Hi all,
I have a situation where I'm trying to use S3 classes as slots in an S4 class
and am running into some problems with inheritance.
My example:
## with S3 classes
a <- function(x) structure(x, class = "a")
b <- function(x) structure(x, class = c("b", "a"))
setOldClass(c("a", "b"))
a_1 <- a("hello")
b_1 <- b("world")
n <- setClass("n", slots = c(s1 = "a"))
n(s1 = a_1) # works as expected
is(b_1, "a") # yields TRUE
n(s1 = b_1) # doesn't work-- unexpected
Error message is:
Error in validObject(.Object) :
invalid class “n” object: invalid object for slot "s1" in class "n": got class
"b", should be or extend class "a"
Digging in, validObject looks like it's calling getClassDef, which gives me
> getClassDef("b")
Virtual Class "b" [in ".GlobalEnv"]
Slots:
Name: .S3Class
Class: character
Extends: "oldClass"
-- it extends "oldClass" but it doesn't extend class "a" despite the fact the
S3 version does.
Now, In this simple example, I could have defined a and b as s4 classes, but in
the real world, I didn't define them and have to use setOldClass.
I'm currently getting around this issue by doing setClassUnion("ab", c("a",
"b")) and then specify to use "ab" as the type for the slot, but since I
already know b inherits from a, this ought not to be necessary (and there are
other reasons this is undesirable).
1. Is there anything I missed, in setting up this simple example, that would
cause n(s1 = b_1) to work properly? Or is there anything I should do other than
setClassUnion?
2. Is this a shortcoming of validObject or getClassDef?
Thank you all!
-Ezra
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