allow users to pass a custom environment to Rcpp::sourceCpp(); fixes h…

…ttp://stackoverflow.com/q/14882486/559676

NEWS.md

@@ -40,6 +40,8 @@
 
 - the function `imgur_upload()` uses Imgur API version 3 now; if you are using the key obtained from version 2, you need to register for your own client id: http://api.imgur.com (#439)
 
+- allow users to pass a custom environment to `Rcpp::sourceCpp()` in the `Rcpp` engine; fixes http://stackoverflow.com/q/14882486/559676
+
 - slight improvement of encoding support in `knit()`
 
 # CHANGES IN knitr VERSION 1.0

R/engine.R

@@ -66,12 +66,11 @@ eng_Rcpp = function(options) {
   code = str_c(options$code, collapse = '\n')
   # engine.opts is a list of arguments to be passed to Rcpp function, e.g.
   # engine.opts=list(plugin='RcppArmadillo')
+  opts = options$engine.opts
+  if (!is.environment(opts$env)) opts$env = knit_global() # default env is knit_global()
   if (options$eval) {
     message('Building shared library for Rcpp code chunk...')
-    do.call(
-      Rcpp::sourceCpp,
-      c(list(code = code, env = knit_global()), options$engine.opts)
-    )
+    do.call(Rcpp::sourceCpp, c(list(code = code), opts))
   }
 
   options$engine = 'cpp' # wrap up source code in cpp syntax instead of Rcpp