[Rcpp-devel] Exposing constructors with same number of arguments with different types using Rcpp Modules

[Jelmer Ypma]

Dear Rcpp-list,

I'm trying to expose multiple constructors of a C++ class to R using
modules. Here is an example of what I want to do based on the Uniform
example

===Begin: example
library('inline')
library('Rcpp')

test_code <-'
using namespace Rcpp;

class Uniform {
    public:
        Uniform(double min_, double max_) : min(min_), max(max_) {}
        Uniform(double max_, std::string dummy_) : min(0.0), max(max_)
{ Rprintf("%s\\n", dummy_.c_str()); }
        Uniform(double max_ ) : min(0.0), max(max_) {}

        NumericVector draw(int n) const {
            RNGScope scope;
            return runif( n, min, max );
        }

        double min, max;
};

double range( Uniform* w) {
    return w->max - w->min;
}

RCPP_MODULE(unif_module) {
    class_<Uniform>( "Uniform" )

    .constructor<double,double>()
    .constructor<double,std::string>()
    .constructor<double>()

    .field( "min", &Uniform::min )
    .field( "max", &Uniform::max )

    .method( "draw", &Uniform::draw )
    .method( "range", &range )
    ;
}
'

fx <- cxxfunction( signature(), "" , include = test_code, plugin = "Rcpp" )
unif_module <- Module( "unif_module", getDynLib(fx) )

show( Uniform )
u1 <- new( Uniform, 0, 10 )
u1$min
u1$max
u1$range()
u1$draw( 10L )

u2 <- new( Uniform, 10, "test" )
u2$min
u2$max
u2$range()
u2$draw( 10L )

u3 <- new( Uniform, 10 )
u3$min
u3$max
u3$range()
u3$draw( 10L )
===End: example

Compilation works fine (on Windows using RTools, Rcpp_0.9.4.1,
inline_0.3.8), but the R code cannot distinguish between two
constructors with the same number of arguments, but with different
types for the arguments and always calls Uniform(double, double). The
output I get is as follows:

> unif_module <- Module( "unif_module", getDynLib(fx) )

> show( Uniform )
C++ class 'Uniform' <02748CF0>
Constructors:
    Uniform(double, double)
    Uniform(double, std::string)
    Uniform(double)

Fields:
    double max
    double min

Methods:
     Rcpp::NumericVector draw(int)  const

     double range()


> u1 <- new( Uniform, 0, 10 )

> u1$min
[1] 0

> u1$max
[1] 10

> u1$range()
[1] 10

> u1$draw( 10L )
 [1] 6.330045 4.637002 6.507183 4.192280 9.560602 3.927548 4.399107
2.332956 8.810553 3.864929

> u2 <- new( Uniform, 10, "test" )
Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) :
  not compatible with REALSXP


Does anyone know of a workaround for this?

Many thanks in advance!
Jelmer