How to set default value to a Rcpp::Function in argument of function in Rcpp?

Question

I want to set default value in argument of function to Rcpp::Function argument.

Just simple assignment, Rcpp::Function func = mean, is not possible. It returns error: no viable conversion from '<overloaded function type>' to 'Rcpp::Function' (aka 'Function_Impl<PreserveStorage>')

Or, I tried something like this: Rcpp::Function func = Function("mean"), but again, it is not working. It returns warning message: Unable to parse C++ default value 'Function("mean")' for argument func of function.

For example, I have my own function to maximum called maxC:

// [[Rcpp::export]]
double maxC(NumericVector x) {
  double max;
  max = *std::max_element(x.begin(), x.end());

  return max;
}

Now, I want to use it (maxC) as default argument to another function, for example like this:

// [[Rcpp::export]]
double aggregate(NumericVector x, Rcpp::Function func = maxC) {
  double agg;
  agg = Rcpp::as<double>(func(x));

  return agg;   
}

But it doesn't work. Any suggestions? Thank you.

Answer 1

I do not believe you can set a default function in this way... The best that can be achieved is setting function to a NULL value and then having the code execute the appropriate default later on. For example...

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericVector func_defaults(Rcpp::NumericVector x,
                                  Rcpp::Nullable<Rcpp::Function> f = R_NilValue) {

  if (f.isNotNull()) {
    Rcpp::NumericVector res = Rcpp::as<Rcpp::Function>(f)(x);
    return res;
  }

  Rcpp::Environment global_funcs = Rcpp::Environment::global_env();
  Rcpp::Function mean_r = global_funcs["mean"];
  return mean_r(x);
}

testing:

func_defaults(c(2.5,3,1))
# [1] 2.166667
func_defaults(c(2.5,3,1), mean)
# [1] 2.166667
func_defaults(c(2.5,3,1), median)
# [1] 2.5