[Rcpp-devel] Extending Rcpp::wrap from the Extending Rcpp vignette

[Romain Francois]

Le 20/06/10 18:27, Douglas Bates a écrit :
>
> I'm confused about where the Rcpp.h file is included when using
> "intrusive" extension of Rcpp::wrap as described in section 2.1 of the
> vignette "Extending Rcpp".
>
> I would like to define the SEXP operator for the C++ classes
> corresponding to the Matrix package S4 classes.  These are declared in
> lme4a/src/MatrixNs.h
>
> It appears to me from the vignette that I need to include RcppCommon.h
> then declare my classes and the operator member SEXP() then include
> Rcpp.h
>
> The problem is that my classes have members that are declared through
> Rcpp.h, so, for example, I need declarations of Rcpp::List,
> Rcpp::NumericVector, etc. to be able to declare my classes.
>
> If it is the case that I must declare my classes and their operator
> SEXP() member before including Rcpp.h, should I use forward
> declarations of the various Rcpp classes that are used in my classes?
>
> I hope the question is clear.  If not, I will provide sample code.
>
> This template stuff is very powerful but very confusing.

Indeed.


This is not something that has been stressed too much so far. I think 
these options might work :

- as you say: forward declare Rcpp classes you intend to use. so 
something in that order:

#include <RcppCommon.h>

// forward declarations of Rcpp classes

// include headers for your classes here

#include <Rcpp.h>


however, perhaps you wish to use classes that are generated by 
templates: NumericVector, etc ...

In that case forward declaring might not work as it might trip up the 
compiler because, e.g NumericVector is just defined by a typedef:

typedef Vector<REALSXP> NumericVector ;

Using Vector<REALSXP> in your headers should work if the Vector template 
is predeclared : I might need to interfere here.



- the other way, which is probably easier, is to forward declare your 
classes and their wrap before you include Rcpp.h

// forward decl
class Foo ;

// needed for the definition of wrap we want to specialize
#include <RcppCommon.h>

namespace Rcpp{
	// forward decl
	template<> SEXP wrap( const& Foo ) ;
}

#include Rcpp.h

// real decl
class Foo {
public:
	Foo( int n ) : vec(n){} ;
	IntegerVector vec ;
} ;

namespace Rcpp{
	template<> SEXP (const& Foo w){
		return w.vec ;
	}
}



In any case, we would like to hear back from your experience, so that we 
can improve the document.

It might be possible to move more things into RcppCommon.h so that Rcpp 
takes care of forward declarations.


Romain

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Romain Francois
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