[Rd] S4: names stripped during instantiation of grandchildren of numeric class

John Chambers jmc at r-project.org
Sat Aug 29 21:24:33 CEST 2009

Vitalie S. wrote:
> Dear All,
> A small inconsistency (it's probably not even a buglet):
>> setClass("A", contains="numeric")
> [1] "A"
>> names(new("A", c(a=23)))
> [1] "a"
>> setClass("B", contains="A")
> [1] "B"
>> names(new("B", c(a=23)))
> This is exactly that kind of behavior S4 was invented for, you expect 
> names to be there, but they are not :(.
You may expect them but you should not:  the  "names" attribute is not a 
slot of class "numeric".

If  you want names to be explicitly required, you have to say so:

 > setClass("A", contains = "numeric", representation(names = "character"))
[1] "A"
 > names(new("A", c(a=23)))
[1] "a"
 > setClass("B", contains="A")
[1] "B"
 > names(new("B", c(a=23)))
[1] "a"

Having said all that, the treatment of "names" is very special and 
buried deep down in the implementation.  While simple treatment of names 
as a slot should work in most cases, it's unlikely that behavior will be 
100% as expected.

Also, the reason you sometimes got  names without specifying them as a 
slot is that the low-level code that assigns names (or other attributes) 
does not check that the S4 class definition has a corresponding slot.  
So the names in the non-S4 vector used as the data part are left in 
place, rather than say producing a warning or error.
> If that behavior is left to be, one would have to implement @names 
> manually for children of basic pseudo-classes, which is quite an 
> overhead given the functionality is already in base R.
> Thanks,
> Vitalie.
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