> library("data.table")
data.table 1.8.3  For help type: help("data.table")
> sessionInfo()
R version 2.15.1 (2012-06-22)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=C                 LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] data.table_1.8.3 fortunes_1.5-1
>
> setClass("DataTable2", contains="data.table")
>
> DT1 <- data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9)
> DT2 <- new("DataTable2", data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9))
>
> ## Everything works!
>
> tables()
     NAME NROW MB COLS  KEY
[1,] DT1     9 1  x,y,v
[2,] DT2     9 1  x,y,v
Total: 2MB
> is(DT2, "data.table")
[1] TRUE
>
> DT1[2]
   x y v
1: a 3 2
> DT2[2]
   x y v
1: a 3 2
>
> (bracketMethods1 <- methods("["))
 [1] [.acf*            [.AsIs            [.bibentry*       [.data.frame
 [5] [.data.table*     [.Date            [.difftime        [.factor
 [9] [.formula*        [.getAnywhere*    [.hexmode         [.ITime*
[13] [.listof          [.noquote         [.numeric_version [.octmode
[17] [.pdf_doc*        [.person*         [.POSIXct         [.POSIXlt
[21] [.raster*         [.roman*          [.simple.list     [.terms*
[25] [.ts*             [.tskernel*

   Non-visible functions are asterisked
>
> DT1[,v]
[1] 1 2 3 4 5 6 7 8 9
> DT2[,v]
[1] 1 2 3 4 5 6 7 8 9
>
> ## These are the packages loaded/imported by RCurl (and [.data.table workds after them).
> ## library("tools")
> ## DT2[,v]
> ## library("bitops")
> ## DT2[,v]
>
> library("RCurl")
Loading required package: bitops
> sessionInfo()
R version 2.15.1 (2012-06-22)
Platform: x86_64-unknown-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
 [7] LC_PAPER=C                 LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
[11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] RCurl_1.95-0     bitops_1.0-4.1   data.table_1.8.3 fortunes_1.5-1

loaded via a namespace (and not attached):
[1] tools_2.15.1
>
> ## This still works
> DT1[,v]
[1] 1 2 3 4 5 6 7 8 9
> ## This no longer works
> DT2[,v]
Error: object 'v' not found
>
> ## No changes in the S3 methods defined for brackets
> (bracketMethods2 <- methods("["))
 [1] [.acf*            [.AsIs            [.bibentry*       [.data.frame
 [5] [.data.table*     [.Date            [.difftime        [.factor
 [9] [.formula*        [.getAnywhere*    [.hexmode         [.ITime*
[13] [.listof          [.noquote         [.numeric_version [.octmode
[17] [.pdf_doc*        [.person*         [.POSIXct         [.POSIXlt
[21] [.raster*         [.roman*          [.simple.list     [.terms*
[25] [.ts*             [.tskernel*

   Non-visible functions are asterisked
> setdiff(bracketMethods1, bracketMethods2)
character(0)
> setdiff(bracketMethods2, bracketMethods1)
character(0)
>