Can Rcpp expose a C++ class method taking a reference to the same class?

Question

Is it possible to use Rcpp to expose a C++ class to R when the class has a member taking an instance of that class?

Example:

#include <Rcpp.h>

class Test {
public:
    Test(int x): x_(x) {}
    int getValue() { return x_; }
    void addValue(int y) { x_ += y; }
    void merge(const Test& rhs) { x_ += rhs.x_; }
private:
    int x_;
};

using namespace Rcpp;

RCPP_MODULE(mod_test) {

    class_<Test>("Test")

    .constructor<int>("sets initial value")

    .method("getValue", &Test::getValue, "Returns the value")
    .method("addValue", &Test::addValue, "Adds a value")
    .method("merge", &Test::merge, "Merges another Test into this object")
    ;
}

Unfortunately that results in the following error:

error: no matching constructor for initialization of 'Test'

After reading and searching for answers, I came up with the idiom of separately including RcppCommon.h and then inserting a block like the following:

namespace Rcpp {
    template <> Test as( SEXP x ) ;
}

Unfortunately that results in the following errors:

Error in dyn.load("/.../sourceCpp_86871.so") : unable to load shared object '/.../sourceCpp_86871.so':
dlopen(/.../sourceCpp_86871.so, 6): Symbol not found: __ZN4Rcpp2asI4TestEET_P7SEXPREC Referenced from: /.../sourceCpp_86871.so Expected in: flat namespace in /.../sourceCpp_86871.so

Is it possible to do this?

Is there an implementation for the 'as' specialization that I need to create? Is there an example somewhere of how to write it?

Alternatively is there an example of how to check and convert a SEXP back into the C++ object it "wraps"?

Answer 1

The proper as conversion seems to be generated by inserting RCPP_EXPOSED_CLASS.

The complete working example becomes:

#include <Rcpp.h>

class Test {
public:
    Test(int x): x_(x) {}
    int getValue() { return x_; }
    void addValue(int y) { x_ += y; }
    void merge(const Test& rhs) { x_ += rhs.x_; }
private:
    int x_;
};

using namespace Rcpp;

RCPP_EXPOSED_CLASS(Test)
RCPP_MODULE(mod_test) {

    class_<Test>("Test")

    .constructor<int>("sets initial value")

    .method("getValue", &Test::getValue, "Returns the value")
    .method("addValue", &Test::addValue, "Adds a value")
    .method("merge", &Test::merge, "Merges another Test into this object")
    ;
}

It works correctly now:

> Rcpp::sourceCpp('test.cpp')
> a = Test$new(2)
> b = Test$new(3)
> a$getValue()
[1] 2
> a$merge(b)
> a$getValue()
[1] 5