Rcpp - Exposing a C++ function that contains pointers to R - rcpp

Question

I am using Rcpp to create a package in R that leverages C++ code. I have read all of the Rcpp vignettes, but I haven't been able to find a solution to the following problem.
One of the C++ classes I'm trying to use contains a pointer. I am exposing the class using a module. When I try to install the package in R, I get the following error.
error: expected unqualified-id before '*' token.field("*w", &ffm_model::*w)
What am I doing wrong?
Code for Class Containing Pointer
typedef float ffm_float;
typedef int ffm_int;
class ffm_model {
public:
ffm_int n; // number of features
ffm_int m; // number of fields
ffm_int k; // number of latent factors
ffm_float *w = nullptr;
bool normalization;
~ffm_model();
};
Code for Corresponding RCPP Module
RCPP_MODULE(ffmModelMod){
using namespace Rcpp;
//Expose class as ffm_model on the r side
class_<ffm_model>( "ffm_model")
.field("n", &ffm_model::n)
.field("m", &ffm_model::m)
.field("k", &ffm_model::k)
.field("*w", &ffm_model::*w)
.field("normalization", &ffm_model::normalization)
.method("~ffm_model",&ffm_model::~ffm_model)
;
}

Answer 1

I had a similar problem, and as Dirk mentions, it is due to types that cannot be mapped automatically, such as float*.
The following workaround works for me:
Do not expose the fields with problematic types to R.
Instead, expose get() and set() functions to the fields above.
Here is an example, in which both the (unproblematic) value field and the (problematic) child field (a pointer to an object of the same class) are hidden:
Class
#include <Rcpp.h>
using namespace Rcpp;
class node
{
public:
double value; // Voluntarily hidden from R
node* child; // Must be hidden from R
// Exposed functions
void setVal(double value);
double getVal();
node* createNode(double value); // return pointer to a node
node* createChild(double value); // set child
node* getChild();
};
Methods
void node::setVal(double value){
this->value = value;
}
double node::getVal(){
return this->value;
}
node* node::createNode(double value){
node* n = new node;
n->value = value;
return n;
}
node* node::createChild(double value){
this->child = createNode(value);
return child;
}
node* node::getChild(){
return this->child;
}
RCPP Module
RCPP_MODULE(gbtree_module){
using namespace Rcpp;
class_<node>("node")
.constructor()
.method("setVal", &node::setVal)
.method("getVal", &node::getVal)
.method("createNode", &node::createNode)
.method("createChild", &node::createChild)
.method("getChild", &node::getChild)
;
}
Usage in R
n <- new(node)
n$setVal(2)
n$getVal()
n2 <- n$createNode(1) # unrelated node
n3 <- n$createChild(3) #child node
n$getChild() #pointer to child node
n3