[R] Making an S3 object act like a data.frame

[hadley wickham]

"[.ggobiDataset" <- function(x, ..., drop=FALSE) {
	x <- as.data.frame(x)
	NextMethod("[", x)
}

"[[.ggobiDataset" <- function(x, ..., drop=FALSE) {
	x <- as.data.frame(x)
	NextMethod("[[", x)
}

"$.ggobiDataset" <- function(x, ..., drop=FALSE) {
	x <- as.data.frame(x)
	NextMethod("$", x)
}

> class(x)
[1] "ggobiDataset" "data.frame"

> x[1:2,1:2]
  total_bill  tip
1      16.99 1.01
2      10.34 1.66

> x[[1]]
  [1] 16.99 10.34 21.01 23.68 24.59 25.29  8.77 26.88 15.04 14.78 10.27 35.26
 [13] 15.42 18.43 14.83 21.58 10.33 16.29 16.97 20.65 17.92 20.29 15.77 39.42
...

> x$total_bill
Error in "$.default"(x, "total_bill") : invalid subscript type


What do I need to do to get "$.ggobiDataset" to imitate a data.frame
like [ and [[ do?

Thanks,

Hadley