Merging Dan's PR into this feature/issue383 branch

Author: eddelbuettel

ChangeLog

@@ -1,3 +1,8 @@
+2015-11-08  Daniel C. Dillon  <[email protected]>
+
+        * inst/include/Rcpp/Nullable.h: No longer prevent assignment of
+        R_NilValue to Nullable<> in function signatures
+
 2015-11-07  Dirk Eddelbuettel  <[email protected]>
 
         * inst/include/Rcpp/vector/Matrix.h (Rcpp): Beginnings of a Matrix

inst/NEWS.Rd

@@ -17,6 +17,8 @@
       by Qiang, fixing a bug reported by Allessandro on the list)
       \item Matrix classes now have a \code{transpose()} function (fixing
       \ghit{383}) 
+      \item \code{Nullable} types can now be assigned \code{R_NilValue} in
+      function signatures. (PR \ghpr{395} by Dan, fixing issue \ghit{394})
     }
     \item Changes in Rcpp Attributes:
     \itemize{

inst/include/Rcpp/Nullable.h

@@ -31,13 +31,6 @@ namespace Rcpp {
 
     template<class T>
     class Nullable {
-    private:
-        template<class U>
-        friend class InputParameter;
-
-        template<class U>
-        friend class traits::Exporter;
-
     public:
 
         /**
@@ -55,8 +48,6 @@ namespace Rcpp {
 
         inline Nullable(const T &t) : m_sexp(t),  m_set(true) {}
 
-    protected:
-
         /**
          * Standard constructor of a Nullable object
          *