[Rcpp-devel] Designing Rcpp modules - allow access to fields or to field accessor methods?

Romain Francois romain at r-enthusiasts.com
Thu Nov 18 22:41:22 CET 2010


Le 18/11/10 22:31, Douglas Bates a écrit :
> My C++ coding style (such as it is) has been influenced by reading
> Frank Bokken's "C++ Annotations".  I keep most data members of classes
> private or protected and usually with names like d_x, d_y, d_weights,
> etc. then provide accessor functions named x, y, weights, etc.
> Generally the accessors return const references.
>
> If I export such a class in an Rcpp module I can't allow field access
> but can allow access from R to the accessor methods.  This seems
> slightly awkward in that the user must know to use
>
> myRefObj$x()
>
> instead of
>
> myRefObj$x
>
> to access the x field.  Any strong opinions about fields access versus
> access to field accessor methods?
>
> As an example, one of my classes ends up looking like this
>
>> getRefClass("Rcpp_lmResp")
> Generator object for class "Rcpp_lmResp":
>
> No fields defined
>
>   Class Methods:
>      "callSuper", "export", "finalize", "import", "initFields",
> "initialize", "mu",
> "offset", "sqrtrwt", "sqrtXwt", "updateMu", "updateWts", "wrss", "wtres", "y"
>
>
>   Reference Superclasses:
>      "envRefClass"
>
> but all those methods except updateMu and updateWts are field accessors.
>
> One advantage of this method is that one can't make changes to those
> fields at the R level, which I like because I am paranoid about
> introducing inconsistencies in the fields.

You can use .property instead of .field to short circuit x back to d_x.

Consider this class (which I believe follows the convention you mention):

class Simple {
public:

     Simple( double x ) : d_x(x){} ;

     const double& x(){ return d_x ;}

private:
     double d_x ;
		
} ;

In R, you'd want to be able to do :

r <- new( Simple, 10 )
r$x

but not :

r$x <- 20



Then .property is what you need:

RCPP_MODULE(mod){

	class_<Simple>( "Simple" )

	    .constructor(init_1<double>())
	    .property( "x" , &Simple::x )
	
	;
}


When the third argument is not given, the property is considered read only.

Romain

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Romain Francois
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