oop - Is there a way to initialize a S4 object so that another object will be returned? -

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i have class hierarchy superclass fb of no objects should exist (i tried virtual classes ran in problem can not initialize objects virtual classes). further, have 2 sub classes (foo, bar) same slots. want make new object, using initialize method superclass returns object of 1 of subclasses based on value:

setclass("fb", representation( x = "numeric"))  setclass("foo", contains = "fb") setclass("bar", contains = "fb")  setmethod("initialize", "fb", function(.object, x) {     if (x < 5) class(.object) <- "foo"     else class(.object) <- "bar"     .object@x <- x     .object })  > new("fb", x = 3) error in initialize(value, ...) :    initialize method returned object of class "foo" instead of required class "fb"

obviously (and reasons) r disallows that. there way achieve want within method , not using if-else construct when create new object?

s4 helps color within lines. fb class should virtual, initialize method shouldn't change class of .object. might write function fb conditional instantiation.

setclass("fb", representation( x = "numeric", "virtual")) setclass("foo", contains = "fb") setclass("bar", contains = "fb")  fb <-     function(x) {     if (x < 5) new("foo", x=x)     else new("bar", x=x) }

fb constructor, more convenient user, , separates interface class hierarchy implementation considered thing.

and it's worth implicit constraint on s4 initialize methods new("foo") (invoking new class name no additional arguments) must work (otherwise there failures when try extend foo). paradigm initialize method along lines of

setmethod(initialize, "foo", function(.object, ..., x=1) {     .object <- callnextmethod(.object, ...)     .object@x <- x     .object })

though (as in case, initialize doing slot assignment) there no need initialize method @ all. note use of ..., positioning of x (requiring argument named in corresponding call new) , use of default value.


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