Error exposing c++ class method with reference parameter using Rcpp modules

Question

My goal is to build a dataset class and a model class, and expose both of them to R. The model class has a train() method that takes a reference to a dataset instance, and this seems to be at the root of my issue. Here's what this looks like

//glue.cpp

#include <Rcpp.h>

class MyData
{
public:
  MyData() = default;
};

class MyModel
{
public:
  MyModel() = default;
  void train(const MyData& data) { Rcpp::Rcout << "training model... "; };
};

// Expose MyData
RCPP_MODULE(MyData){
  Rcpp::class_<MyData>("MyData")
  .constructor()
  ;
}

// Expose MyModel
RCPP_MODULE(MyModel){
  Rcpp::class_<MyModel>("MyModel")
  .constructor()
  .method("train", &MyModel::train)
  ;
}

(I should note that this file, glue.cpp, is embedded in an R package.) When I remove this line, .method("train", &MyModel::train), I can compile this without error via pkgbuild::compile_dll(). With it, I get the nasty error below

─  installing *source* package ‘SimpleCppModel’ ...
   ** libs
   clang++  -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include" -I/usr/local/include  -std=c++14 -fPIC  -Wall -g -O2  -c RcppExports.cpp -o RcppExports.o
   clang++  -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG  -I"/Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include" -I/usr/local/include  -std=c++14 -fPIC  -Wall -g -O2  -c glue.cpp -o glue.o
   In file included from glue.cpp:3:
   In file included from /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp.h:27:
   In file included from /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/RcppCommon.h:168:
   In file included from /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/as.h:25:
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/internal/Exporter.h:31:28: error: no matching constructor for initialization of 'MyData'
                       Exporter( SEXP x ) : t(x){}
                                            ^ ~
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/as.h:87:41: note: in instantiation of member function 'Rcpp::traits::Exporter<MyData>::Exporter' requested here
               ::Rcpp::traits::Exporter<T> exporter(x);
                                           ^
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/as.h:152:26: note: in instantiation of function template specialization 'Rcpp::internal::as<MyData>' requested here
           return internal::as<T>(x, typename traits::r_type_traits<T>::r_category());
                            ^
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/InputParameter.h:72:54: note: in instantiation of function template specialization 'Rcpp::as<MyData>' requested here
           ConstReferenceInputParameter(SEXP x_) : obj( as<T>(x_) ){}
                                                        ^
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/module/Module_generated_CppMethod.h:129:58: note: in instantiation of member function 'Rcpp::ConstReferenceInputParameter<MyData>::ConstReferenceInputParameter' requested here
           typename Rcpp::traits::input_parameter<U0>::type x0(args[0]);
                                                            ^
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/module/Module_generated_CppMethod.h:127:5: note: in instantiation of member function 'Rcpp::CppMethod1<MyModel, void, const MyData &>::operator()' requested here
       CppMethod1( Method m) : method_class(), met(m) {} 
       ^
   /Library/Frameworks/R.framework/Versions/3.5/Resources/library/Rcpp/include/Rcpp/module/Module_generated_method.h:59:27: note: in instantiation of member function 'Rcpp::CppMethod1<MyModel, void, const MyData &>::CppMethod1' requested here
       AddMethod( name_, new CppMethod1<Class,RESULT_TYPE,U0>(fun), valid, docstring);
                             ^
   glue.cpp:30:4: note: in instantiation of function template specialization 'Rcpp::class_<MyModel>::method<void, const MyData &>' requested here
     .method("train", &MyModel::train)
      ^
   glue.cpp:5:7: note: candidate constructor (the implicit copy constructor) not viable: cannot convert argument of incomplete type 'SEXP' (aka 'SEXPREC *') to 'const MyData' for 1st argument
   class MyData
         ^
   glue.cpp:5:7: note: candidate constructor (the implicit move constructor) not viable: cannot convert argument of incomplete type 'SEXP' (aka 'SEXPREC *') to 'MyData' for 1st argument
   glue.cpp:8:3: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
     MyData() = default;
     ^
   1 error generated.
   make: *** [glue.o] Error 1
   ERROR: compilation failed for package ‘SimpleCppModel’
─  removing ‘/private/var/folders/dn/9lp6j6j14t1137ftnnk27wyw0000gn/T/RtmpBqDLoF/devtools_install_4d775ed17ea2/SimpleCppModel’
Error in processx::run(bin, args = real_cmdargs, stdout_line_callback = real_callback(stdout),  : 
  System command error

What gives?

Answer 1

As mentioned by Dirk in the comments, you will need as<>and wrap specializations for MyData. In your case, you can use the easiest solution from the "Extending Rcpp" vignette: RCPP_EXPOSED_CLASS. You just have to be careful when you include which header from Rcpp:

#include <Rcpp.h>

class MyData
{
public:
  MyData() = default;
};
RCPP_EXPOSED_CLASS(MyData)

class MyModel
{
public:
  MyModel() = default;
  void train(const MyData& data) { Rcpp::Rcout << "training model... " << std::endl; };
};

// Expose MyData
RCPP_MODULE(MyData){
  Rcpp::class_<MyData>("MyData")
  .constructor()
  ;
}

// Expose MyModel
RCPP_MODULE(MyModel){
  Rcpp::class_<MyModel>("MyModel")
  .constructor()
  .method("train", &MyModel::train)
  ;
}

/***R
myData <- new(MyData)
myModel <- new(MyModel)
myModel$train(myData) 
 */

As you found out in your answer, this even works after including Rcpp.h. It still makes sense to go through the mentioned gallery article as well the vignette, though.