[SOLVED] Non-intrusive extension of Rcpp with Rcpp::traits::Exporter

Non-intrusive extension of Rcpp with Rcpp::traits::Exporter

I am writing an Rcpp package with classes containing base Rcpp objects that are non-intrusively exported to C++ types (see Extending Rcpp by Eddelbuettel and Francois, and this helpful vignette by coatless).

What this means is that if I have a class foo::bar and a constructor for foo::bar given a SEXP, I can call a function like this:

//[[Rcpp::export]]
void func(foo::bar object){
   // do something
}

My problem is that sourceCpp("foobar.cpp") works exactly as expected, but R CMD build complains about the RcppExports.cpp file missing declarations for this new class.

Here is a minimum reproducible example, containing a foo::bar class holding a single Rcpp::NumericVector. It compiles with sourceCpp() but the package will not build because foobar is not found in RcppExports.cpp:

foobar.cpp:

#include <RcppCommon.h>

// forward declare class
namespace foo {
    class bar;
}

// forward declare Rcpp::as<> Exporter
template <>
class Rcpp::traits::Exporter<foo::bar>;

#include <Rcpp.h>

// now fully declare class, since Rcpp objects 
//   are now loaded from Rcpp.h
namespace foo {
    class bar {
        public:
        Rcpp::NumericVector x;
        bar(Rcpp::NumericVector x) : x(x) {};
    };
}

// now fully declare Rcpp exporter, since we can
//   deal with Rcpp objects from Rcpp.h
namespace Rcpp {
    namespace traits {
        template <>
        class Exporter<foo::bar> {
            Rcpp::NumericVector x_;

            public:
            Exporter(SEXP x) : x_(x) {}

            foo::bar get() {
                return foo::bar(x_);
            }
        };
    }
}

//[[Rcpp::export]]
Rcpp::NumericVector test(foo::bar& A) {
    return A.x;
}

The specific errors I am getting when running R CMD check are all in the src/RcppExports.cpp file:

'foo' has not been declared
'A' was not declared in this scope
'foo' was not declared in this scope
'template argument 1 is invalid
'qualified-id in declaration before 'A'

What needs to happen to get the package to build just like sourceCpp?

Thanks for any solutions!

Solution

RcppExports.cpp file missing declarations for this new class.

See the Attributes vignette. The custom behavior is to have a header named as the package (i.e. foo.h for package foo) and/or foo-types.h) which will be added to the generated RcppExports.cpp.

That is documented but hidden rather too effectively :-/ It's here too so we could close this as a dupe but I don't have time to search that now.