Rcpp: Calling function with object as argument - c++11

Question

I have a function in Rcpp, which creates a very long map-structure within a class. I've given a simple example of it below:
#include <Rcpp.h>
using namespace Rcpp;
class A{
private:
std::map<int, int> m_map;
public:
void fill_map(const size_t limit){
for(size_t i=0; i<limit; ++i){
m_map[i] = i;
}
}
size_t size_map(){return m_map.size();}
};
// [[Rcpp::export]]
void func1(const size_t limit) {
A a;
a.fill_map(limit);
}
/* NOT WORKING */
// [[Rcpp::export]]
void func2(A a)
{
std::cout << a.size_map() << "\n";
}
/* NOT WORKING */
Say I call func1(1e7), which fills up the map in the a-object. I need to pass this A-object to other functions as shown above with func2.
However, my example with func2 doesn't work. Within the Rcpp-framework, what is the correct and most efficient approach to call func2 with an object defined in a previous function?

Answer 1

C++ code
#include <Rcpp.h>
using namespace Rcpp;
class A
{
private:
std::map<int, int> m_map;
public:
void fill_map(const size_t limit)
{
for(size_t i=0; i<limit; ++i)
{
m_map[i] = i;
}
}
size_t size_map(){return m_map.size();}
};
// [[Rcpp::export]]
XPtr<A> func1(const size_t limit)
{
XPtr<A> ptr(new A(), true);
ptr->fill_map(limit);
return(ptr);
}
// [[Rcpp::export]]
void func2(XPtr<A> ptr)
{
Rcout << ptr->size_map() << std::endl;
}
R code
a = func1(10)
func2(a)
a being an External pointer.