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Is there any way to check the class of an ALTREP?

6 messages · Dénes Tóth, Benjamin Christoffersen, Jiefei Wang

Original
Jiefei Wang
# 
Hi all,

I would like to determine if an ALTREP object is from my package, I see
there is a function `ALTREP_CLASS` defined in RInternal.h but its return
value is neither a `R_altrep_class_t` object nor an STRSXP representing a
class name. I do not know how to correctly use it. Any suggestions?

Thanks,
Jiefei
Benjamin Christoffersen
# 
It seems as if you can you use the ALTREP macro as done in this
package: https://github.com/Jiefei-Wang/SharedObject/blob/804b6ac58c63a4bae95343ab43e8b1547b07ee6b/src/C_interface.cpp#L185

and in base R: https://github.com/wch/r-source/blob/54fbdca9d3fc63437d9e697f442d32732fb4f443/src/include/Rinlinedfuns.h#L118

The macro is defined here in Rinternals.h:
https://github.com/wch/r-source/blob/abb550c99b3927e5fc03d12f1a8e7593fddc04d2/src/include/Rinternals.h#L325

Den man. 19. okt. 2020 kl. 10.13 skrev Jiefei Wang <szwjf08 at gmail.com>:
Dénes Tóth
# 
Benjamin,

You happened to send a link which points to the OP's own package :) I 
think Jiefei would like to know how one can "officially" determine if an 
arbitrary ALTERP object belongs to a class that he owns.

Regards,
Denes
On 10/19/20 10:22 AM, Benjamin Christoffersen wrote:
Benjamin Christoffersen
# 
Argh, I am sorry! I did not notice that. My best bet for what I
thought the question was is section 1.1.2 of R Internals where they
note that: `unsigned int alt : 1; /* is this an ALTREP object? */`

This is also what the ALTREP macro checks. However, I also did not
notice the "... if an ALTREP object is from my package" part. I am
sorry.

Sincerely Yours,
Benjamin

Den man. 19. okt. 2020 kl. 11.25 skrev D?nes T?th <toth.denes at kogentum.hu>:
Jiefei Wang
# 
Thank Denes for the clarification, glad to see my package got one citation
from my own question:) Also, thank Benjamin
for sending many useful documents.

Actually, the question is related to the SharedObject package that Benjamin
has pointed to. I wanna
avoid sharing an object that has already been shared, so I need to check
whether an object is an ALTREP
that is defined in my package. Since the ALTREP definition involves an `
R_altrep_class_t` object and a
 class name, I will expect to see a method defined in R to retrieve such
information. I hope this can clarify my question.

Best,
Jiefei

On Mon, Oct 19, 2020 at 5:38 PM Benjamin Christoffersen <boennecd at gmail.com>
wrote:

  
  


  


      

    

    

      
      

        


  

        

      Jiefei Wang
    

    

      
      #
    

  


  

      
Hi all,

By looking at the implementation of R's wrapper. I found a function `
R_altrep_inherits` that can check whether an object belongs to an ALTREP
class. Even though it is a little different from what I am looking for, but
it should be satisfactory for my purpose. I will suspect there is no such
function in R to get the class of an ALTREP object for the SEXP struct is
very compact and this less-important information might not be stored when
an ALTREP object is created.

Best,
Jiefei

        
On Mon, Oct 19, 2020 at 8:37 PM Jiefei Wang <szwjf08 at gmail.com> wrote: