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

[Oliver Madsen]

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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