[R-pkg-devel] accessing call constructors in Cpp

[Oliver Madsen]

Hi Bill thanks for your answer,

The only part left out (which I only realized as I'm trying out your
suggestion) is an #include Rcpp in order to remove 5 errors thrown when
executing Rcpp::sourceCpp. I am guessing that the suggestion from RAPI to
always work with R_NO_REMAP defined is taken literally and that it is
defined in Rcpp.h or its dependencies.

But it is working, thank you! :-)
(continues to dive further down the rabbit hole)

Best Regards
Oliver

On Wed, Jan 6, 2021 at 12:01 AM Bill Dunlap <williamwdunlap using gmail.com>
wrote:

> Try using their official names, Rf_ScalarReal, Rf_eval, Rf_install, etc.
> You may have #defined R_NO_REMAP in code that you did not show us.
>
> -Bill
>
> On Tue, Jan 5, 2021 at 1:42 PM Oliver Madsen <oliver.p.madsen using gmail.com>
> wrote:
>
>> This is maybe better suited for Rcpp-devel or another mailing list. If so,
>> I apologize. :-)
>>
>> In a pet project I've been implementing quite a bit of a package structure
>> within Cpp, including classes to contain and manipulate structures. As a
>> final part of the implementation a user provided R function is to be
>> executed with some of the manipulated data. While my first intuition (and
>> the smart option) was to implement a method in R for extracting the
>> parameters and simply executing the call, I got curious as to how one
>> could
>> do this in Cpp.
>>
>> After researching I found myriad options, one simple method being
>> 1) Create a list of named and unnamed arguments
>> 2) Extract "do.call" as a Function,
>> 3) Test that the function provided is either a CLOSXP (function) or
>> CHARSXP
>> (or maybe STRSXP?) and then
>> 4) execute the function call.
>> but while going further down and checking the source code for do_docall I
>> was curious about trying to recreate the call "the old fashioned way", but
>> in Cpp. After looking over how it was done in R-source and a few examples
>> online I tried implementing a simple call creation and evaluating it with
>> Rf_eval
>>
>> ```cpp
>> #include <R.h>
>> #include <Rinternals.h>
>> //[[Rcpp::export]]
>> SEXP add_10_and_5(SEXP rho){ //rho is an environment (new.env() here)
>> SEXP REALSXP_10 = PROTECT(ScalarReal(10));
>> SEXP REALSXP_5 = PROTECT(ScalarReal(5));
>> SEXP out = PROTECT(LCONS(install("+"), LCONS(
>>   REALSXP_10, LCONS(
>>       REALSXP_5, R_NilValue
>>   )
>> )));
>> UNPROTECT(3);
>> return eval(out, rho);
>> }
>> ```
>> (For those who read the old version of Advanced R, this is a very similar
>> to the R's C interface section on "PairLists")
>>
>> Immediately intellisense warned me that "ScalarReal", "install", "lcons"
>> and "eval" are all undefined. However, a quick look in
>> src/include/Rinternals.h shows that they are forward-declared there, and
>> should (in my understanding) be defined. For good measures I tried
>> including every header iteratively to no avail.
>>
>> While it is likely recommended to not do this (and the final product
>> likely
>> will be implemented in R), I am curious why I seem unable to access these
>> 4
>> from Cpp. I can't seem to spot the reason in the header file itself.
>>
>> Best regards
>> Oliver
>>
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>>
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>>
>

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