Rcpp scoping of free variables

Question

Say that I have a C++ function dosomething(z) that has a single input variable z, but also a "free" variable located within it (say y). I want to be able to design the scoping so that when I run the Rcpp function, it finds the free variable within the R function in which it is called.

function(x) {
y = x^2
Rcpp::sourceCpp('C:/Users/xxx/dosomething.cpp')
dosomething(z)
}

So in the above, I want the dosomething(z) function to use the value of y defined within the R function? How do I use Rcpp's Environment capabilities to achieve this? At present I can only seem to get free variables working by finding them in the global environment (not directly within the R function).

Answer 1

Rcpp does not have something to directly deal with this, but you could query the frames with sys.frames from within the C++ function (you just have to know that Rcpp adds 7 frames just to call sys.frames), e.g:

#include <Rcpp.h>
using namespace Rcpp ;

Environment get_calling_env(){
    Function sys_frames( "sys.frames" ) ;
    List frames = sys_frames() ;

    Environment parent = frames.size() == 7 ? R_GlobalEnv : frames[ frames.size() - 8 ] ;
    return parent ;      
}

// [[Rcpp::export]]
CharacterVector foo(){   
    Environment parent = get_calling_env() ;
    return parent.ls(false) ;
}

get_calling_env gives you the environment from where the function was called. So that we can for example get the list of variables from that environment:

> f <- function(){ y <- 2; foo() }
# called from a function that calls foo, we get y
> f()
[1] "y"
# if called from the top, we just get the global env
> foo()
[1] "f"   "foo"