devtools::document() won't include useDyLib in NAMESPACE - r

Question

I am sure I am missing something stupid, but despite several hours looking online I have not been able to solve this problem.
I am trying to make a simple R package with Rcpp code. The two toy routines are
#' rtest
#'
#' This function does nothing
#' #param x a vector of numbers
#' #return a vector of number
#' #export
#' #examples
#' rtest(1:5)
rtest <- function(x) {
x^2-rtest_cpp(x)
}
and
#include <Rcpp.h>
using namespace Rcpp;
//' another routine that doesn't do much
//'
//' #param x a numeric vector
//' #return a numeric vector
// [[Rcpp::export]]
Rcpp::NumericVector rtest_cpp(Rcpp::NumericVector x) {
int k=x.size();
NumericVector y(k);
for(int i=0;i<k;++i) y[i]=x[i]*x[i];
return y;
}
Also the DESCRIPTION file is
Package: Rtest
Title: Test package
Version: 1.0.0
Authors#R:
person("Wolfgang", "Rolke", , "wolfgang.rolke#upr.edu", role = c("aut", "cre"),
comment = c(ORCID = "0000-0002-3514-726X"))
Description: Package to test various package problems.
License: GPL (>=2)
Encoding: UTF-8
Roxygen: list(markdown = TRUE)
RoxygenNote: 7.2.1
LinkingTo: Rcpp
Imports: Rcpp
Suggests: rmarkdown, knitr
VignetteBuilder: knitr
Now when I run devtools::document() I get the NAMESPACE file with export(rtest) but not the a parts
useDynLib(“Rtest”)
importFrom(Rcpp, sourceCpp)
and so when I build the project and run rtest I get the error message that _Rtest_rtest_cpp is not available.
I can add the usedDynLib .. manually to NAMESPACE and build the project and then everything works fine, but I would much rather have the NAMESPACE file created by document().
What am I missing?
Thanks Limey. I had seen and tried this advice before, however only to add the #useDynLib part, but not to also add the #importFrom. Now it works!
I have a follow-up question, though. This is not the first package I have written with both R and Rcpp code, and I never had this problem before. I also never added those lines to the R file headers, either. And yet devtools::document() created the correct NAMESPACE file anyway. So what else tells devtools::document() to do the right thing? I once thought it was adding "LinkingTo: Rcpp;Imports: Rcpp" to DESCRIPTION , but apparently not.

Answer 1

Thanks Limey. I had seen and tried this advice before, however only to add the #useDynLib part, but not to also add the #importFrom. Now it works! In fact, from reading other things on the internet I created a separate file called extra.R in the R folder with the code
#' #useDynLib Rtestrolke
#' #importFrom Rcpp evalCpp
#' #exportPattern "^[[:alpha:]]+"
NULL
and that does the trick as well.
As I said in the part of the question added later, this just leaves the mystery why this is sometimes needed and sometimes not. Still don't know the answer to that.