Martin,
Martin Morgan wrote:
> The funny effect where class(object) seems to trigger construction of
> a new object is lazy evaluation -- the 'object' argument to
> setValidity is not evaluated until needed, i.e., until class(object)
> (anything would trigger this, including force(object)); only then do
> you see the attempt to create the new object of the previous
> paragraph.
The fact that you can't predict the number of times the "initialize"
method will be called is problematic. Here is a simple example
where "initialize-A" increments a global counter to keep track of the
number of A-objects:
A.GLOBALS <- new.env(parent=emptyenv())
A.GLOBALS[["nobjs"]] <- 0L
setClass("A",
representation(a="character"),
prototype(a="a0")
)
setMethod("initialize", "A",
function(.Object, ...) {
cat("------initialize:A------\n")
A.GLOBALS[["nobjs"]] <- A.GLOBALS[["nobjs"]] + 1
cat("A-object #", A.GLOBALS[["nobjs"]], "\n", sep="")
callNextMethod()
}
)
setValidity("A",
function(object) {
cat("------setValidity:A------\n")
tmp <- class(object)
TRUE
}
)
setClass("B",
contains="A",
representation(b = "character")
)
setMethod("initialize", "B",
function(.Object, ...) {
cat("------initialize:B------\n")
callNextMethod()
}
)
setValidity("B",
function(object) {
cat("------setValidity:B------\n")
TRUE
}
)
Let's try it:
> b1 <- new("B")
------initialize:B------
------initialize:A------
A-object #1
> A.GLOBALS[["nobjs"]]
[1] 1
> b1 <- new("B", b="hello")
------initialize:B------
------initialize:A------
A-object #2
------setValidity:A------
------initialize:A------
A-object #3
------setValidity:B------
> A.GLOBALS[["nobjs"]]
[1] 3
Shouldn't the "initializing" ("constructor" in other languages) code be executed
exactly 1 time per object instanciation? Something that the programmer can
simply
rely on, whatever this code contains and whatever the internal subtilities of
the
programming lamguage are?
Cheers,
H.
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