[Rd] `@<-` modify its argument when slot is externalptr

[Jialin Ma]

Hi Luke,

Thanks a lot for the explanation. I also noted that `@<-` will not
modify the argument when the slot is ".Data". For example

setClass("foo", contains = "numeric")
x <- new("foo", 2)
y <- x
y
# An object of class "foo"
# [1] 2

`@<-`(x, .Data, 42)
# An object of class "foo"
# [1] 42
x
# An object of class "foo"
# [1] 2
y
# An object of class "foo"
# [1] 2

Best regards,
Jialin


On Sun, 2018-03-18 at 09:54 -0500, luke-tierney at uiowa.edu wrote:
> On Sun, 18 Mar 2018, Jialin Ma wrote:
> 
> > Dear all,
> > 
> > I am confused about the inconsistent behaviors of `@<-` operator
> > when
> > used in different ways. Consider the following example:
> > 
> >  library(inline)
> > 
> >  # Function to generate an externalptr object with random address
> >  new_extptr <- cfunction(c(), '
> >    SEXP val = PROTECT(ScalarLogical(1));
> >    SEXP out = PROTECT(R_MakeExternalPtr(&val, R_NilValue, val));
> >    UNPROTECT(2);
> >    return(out);
> >  ')
> > 
> >  setClass("S4ClassHoldingExtptr", contains = "externalptr")
> > 
> >  subs_extptr_1 <- function(x) {
> >    x at .xData <- new_extptr()
> >    x
> >  }
> > 
> >  subs_extptr_2 <- function(x) {
> >    x <- `@<-`(x, ".xData", new_extptr())
> >    x
> >  }
> > 
> > I expect functions "subs_extptr_1" and "subs_extptr_2" should be
> > semantically identical. (is it right?)
> 
> No. The are similar but not identical. It is not a good idea to call
> replacement functions directly and should be avoided. Doing so may
> need
> to become a runtime error or at least a warning in the future.
> 
> Nevertheless, this is a bug in the implementation @<-.A simpler
> example is
> 
> x <- setClass("foo", slots = "bar")
> x <- new("foo", bar = 1)
> y <- x
> y at bar
> ##  [1] 1
> `@<-`(x, bar, 2)
> y at bar
> ##  [1] 2
> 
> Will be fixed soon in R-devel.
> 
> Best,
> 
> luke
> 
> > x <- setClass("foo", slots = "bar")
> > x <- new("foo", bar = 1)
> > y <- x
> > `@<-`(x, bar, 2)
> 
> An object of class "foo"
> Slot "bar":
> [1] 2
> 
> > x
> 
> An object of class "foo"
> Slot "bar":
> [1] 2
> 
> > x$bar
> 
> Error in x$bar : $ operator not defined for this S4 class
> > x at bar
> 
> [1] 2
> 
> > But it seems that "subs_extptr_2" will modify its argument while
> > "subs_extptr_1" won't.
> > 
> >  x <- new("S4ClassHoldingExtptr", new_extptr())
> >  y <- x
> >  y
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60971eb0>
> > 
> >  subs_extptr_1(x)
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60974250>
> >  x
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60971eb0>
> >  y
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60971eb0>
> > 
> > As shown above, "subs_extptr_1" will not modify x or y.
> > 
> >  subs_extptr_2(x)
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60973f30>
> >  x
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60973f30>
> >  y
> >   # An object of class "S4ClassHoldingExtptr"
> >   # <pointer: 0x7ffc60973f30>
> > 
> > However, "subs_extptr_2" will modify both x and y.
> > 
> > Is this behavior expected?
> > 
> > Thanks,
> > Jialin
> > 
> > 
> > > sessionInfo()
> > 
> > R version 3.4.1 (2017-06-30)
> > Platform: x86_64-suse-linux-gnu (64-bit)
> > Running under: openSUSE Tumbleweed
> > 
> > Matrix products: default
> > BLAS: /usr/lib64/R/lib/libRblas.so
> > LAPACK: /usr/lib64/R/lib/libRlapack.so
> > 
> > locale:
> > [1] LC_CTYPE=en_US.UTF-
> > 8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8
> > [4] LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-
> > 8    LC_MESSAGES=en_US.UTF-8
> > [7] LC_PAPER=en_US.UTF-
> > 8       LC_NAME=C                  LC_ADDRESS=C
> > [10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8
> > LC_IDENTIFICATION=C
> > 
> > attached base packages:
> > [1] stats     graphics  grDevices
> > utils     datasets  methods   base
> > 
> > other attached packages:
> > [1] inline_0.3.14 magrittr_1.5
> > 
> > loaded via a namespace (and not attached):
> > [1] compiler_3.4.1 tools_3.4.1    yaml_2.1.18    ulimit_0.0-3
> > 
> > ______________________________________________
> > R-devel at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> > 
> 
>