Rcpp requires a copy constructor - c++

Question

I'm trying to export some C++11 functionality to R, using Rcpp. Some of these functions return their result using a std::unique_ptr. This stuff cannot be copied. The MCVE below uses std::unique_ptr<std::string> to illustrate the errors.
I have created a class around the unique_ptr, in the hopes of making all of this possible, but to no avail. This is the PtrClassOwner below. The function createClassWrapper calls the original createClass, to stick the unique_ptr in a PtrClassOwner object.
#include <RcppCommon.h>
#include <memory>
#include <string>
// Stuff to wrap:
using PtrClass = std::unique_ptr<std::string>;
PtrClass createClass() { return PtrClass{new std::string("boo")}; }
// ---
class PtrClassOwner {
public:
PtrClass string;
};
PtrClassOwner createClassWrapper() { return PtrClassOwner{createClass()}; }
RCPP_EXPOSED_WRAP(PtrClassOwner); // Rcpp-extending vignette says RCPP_EXPORT_WRAP, which doesn't exist.
RCPP_EXPOSED_AS(PtrClassOwner);
#include <Rcpp.h>
RCPP_MODULE(Class){
using namespace Rcpp;
class_<PtrClassOwner>("PtrClass");
function("createClass", &createClassWrapper);
}
This the first error reported by GCC (v 5.4, on Linux):
In file included from /home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/RcppCommon.h:195:0,
from rcpp_module.cpp:1:
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h: In instantiation of ‘SEXPREC* Rcpp::internal::wrap_dispatch(const T&, Rcpp::traits::wrap_type_module_object_tag) [with T = PtrClassOwner; SEXP = SEXPREC*]’:
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap_end.h:30:38: required from ‘SEXPREC* Rcpp::wrap(const T&) [with T = PtrClassOwner; SEXP = SEXPREC*]’
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap_end.h:35:20: required from ‘SEXPREC* Rcpp::module_wrap_dispatch(const T&, Rcpp::traits::normal_wrap_tag) [with T = PtrClassOwner; SEXP = SEXPREC*]’
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:922:40: required from ‘SEXPREC* Rcpp::module_wrap(const T&) [with T = PtrClassOwner; SEXP = SEXPREC*]’
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/module/Module_generated_CppFunction.h:34:50: required from ‘SEXPREC* Rcpp::CppFunction0<RESULT_TYPE>::operator()(SEXPREC**) [with RESULT_TYPE = PtrClassOwner; SEXP = SEXPREC*]’
rcpp_module.cpp:26:1: required from here
/home/cris/R/x86_64-pc-linux-gnu-library/3.2/Rcpp/include/Rcpp/internal/wrap.h:759:54: error: use of deleted function ‘PtrClassOwner::PtrClassOwner(const PtrClassOwner&)’
return Rcpp::internal::make_new_object<T>(new T(object));
^
rcpp_module.cpp:9:7: note: ‘PtrClassOwner::PtrClassOwner(const PtrClassOwner&)’ is implicitly deleted because the default definition would be ill-formed:
class PtrClassOwner {
^
The problem seems to be that, to wrap the object into an R object, it needs to be copied. This other question is from someone running into a similar issue, but there are no answers.
I have created a similar interface to Python, where the Python object just contains a pointer to the C++ object. I find it strange that Rcpp tries to copy the object to wrap it.
Is there a way around this issue? Is it possible to wrap only a pointer to the object in an R type, and somehow still properly manage its lifetime? I'm open to any solutions, I'm not set on using Rcpp, it just seemed the most straightforward method to export this functionality.

Answer 1

The simplest solution I've found so far is to extract the pointer from the std::unique_ptr and create a std::shared_ptr from it. Rcpp will wrap this, as it can be copied.
#include <Rcpp.h>
#include <memory>
#include <string>
// Stuff to wrap:
using PtrClass = std::unique_ptr<std::string>;
PtrClass createClass() { return PtrClass{new std::string("boo")}; }
// ---
using ShPtrClass = std::shared_ptr<PtrClass::element_type>;
class PtrClassOwner {
public:
ShPtrClass shPtrClass;
PtrClassOwner(PtrClass ptr) : shPtrClass(ptr.release()) {}
};
PtrClassOwner createClassWrapper() { return PtrClassOwner{createClass()}; }
std::string getString(PtrClassOwner const& ptr) {
return *(ptr.shPtrClass);
}
RCPP_EXPOSED_CLASS(PtrClassOwner);
RCPP_MODULE(Class){
using namespace Rcpp;
class_<PtrClassOwner>("PtrClass");
function("createClass", &createClassWrapper, "createClass method");
function("getString", &getString, "getString method");
}