Hello,
This question has come up a few times now. How can I use another
constructor than the default constructor in a class exposed by Rcpp
modules.
I've made a few changes to Rcpp to allow that. The new restriction is
that :
- we can only expose one constructor (we need to think about various
ways to implement dispatch)
- the constructor needs to have 0, 1 or 2 arguments. We will handle more
later, but I wanted people to test this first before launching a massive
copy/paste adventure. Also, the pattern is simple enough so that other
people can do it instead of me. (I can explain it a bit more if people
volunteer).
As an illustration, consider this simple class :
class Randomizer {
public:
Randomizer( double min_, double max_) : min(min), max(max_){}
NumericVector get( int n ){
RNGScope scope ;
return runif( n, min, max );
}
private:
double min, max ;
} ;
I can explain what it does, but I feel this is self explanatory.
What is relevant here is the constructor. There is no default
constructor (although the compiler might generate one on our behalf),
and we want to use the constructor that takes two doubles.
This is how we would do it (the syntax might change before Rcpp 0.8.9 is
out):
RCPP_MODULE(mod){
class_< Randomizer, init_2<double,double> >( "Randomizer" )
.method( "get" , &Randomizer::get ) ;
}
The relevant bit here is the second template argument of class_.
Full example below:
require( Rcpp )
require( inline )
inc <- '
class Randomizer {
public:
Randomizer( double min_, double max_) : min(min), max(max_){}
NumericVector get( int n ){
RNGScope scope ;
return runif( n, min, max );
}
private:
double min, max ;
} ;
RCPP_MODULE(mod){
class_< Randomizer, init_2<double,double> >( "Randomizer" )
.method( "get" , &Randomizer::get ) ;
}
'
fx <- cxxfunction( , '', includes = inc, plugin = "Rcpp" )
mod <- Module( "mod", getDynLib( fx ) )
Randomizer <- mod$Randomizer
r <- new( Randomizer, 10.0, 20.0 ) ;
r$get(10)
r$get(5)
Have fun !
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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