Can Rcpp::Function be NULL? - r

Question

I have a cpp function which has an R function as one of its arguments like below:
void mycppfunction(SEXP x, Rcpp::Function func)
func can be a function, or it can be NULL. How can I implement this? If I do this:
void mycppfunction(SEXP x, Rcpp::Nullable<Rcpp::Function> func)
then, the line:
func(x)
gives me the error:
error: no match for call to ‘(Rcpp::Nullable<Rcpp::Function_Impl<Rcpp::PreserveStorage> >) (SEXP)’
On the other hand, if I just define mycppfunction as:
void mycppfunction(SEXP x, Rcpp::Function func)
then running the code with a NULL value for func results in a segfault: "memory not mapped".

Answer 1

Yes you can do this but I would advise that you are careful about verifying the compatibility of the function passed in and the argument. Here's a small example, with minimal defensive code to ensure that f is a valid function:
#include <Rcpp.h>
typedef Rcpp::Nullable<Rcpp::Function> nullable_t;
// [[Rcpp::export]]
SEXP null_fun(Rcpp::NumericVector x, nullable_t f = R_NilValue) {
if (f.isNotNull()) {
return Rcpp::as<Rcpp::Function>(f)(x);
}
return Rcpp::wrap((double)Rcpp::sum(x));
}
/*** R
null_fun(1:5)
#[1] 15
null_fun(1:5, mean)
#[1] 3
null_fun(1:5, min)
#[1] 1
null_fun(1:5, max)
#[1] 5
*/
Note that the use of SEXP as a return type and the Rcpp::wrap((double)...) was just to quiet the compiler in this specific example, and won't necessarily apply to your use case(s).