Rcpp
interoperability with std::array (et al.)Rcpp is a mature and
very widely used package providing seamless interoperability between C++
and the R language, which has a
native C API. This package facilitates conversion between R data
structures and the templated std::array
class introduced in C++11, by using the interface provided in
Rcpp. It originated with this
Stack Overflow question.
Support for std::tuple
was added in package version 0.3.0. This data structure, also introduced
in C++11, combines a fixed set of elements of potentially different
types.
std::array is a templated container type with a fixed
number of elements, an object-orientated analogue of a C-style array
type like int[3]. Client packages can interface this type
with R if they add RcppArray to LinkingTo and
include the header. (A simple example
package is provided.)
// No need to include "Rcpp.h" as well
#include "RcppArray.h"
// [[Rcpp::export]]
RObject test() {
Rcpp::NumericVector vec = Rcpp::NumericVector::create(1,2,3);
std::array<double,3> arr = Rcpp::as<std::array<double,3>>(vec);
// Do something with the array
std::array<unsigned int,3> result = { 1, 2, 3 };
return result; // Implicitly Rcpp::wrap(result)
}In either direction the element type of the array can be any atomic
type that Rcpp knows how to convert, and isn’t limited to
the types that R uses internally. In the example above,
unsigned int is used even though it doesn’t directly
correspond to an R vector mode: the wrap() function will
convert it to an R numeric vector, i.e.,
double.
A tuple, declared in C++ via a variadic template, as in
std::tuple<int, double, std::string>, can be
converted similarly using Rcpp::as() and
Rcpp::wrap(). In this case the R analogue is a list rather
than an atomic vector, since the element types can vary.
There is also experimental support for std::span,
introduced in C++20. This is a typed container of fixed or dynamic
length which provides a “view” onto a contiguous block of data owned by
another object. Once again, there is a simple example
client package showing usage of this facility.
In this case there are some notable caveats. Client packages must
obviously request C++20 support, and a
configure script may well be needed to check for
span availability. Moreover, since a span does
not own the memory it points to,
as<span<T,D>>() will only compile where the
requested type T matches a type that R uses internally
(viz. int, double, etc.). The latter
limitation does not apply to wrap, however, because a new
vector is created and the data converted.