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

[Bill Dunlap]

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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