[Rd] What is the best way to loop over an ALTREP vector?

[Wang Jiefei]

Hi devel team,

I'm working on C/C++ level ALTREP compatibility for a package. The package
previously used pointers to access the data of a SEXP, so it would not work
for some ALTREP objects which do not have a pointer. I plan to rewrite the
code and use functions like get_elt, get_region, and get_subset to access
the values of a vector, so I have a few questions for ALTREP:

1. Since an ALTREP do not have to define all of the above
functions(element, region, subset), is there any way to check which
function has been defined for an ALTREP class? I did a search on
RInternal.h and altrep.c but did not find a solution for it. If not, will
it be added in the future?

2. Given the diversity of ALTREP classes, what is the best way to loop over
an ALTREP object? I hope there can be an all-in-one function which can get
the values from a vector as long as at least one of the above functions has
been defined, so package developers would not be bothered by tons of
`if-else` statement if they want their package to work with ALTREP. Since
it seems like there is no such function exist, what could be the best way
to do the loop under the current R version?

Best,
Jiefei

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