[Rcpp-devel] CRTP and Modules

[FHRB Toledo]

Dear All,

I am trying to deploy a way to enable user extensions for a package that
uses Rcpp. In that sense, I have found a vignette from dplyr package
("Hybrid Evaluation") where it is used CRTP.

Thus, I am trying to reproduce e toy example of that to be exposed by Rcpp
Modules. Digging on the matter I found about the package RcppAnnoy which
have partially solved the issue. But, I still have a question: How to have
different implementations one for each Derived classes exposed by the Rcpp
Modules, specifically, how to expose the CRTP interface?

Following you can find What I did up to the moment. I also read an recent
thread in stackoverflow where Dirk mentioned RcppAnnoy but, I can't find
the way to expose the especializations.

Many thanks in advance for your help over that. By the way, I would be glad
with any comment or suggestion to fullfill the requirements for the CTRP.
even out of the scope of the Modules.

[CODE:]


#include <Rcpp.h>

template < class Derived >
class Base {

  public :

    Base< Derived > ( void ) { } ;
    virtual ~Base< Derived > ( void ) { } ;

    // CRTP interface
    virtual double operation ( void )
      { return static_cast< Derived * >( this ) -> implementation ( ) ; }

  private :

} ;

class Multiply : public Base< Multiply > {

  public :

    Multiply ( double, double ) ;

    double implementation ( void ) ;

  private :

    double x, y ;

} ;

class Sum : public Base< Sum > {

  public :

    Sum ( double, double ) ;

    double implementation ( void ) ;

  private :

    double x, y ;

} ;

Multiply::Multiply ( double x_, double y_ ) : x ( x_ ), y ( y_ ) { }

double Multiply::implementation ( void ) { return x * y ; }

Sum::Sum ( double x_, double y_ ) : x ( x_ ), y ( y_ ) { }

double Sum::implementation ( void ) { return x + y ; }

RCPP_EXPOSED_CLASS_NODECL( Base< Multiply > )
RCPP_EXPOSED_CLASS_NODECL( Base< Sum > )
RCPP_EXPOSED_CLASS( Multiply )
RCPP_EXPOSED_CLASS( Sum )

RCPP_MODULE( CRTP ) {


    // needs templated parameter -- doesn't work
    Rcpp::class_< Base >( "Base" )

      // exposing interface specialization member function
      .method( "operation", & Base::operation )

    ;


    // exposing the class
    Rcpp::class_< Multiply >( "Multiply" )

      // explicit inheritance
      // .derives< Base >( "Base" )

      // exposing the default constructor
      .constructor< double, double >( )

      // exposing Base member functions
      // .method( "operation", & Multiply::operation ) // doesn't work

    ;

    // idem above
    Rcpp::class_< Sum >( "Sum" )

      // explicit inheritance
      // .derives< Base >( "Base" )

      .constructor< double, double >( )

      .method( "operation", & Sum::implementation ) // works :) but it
isn't CRTP

    ;

}

Cheers,
FH
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