Communication between R and C++

Forget about .C. That is clunky. Perhaps using .C over .Call or .External made sense before Rcpp. But now with the work we've put in Rcpp, I really don't see the point of using .C anymore. Just use .Call.

Better still, with attributes (sourceCpp and compileAttributes), you don't even have to see the .Call anymore, it just feels like you are using a c++ function.

Now, if I wanted to do something that preserves states, I'd use a module. For example, your application is this Test class. It has methods do_something and do_something_else and it counts the number of times these methods are used:

#include <Rcpp.h>
using namespace Rcpp ;

class Test {
public:
    Test(): count(0){}

    void do_something(){
        // do whatever
        count++ ;
    }
    void do_something_else(){
        // do whatever
        count++ ;
    }

    int get_count(){
        return count ;
    }

private:
    int count ; 
} ;

This is pretty standard C++ so far. Now, to make this available to R, you create a module like this :

RCPP_MODULE(test){
    class_<Test>( "Test" )
        .constructor()
        .method( "do_something", &Test::do_something )
        .method( "do_something_else", &Test::do_something_else )

        .property( "count", &Test::get_count )
    ;
}

And then you can just use it :

app <- new( Test )
app$count
app$do_something()
app$do_something()
app$do_something_else()
app$count