В Thu, 24 Apr 2025 12:36:57 +0000 Tony Wilkes <tony_a_wil...@outlook.com> пишет:
> One use-case for this is to check if it's save to modify an object by > reference. I think you might need NOT_SHARED() for that, except it's far from being simple, especially once you pass arbitrary expressions (not just variable names) as arguments to your function: https://developer.r-project.org/Refcnt.html > For example, when saying x <- base::letters, x is not save > to modify by reference: Although the binding of x itself is not > locked, the binding of base::letters is locked, and we obviously > don't want to modify base::letters. The situation is more dangerous than that. When 'x' references the same object as base::letters (which is a locked binding), you can still modify this object from C code, given its SEXP address, and see the changes from either binding; R won't be able to stop you. (That's the danger of by-reference operations like those implemented in data.table.) On the other hand, if you assign a new value to the 'x' binding inside a given environment, base::letters and any other objects referencing it will retain their original values; there is no need to worry about other bindings being locked. > R has a few pass-by-reference classes (like environments) These are different. You can lock a _binding_ to an environment and still change the _value_ of that environment precisely because it's a mutable, pass-by-reference object: x <- new.env() y <- x lockBinding('x', environment()) # x$foo <- 'bar' # would signal an error, but: assign('foo', 'bar', envir = x) # works x$foo # [1] "bar" identical(x, y) # [1] TRUE R switched to reference counting by default in version 4.0. One downside of the reference counts as currently implemented is that when an object containing references to other objects is garbage-collected, the reference counts of the child objects are not decremented: x <- 1 invisible(replicate(999, list(x))) gc(full = TRUE) .Internal(inspect(x)) # @5618cd515488 14 REALSXP g1c1 [MARK,REF(2000)] (len=1, tl=0) 1 'x' retains a reference count of 2000 despite the 999 lists that previously referenced it no longer exist. This is possible to change, but at the cost of slowing down the garbage collector. -- Best regards, Ivan ______________________________________________ R-package-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-package-devel
