for when you want to return a std::function to R wrapped in a struct to be evaluated later

std_function_RcppXPtr.cpp

#include <functional>

#include <Rcpp.h>

// [[Rcpp::plugins(cpp14)]]

using my_lambda = std::function<double(const double)>;

typedef struct lambda_st {
  my_lambda func;
  lambda_st(){Rcpp::Rcout << "lambda_st ctor called at " << this << "\n";}
  ~lambda_st(){Rcpp::Rcout << "lambda_st dtor called at " << this << "\n";}
} lambda_st;

// make a silly lambda
my_lambda make_my_lambda(const double x0){
  
  my_lambda lambda = [x0=x0](const double x1) -> double {
    return x0 + x1;
  };
  
  return lambda;
}

// the R interface
// [[Rcpp::export]]
Rcpp::XPtr<lambda_st> make_my_lambdaR(const double x0){
  
  lambda_st* lambdastruct = new lambda_st;
  lambdastruct->func = make_my_lambda(x0);
  
  Rcpp::XPtr<lambda_st> xptr(lambdastruct);
  return xptr;
}

// [[Rcpp::export]]
double evaluate_my_lambdaR(SEXP xptr, const double x1){
  Rcpp::XPtr<lambda_st> lambda_st_xptr(xptr);
  double out = lambda_st_xptr->func(x1);
  return out;
}