'unlockEnvironment' implemented via 'Rcpp' instead of 'inline'

Question

Actual question

Could someone get me started on what I need to do to implement the code of unlockEnvironment below in Rcpp?

Background

Answer 1

It sounds like your question basically amounts to, 'how do I use Rcpp::attributes'? And I would suggest that you go over many of the examples in the Rcpp Gallery to learn a bit more.

The main idea: write some C++ code, put it a .cpp file, then call Rcpp::sourceCpp(<file>) to load it. For this particular example:

#include <Rcpp.h>
using namespace Rcpp;

/* This is taken from envir.c in the R 2.15.1 source
https://github.com/SurajGupta/r-source/blob/master/src/main/envir.c
*/
#define FRAME_LOCK_MASK (1<<14)
#define FRAME_IS_LOCKED(e) (ENVFLAGS(e) & FRAME_LOCK_MASK)
#define UNLOCK_FRAME(e) SET_ENVFLAGS(e, ENVFLAGS(e) & (~ FRAME_LOCK_MASK))

// [[Rcpp::export]]
bool unlock_environment(Environment env) {
  UNLOCK_FRAME(env);
  return FRAME_IS_LOCKED(env) == 0;
}

/*** R
env <- new.env()
lockEnvironment(env)
try(env$a <- 1) ## error
unlock_environment(env)
env$a <- 1
*/

Calling Rcpp::sourceCpp() on a file with these contents gives me:

> Rcpp::sourceCpp('~/scratch/unlock.cpp')

> env <- new.env()

> lockEnvironment(env)

> try(env$a <- 1) ## error
Error in env$a <- 1 : cannot add bindings to a locked environment

> unlock_environment(env)
[1] TRUE

> env$a <- 1 ## success!

The main little features here:

1. You can provide signatures using base / STL C++ types, or Rcpp types as well. Note that bool is the C++ bool type, and Environment is an Rcpp type encompassing environments.
2. Rcpp Attributes handles automatic conversion of these C++ return types to R's SEXP.

You might like Hadley's adv-r introduction as well.