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

[Romain Francois]

Le 18/11/10 22:41, Romain Francois a écrit :
> 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

For convenience, here is the full example :

require( Rcpp )
require( inline )
inc <- '

class Simple {
public:

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

     const double& x(){ return d_x ;}

private:
     double d_x ;
		
} ;

RCPP_MODULE(mod){

	class_<Simple>( "Simple" )

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

}
'

fx <- cxxfunction( , '', includes = inc, plugin = "Rcpp" )
mod <- Module( "mod", getDynLib( fx ) )
Simple <- mod$Simple
r <- new( Simple, 10 )
r$x
r$x <- 20





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Romain Francois
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http://romainfrancois.blog.free.fr
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