Rcpp, making functions available to R from DLL - c++

Question

New to C++, I would like to make functions compiled in a DLL available in R.
So far, I managed to do that for a basic function taking integer as input and returning the same, by following these steps in VisualStudio, then using dyn.load from R to load the created dll.
However, my C++ functions will need to handle R data.frame objects, and I am not sure how to make that possible. I saw from the Rcpp gallery that Rcpp might include some kind of translations between R and c++ data types including data.frame objects, but I don't know if I can generate a dll using Rcpp that I can then include in R using dyn.load.
From this answer by Dirk Eddelbuettel, it seems possible to generate a "dynamic library" using Rcpp, however, I could not find any dll when I tried generating a package with a source .cpp file using rcpp.package.skeleton(). The function I'd like to have a dll for is from the Rcpp gallery
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
DataFrame modifyDataFrame(DataFrame df) {
// access the columns
IntegerVector a = df["a"];
CharacterVector b = df["b"];
// make some changes
a[2] = 42;
b[1] = "foo";
// return a new data frame
return DataFrame::create(_["a"]= a, _["b"]= b);
}
I tried to just paste that code into VisualStudio to try and generate that DLL, however I have the error "cannot find Rcpp.h" which I quite expected.
I then followed these steps in RStudio:
Create new project / Package
Include this cpp source file as a source file for this package
include Rcpp and enter Rcpp.package.skeleton("mypackage") so far, no DLL in the package folders
Tried to build the package in RStudio by going to Build/Install and Restart, but then I get an error message "Building R Packages needs installation of additional build tools, do you want to install them?" However I already have RbuildTools 3.4 installed, and when I click "YES" in RStudio nothing happens.
PS: Happy to hear about other methods but here the DLL format should be used if possible. Any piece of info is greatly appreciated since I have basically no idea of how Rcpp or C++ work

Answer 1

You need to figure out why your setup is hosed. This is meant to be easy and it is easy. Watch:
R> Rcpp::cppFunction('DataFrame modDF(DataFrame df) { IntegerVector a = df["a"]; CharacterVector b = df["b"]; a[2] = 42; b[1] = "foo"; return DataFrame::create(Named("a")=a, Named("b")=b); } ')
R> df <- data.frame(a=1:3, b=letters[24:26])
R> df
a b
1 1 x
2 2 y
3 3 z
R> modDF(df)
a b
1 1 x
2 2 foo
3 42 z
R>
Now, I obviously don't recommend writing this way in a long one-liner. You are already on the right track setting up a package. But you need to sort out what is holding up your tools.
And as a PS the one-liner above with better indentation:
R> Rcpp::cppFunction('DataFrame modDF(DataFrame df) { \
IntegerVector a = df["a"]; \
CharacterVector b = df["b"]; \
a[2] = 42; \
b[1] = "foo"; \
return DataFrame::create(Named("a")=a, Named("b")=b); \
} ')

Answer 2

The following seems to work:
from R, ran Rcpp.package.skeleton("dfcpp4", cpp_files="modifyDataFrame.cpp"). The second argument is required in order for the modifyDataFrame function to be available from the dll using dyn.load.
From the command line ran R CMD build dfcpp4
ran R CMD check dfcpp4 --no-manual from the command line.
The dll file in now present in the src-x64 folder
I am now able to call this function using
dyn.load("dfcpp4/src-x64/dfcpp4.dll")
df <- data.frame(a = c(1, 2, 3),
b = c("x", "y", "z"))
.Call("_dfcpp4_modifyDataFrame", df)
a b
1 1 x
2 2 foo
3 42 z
What I don't get is why in this case .Call should be used instead of .C...