Rcpp exported module not exposed

Question

I have an R package called multicool which handles permutations of multisets. Currently, internally, there exists a C++ class, and a call to initMC creates a new object of class Multicool which then can do all the things I need it to do. However, there is no simple way to release the memory allocated to this object. It doesn't matter for simple uses, but I have an application which might call this hundreds of thousands of times.

The solution, I think, is to expose the class to R using an Rcpp Module. However, I have tried following the instructions and I get the error:

Error: object 'mcModule' not found

Initially - I would just like to expose the object and its constructor. This is my class definition

#include <Rcpp.h>

using namespace Rcpp;
using namespace std;

class Multicool{
  struct list_el {
    int v;
    struct list_el * n;
  };
  typedef struct list_el item;

  item *h;
  item *t;
  item *i;

  int *m_pnInitialState; 
  int *m_pnCurrState;
  int m_nLength;
  bool m_bFirst;

  public:
   // constructor
  Multicool(IntegerVector x){
    int nx = (int)x.size();
  }
};

and then I export the class and the constructor with

RCPP_MODULE(mcModule){
  using namespace Rcpp;

  class_<Multicool>("Multicool")

  .constructor<IntegerVector>()
  ;
}

I have added the line

import(Rcpp)

to my NAMESPACE file

I have added

RcppModules: mcModule

to my DESCRIPTION file

and I have added a call to loadRcppModules in the .onLoad function

.onLoad <- function(libname, pkgname) {
  loadRcppModules()
}

All of this compiles and the package builds without complaint. But when I got to create a new Multicool object I get the aforementioned error

> library(multicool)
> Multicool = mcModule$Multicool
Error: object 'mcModule' not found

Any help or advice would be appreciated

Answer 1

You generally need a new call first. See in my RcppRedis package:

RCPP_MODULE(Redis) {
    Rcpp::class_<Redis>("Redis")   

        .constructor("default constructor")  
        [...stuff omitted for brevity...]

    ;
}

where the R code (eg in the demo/ directory) does

suppressMessages(library(RcppRedis))

redis <- new(Redis)

after which you can access functions redis$foo() etc that are part of the module.

So try adding

mcModule <- new(mcModule)

before accessing mcModule$Multicool.

Edit: Looks like you were missing the loadModule("mcModule", TRUE) call.

Edit 2: To be more explicit, I added your file to the (working) testRcppModule from the package and made the one change you made (to DESCRIPTION) and one more to an R file to load the module:

edd@max:/tmp/rcpp/module$ diff -ru ~/git/rcpp/inst/unitTests/testRcppModule/  testRcppModule/ 
diff -ru /home/edd/git/rcpp/inst/unitTests/testRcppModule/DESCRIPTION testRcppModule/DESCRIPTION
--- /home/edd/git/rcpp/inst/unitTests/testRcppModule/DESCRIPTION        2015-08-26 15:53:03.891830292 -0500
+++ testRcppModule/DESCRIPTION  2015-10-22 21:34:23.716959638 -0500
@@ -10,6 +10,6 @@
 LazyLoad: yes
 Depends: methods, Rcpp (>= 0.8.5)
 LinkingTo: Rcpp
-RcppModules: RcppModuleWorld, stdVector, NumEx
+RcppModules: RcppModuleWorld, stdVector, NumEx, mcModule
 Packaged: 2010-09-09 18:42:28 UTC; jmc

diff -ru /home/edd/git/rcpp/inst/unitTests/testRcppModule/R/zzz.R testRcppModule/R/zzz.R
--- /home/edd/git/rcpp/inst/unitTests/testRcppModule/R/zzz.R    2015-08-26 15:53:03.891830292 -0500
+++ testRcppModule/R/zzz.R      2015-10-22 21:41:41.468532838 -0500
@@ -8,4 +8,5 @@
 loadModule("RcppModuleNumEx", TRUE)
 loadModule("RcppModuleWorld", TRUE)
 loadModule("stdVector", TRUE)
+loadModule("mcModule", TRUE)

Only in testRcppModule/src: multicool.cpp
edd@max:/tmp/rcpp/module$ 

With that, all is good:

$ r --package testRcppModule --eval 'm <- new(mcModule); print(m)'
C++ object <0x757d18> of class 'mcModule' <0x1adeab0>
$