Skip to content

[Rcpp-devel] Yet another design question regarding Rcpp

2 messages · Douglas Bates, Romain Francois

Original
Douglas Bates
# 
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.

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

Opinions?
Romain Francois
# 
Le 04/05/10 22:26, Douglas Bates a ?crit :
yes
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.