c++ documentation in an R package

Question

my R package uses Rcpp and RcppArmadillo. I built it in Rstudio. I am following H.Wickhams guide (R-packages). The package builds (with one warning, see below), installs and runs fine.

cpp scripts have been documented using the //' headers

I have several questions that i think are all related:

1. If i add //'@export to a cpp file then an entry of export() is added to NAMESPACE . Shouldn't it be export(filename)?

2. The RcppExports.R file contains a NULL value and during build i get a warning "Warning: RcppExports.R:18 : Missing name" . Why is this? How do i rectify it?

3. How do i read the help file for the cpp script. ?filename doesn't seem to work like it does for R files in the package?

EDIT: snippet of code from .cpp

//' @export
#include <RcppArmadillo.h>
// [[Rcpp::export]]
arma::mat myfunc(int nSize, ... ,arma::cube suitability) { 
   do some stuff
}

Code from RcppExports.R (This NULL is the warning mentioned above)

#' @export
NULL
myfunc <- function(nSize, ..., suitability) {
   .Call('_myfunc', PACKAGE = 'mypackage', nSize, ... , suitability)
}

and code from RccpExports.cpp

// Generated by using Rcpp::compileAttributes() -> do not edit by hand
// Generator token: 10BE3573-1514-4C36-9D1C-5A225CD40393
#include <RcppArmadillo.h>
#include <Rcpp.h>

using namespace Rcpp;

// myfunc
arma::mat myfunc(int nSize, ..., arma::cube suitability);
RcppExport SEXP _myfunc(SEXP nSizeSEXP, SEXP suitabilitySEXP) {
BEGIN_RCPP
  Rcpp::RObject rcpp_result_gen;
  Rcpp::RNGScope rcpp_rngScope_gen;
  Rcpp::traits::input_parameter< int >::type nSize(nSizeSEXP);
  Rcpp::traits::input_parameter< arma::cube >::type suitability(suitabilitySEXP);
  rcpp_result_gen = Rcpp::wrap(myfunc(nSize, ..., suitability));
  return rcpp_result_gen;
END_RCPP
}

static const R_CallMethodDef CallEntries[] = {
    {"_myfunc", (DL_FUNC) &_myfunc, 9},
   {NULL, NULL, 0}
};

RcppExport void R_init_mypackage(DllInfo *dll) {
   R_registerRoutines(dll, NULL, CallEntries, NULL, NULL);
   R_useDynamicSymbols(dll, FALSE);
}

SOLUTION : Thanks to Anders and Ralf

#include <RcppArmadillo.h>
//' calculates stuff
//'
//' Calculates the stuff
//'
//'@param nSize number of some stuff
//'@param p2
//'@param p3
//'@param p4
//'@param p5
//'@param p6
//'@param p7
//'@param p8
//'@param suitability 3D array 
//' @export
// [[Rcpp::export]]
arma::mat myfunc(int nSize, ... ,arma::cube suitability) { 
  do some stuff
}

Answer 1

You need to have something like

//' @export
// [[Rcpp::export]]

right before your function definition. The first is Roxygen saying that the function is to be exposed to the user-interface. The second line is telling Rcpp to export to the R-side---ie the functions are simply put/exported into RcppExports.Rcpp and RcppExports.R . Two different things where the former only makes sense if the latter is provided too.

The Roxygen comments \\\\' are just carried on to the RcppExports.R file.

Edit: From your comments, I see you metion RcppArmadillo. You only need to #include <RcppArmadillo.h> once at the top of your .cpp file. The // [[Rcpp::depends(RcppArmadillo)]] is not needed.

Answer 2

The roxygen comment has to be next to the function(s) it applies to. Different combination of such comments are possible:

#include <RcppArmadillo.h>

//' @export
// [[Rcpp::export]]
arma::mat myfunc(int nSize, ... ,arma::cube suitability) { 
   do some stuff
}

//' @export
// [[Rcpp::export]]
arma::mat myotherfunc(int nSize, ... ,arma::cube suitability) { 
   do some other stuff
}

// C++ internal function, i.e. usable in the packages C++ code
arma::mat cppinternal(int nSize, ... ,arma::cube suitability) { 
   do some other stuff
}

// R internal function, i.e. usable in the packages R code 
// [[Rcpp::export]]
arma::mat Rinternal(int nSize, ... ,arma::cube suitability) { 
   do some other stuff
}