Hi,
I have some questions regarding extending 'as' and 'wrap' for custom
classes.
In my simple example I have declared a new class which has a single member.
This member is of type NumericVector, i.e. it is an Rcpp class. I have
written simple 'as' and 'wrap' extensions using non-intrusive extension. My
simple example compiles and appears to work as expected.
My question is about the placement of #include <Rcpp.h>. In the
Rcpp-extending vignette, it says that for non-intrusive extension, the
#include <Rcpp.h> must appear after the specialisation otherwise the
specialisation will not be seen by Rcpp types. However, if I do this
compilation fails because the NumericVector member is not recognised by the
compiler in the class definition.
As an experiment, I moved the #include <Rcpp.h> to before the class
definition. My example then compiles and appears to run correctly. I was
surprised because what I did appears to contradict the advice given the
vignette.
So my questions are:
Should I be surprised that this example works even though it appears I have
contradicted the advice in the vignette?
Although my example appears to work, because I have contradicted the advice
in the vignette, is there now something bad lurking in the application.
i.e. things appear fine with my simple example but will something blow up
if I continue to develop my class?
My example cpp code is as follows:
// File: DummyClass_with_Rcpp_member_example.cpp
#include <RcppCommon.h>
// If #include <Rcpp.h> not placed here then compilation fails
#include <Rcpp.h>
class DummyClass {
public:
Rcpp::NumericVector data;
};
namespace Rcpp {
// non-intrusive extension via template specialisation
template <> DummyClass as(SEXP dc_sexp);
// non-intrusive extension via template specialisation
template <> SEXP wrap(const DummyClass &flq);
}
// Compilation fails if #include<Rcpp.h> placed here
//#include <Rcpp.h>
// define template specialisations for as and wrap
namespace Rcpp {
template <> DummyClass as(SEXP dc_sexp) {
S4 dc_s4 = Rcpp::as<S4>(dc_sexp);
DummyClass dc;
dc.data = dc_s4.slot(".Data");
return dc;
}
template <> SEXP wrap(const DummyClass &dc) {
Rcpp::S4 dc_s4("DummyClass");
dc_s4.slot(".Data") = dc.data;
return Rcpp::wrap(dc_s4);
}
}
// [[Rcpp::export]]
DummyClass test_DC_as_wrap(DummyClass dc, double multiplier){
DummyClass new_dc;
new_dc.data = dc.data;
new_dc.data(0) = new_dc.data(0) * multiplier;
return new_dc;
}
The R code that runs the example is:
library(Rcpp)
sourceCpp("DummyClass_with_Rcpp_member_example.cpp")
setClass("DummyClass", representation(.Data = "numeric"))
dc <- new("DummyClass")
dc@.Data <- 1:10
test_DC_as_wrap(dc, 4)
Yours
Finlay
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