This has nothing to do with printing or dispatch per se. It is the
result of an internal register (R_ReturnedValue) being protected. It
gets rewritten whenever there is a jump, e.g. by an explicit return
call. So a simplified example is
new_foo <- function() {
e <- new.env()
reg.finalizer(e, function(e) message("Finalizer called"))
e
}
bar <- function(x) return(x)
bar(new_foo())
gc() # still in .Last.value
gc() # nothing
UseMethod essentially does a return call so you see the effect there.
The R_ReturnedValue register could probably be safely cleared in more
places but it isn't clear exactly where. As things stand it will be
cleared on the next use of a non-local transfer of control, and those
happen frequently enough that I'm not convinced this is worth
addressing, at least not at this point in the release cycle.
Best,
luke
On Mon, 26 Mar 2018, Iñaki Úcar wrote:
Hi,
I initially opened an issue in the R6 repo because my issue was with
an R6 object. But Winston (thanks!) further simplified my example, and
it turns out that the issue (whether a feature or a bug is yet to be
seen) had to do with S3 dispatching.
The following example, by Winston, depicts the issue:
print.foo <- function(x, ...) {
cat("print.foo called\n")
invisible(x)
}
new_foo <- function() {
e <- new.env()
reg.finalizer(e, function(e) message("Finalizer called"))
class(e) <- "foo"
e
}
new_foo()
gc() # still in .Last.value
gc() # nothing
I would expect that the second call to gc() should free 'e', but it's
not. However, if we call now *any* S3 method, then the object can be
finally gc'ed:
print(1)
gc() # Finalizer called
So the hypothesis is that there is some kind of caching (?) mechanism
going on. Intended behaviour or not, this is something that was
introduced between R 3.2.3 and 3.3.2 (the first succeeds; from the
second on, the example fails as described above).
Regards,
Iñaki
PS: Further discussion and examples in https://github.com/r-lib/R6/issues/140
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Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa Phone: 319-335-3386
Department of Statistics and Fax: 319-335-3017
Actuarial Science
241 Schaeffer Hall email: luke-tier...@uiowa.edu
Iowa City, IA 52242 WWW: http://www.stat.uiowa.edu
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