How do I check if an externalptr is NULL from within R

Question

I\'m using SWIG to generate wrapper code to access C code from within the R language. The wrapper code uses the R externalptr type to hold references to C pointers

Answer 1

The C solution above is most elegant, but a compiler might not always be available. A solution that does not require compiled code could be something like:

identical(pointer, new("externalptr"))

However this would not work if the object has custom attributes. If this is the case, you could do:

isnull <- function(pointer){
  a <- attributes(pointer)
  attributes(pointer) <- NULL
  out <- identical(pointer, new("externalptr"))
  attributes(pointer) <- a
  return(out)
}

Again a bit more involved than the C solution, but will work in a simple script on any platform.

Answer 2

For completeness, here's the solution I used. It required a function on the C side, and one on the R side, as suggested by @DirkEddelbuettel. The C function is:

#include <Rinternals.h>

SEXP isnull(SEXP pointer) {
  return ScalarLogical(!R_ExternalPtrAddr(pointer));
}

And the wrapper function in R is:

is.null.externalptr <- function(pointer) {
  stopifnot(is(pointer, "externalptr"))
  .Call("isnull", pointer)
}

For an example of use from within R:

> p <- new("externalptr")
> p
<pointer: (nil)>
> is.null.externalptr(p)
[1] TRUE