[Rcpp-devel] Rcpp modules and S3 dispatch on rbind/cbind

[Louis Aslett]

I've encountered a problem when trying to perform S3 method dispatch
for rbind() with an Rcpp module I've written.  Obviously Rcpp modules
are S4, but as per many Google-able discussions, rbind/cbind can't
support S4 method dispatch due to the first argument being a
dot-dot-dot one, so S3 on the first argument type is the best that can
be done.  I think this is good enough for my problem as my types can't
be mixed with base types so I've a fairly short list of possible first
argument types and can disentangle in my own code.

I won't bog the list down with my real problem which is hundreds of
lines of code ... I've managed to narrow it down to the following
minimal working example which produces the same issue (description to
follow below).

== Start content of test.cpp ==
#include <Rcpp.h>
using namespace Rcpp;

class TestClass1 {
  public:
    TestClass1() {}
    void hello() const {
      Rcout << "Hello world!" << std::endl;
    }
};

class TestClass2 {
  public:
    TestClass2() {}
    void hello() const {
      Rcout << "Hello world!" << std::endl;
    }
};

RCPP_MODULE(test_mod) {
  class_<TestClass1>( "TestClass1" )
  .constructor()
  .method("hello", &TestClass1::hello)
  ;

  class_<TestClass2>( "TestClass2" )
  .constructor()
  .method("hello", &TestClass2::hello)
  ;
}
== End content of test.cpp==

== Start example R script ==
Rcpp::sourceCpp('test.cpp')

# Define S3 method for the Rcpp class types
rbind.Rcpp_TestClass1 <- function(...) {
  args <- list(...)
  cat("Calling type 1 hello\n")
  args[[1]]$hello()
}
rbind.Rcpp_TestClass2 <- function(...) {
  args <- list(...)
  cat("Calling type 2 hello\n")
  args[[1]]$hello()
}

a <- new(TestClass1)
b <- new(TestClass2)

# OK, this goes to plan ... I can has an Rcpp module, many of same
# type or mix with R types
rbind(a)
rbind(a,a)
rbind(a,2,matrix(1:4,2))
rbind(b)
rbind(b,b)
rbind(b,2,matrix(1:4,2))

# ... but I can't mix two Rcpp module types
rbind(a,b)
rbind(b,a)

# Get rid of one of the S3 methods and it works to mix Rcpp module types
rm(rbind.Rcpp_TestClass2)
rbind(a,b)
== End example R script ==

The output for the problem part is:

> # ... but I can't mix two Rcpp module types
> rbind(a,b)
Error in rbind(a, b) : environments cannot be coerced to other types
> rbind(b,a)
Error in rbind(b, a) : environments cannot be coerced to other types

All other lines in the script behave as expected.

So in a nut shell the problem seems to be that one can't have first
argument S3 method dispatch defined for more than one Rcpp module type
at a time.  I am mystified why this should be?  Any insights greatly
appreciated!

All the best,

Louis


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