Error when I call cpp function from another .cpp file

Question

I tried to call c++ function from another .cpp file. I used .h header. See below what I did.

I have a fh file:

#ifndef PACKAGENAME_ADD_H
#define PACKAGENAME_ADD_H

#include <Rcpp.h>
Rcpp::NumericVector f(Rcpp::NumericVector x) ;

#endif

f.cpp file:

#include <Rcpp.h>
using namespace Rcpp;

NumericVector f(NumericVector x) {
  return x * 2;
}

g.cpp file:

#include <Rcpp.h>
#include <f.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector g(NumericVector x) {
  return f(x);
}

The three files are in the same folder I got this error when I run g.cpp :

Rcpp::sourceCpp('~/g.cpp')

Error in dyn.load("/tmp/Rtmpdu4AWp/sourceCpp-x86_64-pc-linux-gnu-0.12.17/sourcecpp_260f5e1a9ebc/sourceCpp_9.so") : unable to load shared object '/tmp/Rtmpdu4AWp/sourceCpp-x86_64-pc-linux-gnu-0.12.17/sourcecpp_260f5e1a9ebc/sourceCpp_9.so': /tmp/Rtmpdu4AWp/sourceCpp-x86_64-pc-linux-gnu-0.12.17/sourcecpp_260f5e1a9ebc/sourceCpp_9.so: undefined symbol: _Z1fN4Rcpp6VectorILi14ENS_15PreserveStorageEEE

Can someone help me? I work on ubuntu 18.04 and I have R 3.4.4 version.

Answer 1

The way I am most familiar with dealing with this issue in the context of Rcpp is by creating a package. In the case you present in the original post, as pointed out by Ralf Stubner it isn't really necessary; after changing the brackets( <> ) around fh in g.cpp to quotes ( "" ), your code compiled fine for me with sourceCpp() :

Rcpp::sourceCpp("g.cpp")
g(1:10)
# [1]  2  4  6  8 10 12 14 16 18 20

(for details see section 1.10 of the Rcpp Attributes vignette ).

However, if you are eventually needing multiple .cpp files to compile (ie, not just one that relies on an implementation in another), the way to go is creating a package. This might sound involved or intimidating, but with the tools provided by Rcpp , it's really quite simple. Here are the steps I took to turn your code into a package:

1. From R, run Rcpp::Rcpp.package.skeleton("SOanswer", example_code = FALSE)
2. Delete the file Read-and-delte-me .
3. Add the C++ files from your original post into the src/ folder (with only one minor edit -- changing the brackets( <> ) around fh in g.cpp to quotes ( "" )).
4. From R, run Rcpp::compileAttributes("SOanswer/") and devtools::install("SOanswer/")

Then it should compile nicely and you can run g() from R:

SOanswer::g(1:10)
# [1]  2  4  6  8 10 12 14 16 18 20

I will say that I would add there a step 0: Read the vignettes at https://cran.r-project.org/package=Rcpp , in particular the Rcpp Introduction and Rcpp Package vignettes. You can also check out this lovely example of a package with headers in src/ provided by coatless in the comments.