Repository navigation
Enhance XPtr to set and check type info #1212
Description
The XPtr class wraps somewhat narrowly around R's own external pointers. The class actually has three fields: the pointer, a tag and a protect field. WRE, Section 5.13.1 has an example of setting the tag (of type SEXP, so can be anything) to a string. Searching the CRAN org here at GitHub (with its effective CRAN mirror) shows a few packages using this ... for various purposes, sometimes even second payloads.
Now, because the external pointer is effectively a naked C pointer, it would be useful to also offer some type checking. Following some initial discussion (thanks, @ihnorton) I am currently prototyping use with the tiledb package. But as this is generally useful we should support it here in time (maybe the July release, maybe next January).
Among the obvious missing features are
- a tag getter in the
XPtrclass - an alternate
XPtrcreation helper setting pointer and tag - and likely more
Following some quick discussion with @Enchufa2 there is also an idea to automate this further via typeid() and std::type_info() (though that may not work for user-defined classes, where I also ran into some potholes on initial probing with C++17-ish features).
In any event, this seems like a fruitful avenue to venture down so opening this ticket to collect more ideas.
Activity
Enchufa2 commentedon Apr 2, 2022
On second thought, a drawback of automatic type checking is that it wouldn't work with polymorphism. And I use this quite a lot.
eddelbuettel commentedon Apr 2, 2022
Good catch. Optional extension in derived class as we riffed may be the way to go.
eddelbuettel commentedon Apr 14, 2022
FWIW that is now in release 0.12.0 of package
tiledb-- but we use C++17 there and I haven't checked if what I use there can be used with older compilation standards too.github-actions commentedon Apr 15, 2023
This issue is stale (365 days without activity) and will be closed in 31 days unless new activity is seen. Please feel free to re-open it is still a concern, possibly with additional data.
eddelbuettel commentedon Apr 16, 2023
Still on the TODO list...
eddelbuettel commentedon Jan 13, 2024
The tagging idea I had (and use in
tiledbdoes not fully generalise. Packagenanoarrowfor example uses theSEXPfield to store another external pointer (!!) as Arrow data structures prefer to come in pair: 'data' (usually called 'array') along with 'schema'.So maybe a better idea is to use an additional attribute?
github-actions commentedon Jan 18, 2025
This issue is stale (365 days without activity) and will be closed in 31 days unless new activity is seen. Please feel free to re-open it is still a concern, possibly with additional data.