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

[Louis Aslett]

Sorry hit send prematurely ....

Just for the list record I think I've figured this out.  It turns out
that there is an S4 approach to this now, whereby one actually sets
appropriate methods for the rbind2 and cbind2 functions in the methods
package and then call:

methods:::bind_activation(on = TRUE)

This then recursively calls rbind2 and cbind2 with pairs of arguments
whenever the base rbind/cbind are called with S4 arguments matching
the signature.  Of course, it means all the other functions such as
nrow/ncol/rownames/... must be overridden too for the classes used, in
order that the knitting together that the methods package does will
work correctly.  See the rbind.R source file in the methods package
for details.

Hope that helps if anyone encounters this in future,

Louis


On 12 January 2015 at 13:44, Louis Aslett <aslett at stats.ox.ac.uk> wrote:
> 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
>
>
> PS In case this arrives twice in error I resent from my subscribed address.