[Rcpp-devel] using an unloaded package's lazy-load data in Rcpp function

Dear all,

It seems I can reference lazy-loaded data from an unloaded, unattached
package from R, but not using the equivalent Rcpp as described in the
quickref document. I checked out: http://dirk.eddelbuettel.com/
code/rcpp/html/Environment_8h_source.html and even the scant references
to environments in the C API section of "Writing R Extensions" and doesn't
see what I'm missing. I suspect this is a limitation of R's C interface,
not necessarily Rcpp itself.

I use the package nycflights13 as an example. Running `sessionInfo()`
shows the package nycflights13 is not loaded, then I run:
R --vanilla

a <- nycflights13::flights

Which completes without error. In Rcpp, I cannot even get a handle on the
package namespace if it is not loaded.

Rcpp::cppFunction('Rcpp::Environment getUnloaded() { Rcpp::Environment
env("package:nycflights13"); return env;}')
getUnloaded()
Error in getUnloaded() :
  Cannot convert object to an environment: [type=character; target=ENVSXP].

library(nycflights13)
Rcpp::cppFunction('Rcpp::Environment getUnloaded() { Rcpp::Environment
env("package:nycflights13"); return env;}')
getUnloaded()
<environment: package:nycflights13>
attr(,"name")
[1] "package:nycflights13"
attr(,"path")
[1] "/Library/Frameworks/R.framework/Versions/3.4/Resources/library/
nycflights13"

I also found that simple using `loadNamespace` was insufficient for the
Rcpp approach to work, but after `library` or `attachNamespace` the above
function does work.

Am I treading in another rare code path? All I want to do is use a
lazy-loaded data set in an Rcpp function when the package may not be loaded
when called. Should I just be passing it as a parameter to the function?
This seems ugly.

Using Rcpp 0.12.16, R 3.4.3 on Mac.

Any advice appreciated. Thanks,

Jack