Passing a vector or matrix from R to a C function using Rcpp without a local copy? - rcpp

Question

Here's an illustration of what I am currently doing with two functions.
In both cases I am creating a local copies: either an instance of std::vector<GLfloat> or of std::vector<GLdouble>.
Isn't there a shortcut not involving copies, where I could go from Rcpp::NumericMatrix to GLfloat */GLdouble * in a more direct way?
#include <Rcpp.h>
#include <GLFW/glfw3.h>
using namespace Rcpp;
//' #export
// [[Rcpp::export("glLoadMatrixf")]]
void gl_load_matrixf(Rcpp::NumericMatrix m) {
std::vector<GLfloat> float_v = Rcpp::as<std::vector<GLfloat> >(m);
const GLfloat * _float_v = &float_v[0];
glLoadMatrixf(_float_v);
}
//' #export
// [[Rcpp::export("glLoadMatrixd")]]
void gl_load_matrixd(Rcpp::NumericMatrix m) {
std::vector<GLdouble> double_v = Rcpp::as<std::vector<GLdouble> >(m);
const GLdouble * _double_v = &double_v[0];
glLoadMatrixd(_double_v);
}

Answer 1

On my system GLfloat and GLdouble are defined as:
typedef float GLfloat;
typedef double GLdouble;
And R's numeric data type is always a double. So you can use &m[0] or m.begin() directly to get something convertible to a GLdouble * without the need to copy the data. For GLfloat this is not possible, since it requires a (lossy) cast to go from the (64 bit) double used by R to a (32 bit) float.
Some code as illustration:
Sys.setenv(PKG_LIBS="-lGL -lglfw")
Rcpp::sourceCpp(code = '
#include <Rcpp.h>
#include <GLFW/glfw3.h>
using namespace Rcpp;
// [[Rcpp::export("glLoadMatrixd")]]
void gl_load_matrixd(Rcpp::NumericMatrix m) {
const GLdouble * _double_v = &m[0];
glLoadMatrixd(_double_v);
}
')
glLoadMatrixd(matrix(runif(10), 2, 5))
BTW, I have no idea what dimensions such a matrix should have. 2x5 is probably incorrect ...