(x = 'tmp_class'), function(x) dim(x#df))
But I'm limited to only the functions I can think of, and any function invented by a user wouldn't work.
It is a simple matter to write a sort of wrapper/closure to modify a function to work on my class, like this:
tmp_classize <- function(func){
function(tmp, ...){ func(tmp#df, ...) }
}
So, rather than writing methods for, say, colnames() or ncol(), I could just run:
tmp_classize(colnames)(test_object)
or
tmp_classize(ncol)(test_object)
But what I'd like to do is somehow evoke my "tmp_classize" function on any function applied to my class, automatically. I can't figure out how to do it. I was thinking that if could somehow call a "universal method" with an input signature of class "tmp_class", and then use sys.function() to grab the actual function being called, maybe I could make something work, but A) there are recursion problems B) I don't know how to call such a "universal" method. It seems to me that the solution, if it exists at all, might necessitate non-standard evaluation, which I'd rather avoid, but might use if necessary.
Thanks!
P.S. I realize this undertaking may be unwise/poor programming technique, and I may never actually implement it in a package. Still I'm curious to know if it is possible.
P.P.S. I'd also be interested in the same idea applied to S3 classes!

Answer 1

In principal what you could do is make a classUnion for your class and data.frame and write methods for your class that deal with all of the ways to read and write to data.frames such as $, [, dim(), <- and many more. Then when other functions seek to use your new class as data.frame there will be methods for this to work. This is somewhat explained in John Chambers "Software for Data Analysis" starting on page 375. That said this system may be very difficult to implement.
A simpler system may be to just add an extra attribute to your data.frame with the extra info you need. For example:
x<-data.frame(a=1:3,b=4:6)
attr(x,"Info")<-"Extra info I need"
attributes(x)$Info
[1] "Extra info I need"
This is not as elegant as a S4 class but will do everything a data.frame does. I suspect that someone who is familiar with S3 classes could improve on this idea quite a bit.

Answer 2

The simplest solution is to have your class contain data.frame instead of having it as one of the slots. For example here is a data.frame with a timestamp:
setclass(
"timestampedDF",
slots=c(timestamp="POSIXt"),
contains="data.frame"
)
Now all functions which work for a data.frame (such as head) will automatically work for timestampedDF objects. If you need to get at the "data frame part", then that is held in a hidden slot object#.Data.