[R] import_from_c() / export_to_c() methods should accept external pointers

[asfimport]

The import and export to C methods for Field, Shema, DataType, Array, RecordBatch, and RecordBatchReader all require the pointer to be passed as a double (casted from uintptr_t casted from *. I imagine there's good reason for this path to exist; however, the R externalptr type and 64-bit integer are more natural types and I think it would not be hard to allow pointers to be passed in this way. A short discussion of this occurred when updating the Python docs on how to pass Array pointers to R.

The implementation lives in arrow_cpp11.h and it seems like it would be easy to allow a user to pass something more R-like while still keeping the double route open for the packages that need it.

I see that this is getting used for pointers that R6 objects are storing for the underlying C++ objects...I imagine there's a good reason for this but I wonder if there's a way to make the places where this interacts with other packages more intuitive.

Reporter: Dewey Dunnington / @paleolimbot
Assignee: Alessandro Molina / @amol-

Related issues:

• [Python] _import_from_c() / _export_to_c() methods should accept capsules (is depended upon by)

PRs and other links:

• GitHub Pull Request #11919

_{Note: This issue was originally created as ARROW-14804. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Not only dedicated pointer types may be "more natural", but double is actually insufficient. A double precision floating-point number has 53 bits of mantissa, which is not enough to represent every possible 64-bit pointer.

I suppose pointer values are often clustered around 0, which makes them exactly representable as a double, but that's just luck and there's no guarantee that pointers will always be like that.

[asfimport]

Jonathan Keane / @jonkeane:
Issue resolved by pull request 11919
#11919