[Rcpp-devel] Yet another design question regarding Rcpp

[Romain Francois]

Le 04/05/10 22:26, Douglas Bates a écrit :
>
> In the files src/MatrixNs.{h,cpp} of the lme4a package (the
> development version of lme4, available only from R-forge) I create C++
> classes that parallel some of the S4 classes in the Matrix package for
> R.  Most of the time I apply operations in those classes to numeric
> vectors passed as arguments or components of arguments from R.  Thus
> many of the function signatures include arguments of the form
>
> NumericVector const&
>
> If I have a more complex operation to perform in C++ using temporary
> storage I could allocate the storage by declaring
>
>   NumericVector bar(foo.size());
>
> or something like that, even though this vector will never be touched
> by R.  I think this is the simplest approach so that I only need to
> use one set of function signatures in the MatrixNs.h declarations.
>
> Alternatively I could declared multiple function signatures for both
> NumericVector const&  and std::vector<double>  const&  arguments and
> allocate the temporary storage using std::vector<double>  instead of
> NumericVector.
>
> Or I could even declare all the function arguments as
> std::vector<double>  const&  and convert the NumericVector objects to
> that form (although I think that might involve some copying) before
> passing them.

yes

> I am currently favoring using NumericVector to allocate the
> temporaries, even though the storage must be allocated and freed
> through R's mechanisms.
>
> Opinions?

It depends what you do with the data. If you change the size of the 
vector from time to time, add elements, remove elements, I'd go for 
std::vector.

For fixed size, I'd use R(cpp) data structures, which perform just as 
good as std::vector, both really are glorified C arrays. And then you 
have the option of returning the data to R cheaply.

One other way if you really wanted to use std::vector would be to wrap 
them in external pointers. but that's a bit more work.

-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
|- http://bit.ly/9aKDM9 : embed images in Rd documents
|- http://tr.im/OIXN : raster images and RImageJ
|- http://tr.im/OcQe : Rcpp 0.7.7