The RcppXPtrUtils package provides the means to compile user-supplied C++ functions with ‘Rcpp’ and retrieve an XPtr that can be passed to other C++ components.

Installation

Install the release version from CRAN:

install.packages("RcppXPtrUtils")

The installation from GitHub can be done with the remotes package:

remotes::install_github("Enchufa2/RcppXPtrUtils")

Use case

Let’s suppose we have a package with a core written in C++, connected to an R API with Rcpp. It accepts a user-supplied R function to perform some processing:

#include <Rcpp.h>
using namespace Rcpp;
template <typename T>
NumericVector core_processing(T func, double l) {
  double accum = 0;
  for (int i=0; i<1e3; i++)
    accum += sum(as<NumericVector>(func(3, l)));
  return NumericVector(1, accum);
}
// [[Rcpp::export]]
NumericVector execute_r(Function func, double l) {
  return core_processing<Function>(func, l);
}

But calling R from C++ is slow, so we can think about improving the performance by accepting a compiled function. In order to do this, the core can be easily extended to accept an XPtr to a compiled function:

typedef SEXP (*funcPtr)(int, double);
// [[Rcpp::export]]
NumericVector execute_cpp(SEXP func_, double l) {
  funcPtr func = *XPtr<funcPtr>(func_);
  return core_processing<funcPtr>(func, l);
}

To easily leverage this feature, the RcppXPtrUtils package provides cppXPtr(), which compiles a user-supplied C++ function using Rcpp::cppFunction() and returns an XPtr:

# compile the code above
# Rcpp::sourceCpp(code='...')
library(RcppXPtrUtils)
func_r <- function(n, l) rexp(n, l)
func_cpp <- cppXPtr("SEXP foo(int n, double l) { return rexp(n, l); }")
microbenchmark::microbenchmark(
  execute_r(func_r, 1.5),
  execute_cpp(func_cpp, 1.5)
)
#> Unit: microseconds
#>                        expr       min        lq       mean     median        uq
#>      execute_r(func_r, 1.5) 14910.161 16261.928 17628.8078 17468.1140 18635.388
#>  execute_cpp(func_cpp, 1.5)   213.123   223.125   310.2708   237.0265   279.808
#>        max neval cld
#>  22657.568   100   b
#>   2417.878   100  a

The object returned by cppXPtr() is just an externalptr wrapped into an object of class XPtr, which stores the signature of the function. If you are a package author, you probably want to re-export cppXPtr() and ensure that user-supplied C++ functions comply with the internal signatures in order to avoid runtime errors. This can be done with the checkXPtr() function:

func_cpp
#> 'SEXP foo(int n, double l)' <pointer: 0x55909eb28830>
checkXPtr(func_cpp, "SEXP", c("int", "double")) # returns silently
checkXPtr(func_cpp, "int", c("int", "double"))
#> Error in checkXPtr(func_cpp, "int", c("int", "double")): Bad XPtr signature:
#>   Wrong return type 'int', should be 'SEXP'.
checkXPtr(func_cpp, "SEXP", c("int"))
#> Error in checkXPtr(func_cpp, "SEXP", c("int")): Bad XPtr signature:
#>   Wrong number of arguments, should be 2'.
checkXPtr(func_cpp, "SEXP", c("double", "int"))
#> Error in checkXPtr(func_cpp, "SEXP", c("double", "int")): Bad XPtr signature:
#>   Wrong argument type 'double', should be 'int'.
#>   Wrong argument type 'int', should be 'double'.