[Rcpp-devel] Question about custom as and wrap functions

Thanks for the reply. I followed your suggestion and changed the name of
the header file in /inst/include to 'asWrapExample.h' (the name of the
package). I then changed the #include in the 'DummyClass_example.cpp' file
to reflect that change. I also added:

PKG_CXXFLAGS=-I../inst/include

To the top of the Makevars file. However, I still get the same error. I
have the feeling I'm missing something obvious. Thanks for the help so far.

Yours

Finlay



On Wed, Apr 24, 2013 at 5:41 PM, JJ Allaire <jj.allaire at gmail.com> wrote:

Hi Finlay,

If you name your include file the same name as your package then it will
be included automatically in RcppExports.cpp. The convention at work here
is that any custom as/wrap handlers should be accumulated (or referenced
from) that single file. This mirrors the existing convention used by

Rcpp,

RcppArmadillo, RcppEigen, etc. to have a single global header file for
their C++ API.

J.J.


On Wed, Apr 24, 2013 at 8:22 AM, Finlay Scott <drfinlayscott at gmail.com
wrote:

Hi
First of all I want to say how impressed I am with Rcpp. I think it is
going to be very useful for some of the packages I am developing. Thank

you

very much for developing it.

I have a question regarding writing custom as and wrap functions for my
own classes. Following the example in:

http://gallery.rcpp.org/articles/custom-as-and-wrap-example/

I can get my own minimal example to work with a very simple class, and
using the sourceCpp() function.

The cpp code saved as a *.cpp file:

    #include <RcppCommon.h>

    class DummyClass {
        public:
            double value;
    };

    namespace Rcpp {
        // non-intrusive extension via template specialisation
        template <> DummyClass as(SEXP dt);
        // non-intrusive extension via template specialisation
        template <> SEXP wrap(const DummyClass &d);
    }

    #include <Rcpp.h>

    // define template specialisations for as and wrap
    namespace Rcpp {
        template <> DummyClass as(SEXP dtsexp) {
        S4 dc_s4 = Rcpp::as<S4>(dtsexp);
        DummyClass dc;
        dc.value = dc_s4.slot("value");
        return dc;
        }

        template <> SEXP wrap(const DummyClass &d) {
        Rcpp::S4 dc_s4("DummyClass");
        dc_s4.slot("value") = d.value;
        return Rcpp::wrap(dc_s4);
        }
    }

    // [[Rcpp::export]]
    DummyClass test_as_wrap(DummyClass dc, double multiplier){
        DummyClass dc_out;
        dc_out.value = dc.value * multiplier;
        return dc_out;
    }


And the following R code compiles and calls the function:

    library(Rcpp)
    sourceCpp("DummyClass_example.cpp")
    setClass("DummyClass", representation(value = "numeric"))
    dc <- new("DummyClass")
    dc at value <- 23
    test_as_wrap(dc, 4)

This works just fine (like magic!) and the test_as_wrap() function is
happily called from R and returns an object of type DummyClass. I want

to

use a similar approach in a package, so I made a minimal package using:

Rcpp.package.skeleton("asWrapExample",attributes=TRUE)

I then split my original cpp file above into header and source code
files. In the /inst/include directory I placed a file
'DummyClass_example.h' which has:

    #include <RcppCommon.h>

    class DummyClass {
        public:
            double value;
    };

    namespace Rcpp {
        // non-intrusive extension via template specialisation
        template <> DummyClass as(SEXP dt);
        // non-intrusive extension via template specialisation
        template <> SEXP wrap(const DummyClass &d);
    }

In the /src directory I placed a file 'DummyClass_example.cpp' which

has:

    #include "../inst/include/DummyClass_example.h"
    #include <Rcpp.h>

    // define template specialisations for as and wrap
    namespace Rcpp {
        template <> DummyClass as(SEXP dtsexp) {
        S4 dc_s4 = Rcpp::as<S4>(dtsexp);
        DummyClass dc;
        dc.value = dc_s4.slot("value");
        return dc;
        }

        template <> SEXP wrap(const DummyClass &d) {
        Rcpp::S4 dc_s4("DummyClass");
        dc_s4.slot("value") = d.value;
        return Rcpp::wrap(dc_s4);
        }
    }

    // [[Rcpp::export]]
    DummyClass test_as_wrap(DummyClass dc, double multiplier){
        DummyClass dc_out;
        dc_out.value = dc.value * multiplier;
        return dc_out;
    }

When I try to compile the package I get this error message:

    RcppExports.cpp:9:1: error: 'DummyClass' does not name a type

This is probably caused by the RcppExports.cpp not having an #include

for

my DummyClass_example.h.
I understand the RcppExports.cpp file is automatically generated by the
magic of Rcpp so there is no point in adding it there by hand.
I've looked at the documentation but it is not clear to me how I can

tell

RcppExports to also include my header file (if this is the source of the
problem).
Have I missed something in the documentation, or is there an example I
can follow?
Any help is appreciated.

Yours

Finlay