setMethod("initialize","test2",
function(.Object,...){
.Object<-callNextMethod(.Object,...)
validity is checked in the preceeding line
.obj...@b<-.obj...@a ## here "b" is initialized ##
.Object
})
new("test2")
Error in if (obj...@a != obj...@b) print("values must be equal!!")
else TRUE : argument is of length zero
This could mean only one thing, validity of "test2" is checked before
it gets to point when b is initialized (i.e. in callNextMethod). But
this is absurd (at leas from my point): callNextMethod constructs
object of type "test" and should have nothing to do with validity of
"test2". Shouldn't validity be checked only after the object is
created?
The class of .Object is test2, and initialize should return a (valid)
instance of test2, right? So the validity check on test2 has to have
been performed before returning from callNextMethod.
Oh yes, that is indeed meaningful. So it's default coercion method from test1 to test2 which is called.
Second my slot "a" is initialized in "test1"'s constructor and "b"
must be assigned only after the callNextMethod as in above
code. Seemingly unsolvable paradox? How to be?
My own solution would avoid using initialize (because as you're
discovering the contract is hard to satisfy in general) and instead
use constructors
I am sure this "contract" and all this
initialization/coercion/creation-sequence stuff must be documented somewhere. Though all
what I can find is a collection of your responses in R-help archive. Could you please
suggest something to read (except R-core code)?
For now I am pretty much clarified. Thanks a lot!
test2 <- function(a=runif(1), b=a, ...) new("test2", a=a, b=b, ...)
or similar, which I think helps to provide a bit of an interface
between the class and it's implementation.
One could think of additional solutions (creating a new 'test' inside
'test2', and not using callNextMethod(), for instance) but this is
unlikely to be robust (e.g., when derived classes are relying on
initialize,ANY-method to fill in slots with supplied arguments).
Martin
Many thanks.
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