[Rcpp-devel] Linking to c++ files in a package without Rcpp overhead for faster computation

[John Tipton]

Hey Rcpp developers,

I am brand spanking new to the Rcpp community and I want to introduce
myself with what is likely a very simple question that I haven't found an
answer for yet. My question is on package development and linking c++ files
from one package directly to another package without having to create a
CCallable function . As a relatively experienced R programmer but novice
c++ programmer, I am still learning as I go so I created a minimal example
below that I believe better illustrates my question.

I am building a package with RStudio and I want to make available to
another package the .h header files - (more explicitly the c++ subroutines
in those headers) without the loss of computation speed shown below.

My question can best be summarized with two parts

1) Why when I directly source testPack.h is the function call so much
slower than when I define the function in the same .cpp file (i.e. why is
fun1.cpp so much slower (~40%) than fun2.cpp in test.R)

2) How do I build a library in an automated fashion (maybe there is a
CPPFLAGS variable?) so I can use the functionality of something like
#include “path/to/file/hello_world2.h” in fun2.cpp without using an
absolute path? I am building a large MCMC sampler so every bit of marginal
speed gain is important. In other words, how can I replicate the behavior
of fun2.cpp in a function that sources my library testPack


I have the following files (outside my package) that provide testing.
Thanks for any help.

test.R
fun1.cpp <- includes <testPack.h> header, runs slow
fun2.cpp <- includes “hello_world2.h” header, runs faster

I built a package skeleton in RStudio using Rcpp. Within the src directory
I have the files
hello_world2.h
cpp_hello_world.cpp


The files are as follows
##
##
##
———— begin test.R script ————

library(rbenchmark)
n <- 10000
Rcpp::sourceCpp('~/test/fun1.cpp’)
Rcpp::sourceCpp('~/test/fun2.cpp’)

benchmark(fun1(n), fun2(n), replications = 500)

———— end test.R script ————
##
##
##
———— begin fun1.cpp ————

#include <RcppArmadillo.h>
#include <testPack.h>
// [[Rcpp::depends(testPack)]]
// [[Rcpp::depends(RcppArmadillo)]]

//using namespace Rcpp;

//[[Rcpp::export]]
void fun1(const int& n) {
   for(int i = 0; i < n; i++){
    testPack::rcpp_hello_world();
   }
}

———— end fun1.cpp ————
##
##
##
———— begin fun2.cpp ————

#include <RcppArmadillo.h>
#include <path/to/file/testPack/src/hello_world2.h>

// [[Rcpp::depends(RcppArmadillo)]]

//[[Rcpp::export]]
void fun2(const int& n) {
   for(int i = 0; i < n; i++){
    rcpp_hello_world2();
   }
}

———— end fun2.cpp ————
##
##
##
———— begin hello_world2.h ————

Rcpp::List rcpp_hello_world2() {

    Rcpp::CharacterVector x = Rcpp::CharacterVector::create("foo", "bar");
    Rcpp::NumericVector y   = Rcpp::NumericVector::create(0.0, 1.0);
    Rcpp::List z            = Rcpp::List::create(x, y);

    return z ;
}

———— end hello_world2.h ————
##
##
##
———— begin rcpp_hello_world.cpp ————

#include <RcppArmadillo.h>

// [[Rcpp::depends(RcppArmadillo)]]
// [[Rcpp::interfaces(cpp)]]

using namespace Rcpp;

//[[Rcpp::export]]
List rcpp_hello_world() {

    CharacterVector x = CharacterVector::create( "foo", "bar" )  ;
    NumericVector y   = NumericVector::create( 0.0, 1.0 ) ;
    List z            = List::create( x, y ) ;

    return z ;
}

———— end rcpp_hello_world.cpp ————
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