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

[luke-tierney at uiowa.edu]

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
>

-- 
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-tierney at uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu