Passing R functions to C routines using rcpp - r

Question

I have a C function from a down-stream library that I call in C like this
result = cfunction(input_function)
input_function is a callback that needs to have the following structure
double input_function(const double &x)
{
return(x*x);
}
Where x*x is a user-defined computation that is usually much more complicated. I'd like to wrap cfunction using Rcpp so that the R user could call it on arbitrary R functions.
NumericVector rfunction(Function F){
NumericVector result(1);
// MAGIC THAT I DON'T KNOW HOW TO DO
// SOMEHOW TURN F INTO COMPATIBLE input_funcion
result[0] = cfunction(input_function);
return(result);
}
The R user then might do rfunction(function(x) {x*x}) and get the right result.
I am aware that calling R functions within cfunction will kill the speed but I figure that I can figure out how to pass compiled functions later on. I'd just like to get this part working.
The closest thing I can find that does what I need is this https://sites.google.com/site/andrassali/computing/user-supplied-functions-in-rcppgsl which wraps a function that uses callback that has an oh-so-useful second parameter within which I could stuff the R function.
Advice would be gratefully received.

Answer 1

One possible solution would be saving the R-function into a global variable and defining a function that uses that global variable. Example implementation where I use an anonymous namespace to make the variable known only within the compilation unit:
#include <Rcpp.h>
extern "C" {
double cfunction(double (*input_function)(const double&)) {
return input_function(42);
}
}
namespace {
std::unique_ptr<Rcpp::Function> func;
}
double input_function(const double &x) {
Rcpp::NumericVector result = (*func)(x);
return result(0);
}
// [[Rcpp::export]]
double rfunction(Rcpp::Function F){
func = std::make_unique<Rcpp::Function>(F);
return cfunction(input_function);
}
/*** R
rfunction(sqrt)
rfunction(log)
*/
Output:
> Rcpp::sourceCpp('57137507/code.cpp')
> rfunction(sqrt)
[1] 6.480741
> rfunction(log)
[1] 3.73767