By default, `names<-` alters S4 objects

Hi,

I was stumped by this. The two S4 objects below looked exactly the same:

   > a1
   An object of class "A"
   Slot "aa":
   integer(0)
   > a2
   An object of class "A"
   Slot "aa":
   integer(0)

   > str(a1)
   Formal class 'A' [package ".GlobalEnv"] with 1 slots
     ..@ aa: int(0)
   > str(a2)
   Formal class 'A' [package ".GlobalEnv"] with 1 slots
     ..@ aa: int(0)

But they were not identical:

   > identical(a1,a2)
   [1] FALSE

Then I found that one had a "names" attribute but not the other:

   > names(attributes(a1))
   [1] "aa"    "class" "names"
   > names(attributes(a2))
   [1] "aa"    "class"

   > names(a1)
   NULL
   > names(a2)
   NULL

Which explained why they were not reported as identical.

After tracking the history of 'a1', I found that it was created with
something like:

   > setClass("A", representation(aa="integer"))
   [1] "A"
   > a1 <- new("A")
   > names(a1) <- "K"
   > names(a1)
   NULL

So it seems that, by default (i.e. in the absence of a specialized
method), the `names<-` primitive is adding a "names" attribute to the
object. Could this behaviour be modified so it doesn't alter the object?

Thanks,
H.

Herv? Pag?s

Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M1-B514
P.O. Box 19024
Seattle, WA 98109-1024

E-mail: hpages at fhcrc.org
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