Try cleaning up Shield (following from Rcpp11)

Author: kevinushey

inst/include/Rcpp/api/meat/protection.h

@@ -17,27 +17,22 @@
 
 #ifndef Rcpp_protection_meat_H
 #define Rcpp_protection_meat_H
-      
+
 namespace Rcpp{
 
     template <typename T>
     template <typename U>
     Armor<T>::Armor( U x ) : data() {
         init( wrap(x) ) ;
-    }       
-        
+    }
+
     template <typename T>
     template <typename U>
     inline Armor<T>& Armor<T>::operator=( const U& x ){
         REPROTECT(data = wrap(x), index) ;
         return *this ;
     }
-         
-    template <typename T>
-    template <typename U>
-    inline Shield<T>::operator U() const { 
-        return as<U>(t) ; 
-    }
+
 }
 
 #endif

inst/include/Rcpp/protection/Shield.h

@@ -1,4 +1,4 @@
-// Copyright (C) 2013    Romain Francois
+// Copyright (C) 2013 Romain Francois and Kevin Ushey
 //
 // This file is part of Rcpp.
 //
@@ -15,34 +15,34 @@
 // You should have received a copy of the GNU General Public License
 // along with Rcpp.  If not, see <http://www.gnu.org/licenses/>.
 
-#ifndef Rcpp_protection_Shield_H
-#define Rcpp_protection_Shield_H
+#ifndef Rcpp__protection_Shield_h
+#define Rcpp__protection_Shield_h
+
+namespace Rcpp{
+
+    inline SEXP Rcpp_protect(SEXP x){
+        if( x != R_NilValue ) PROTECT(x) ;
+        return x ;
+    }
 
-namespace Rcpp {
-    
     template <typename T>
     class Shield{
     public:
-        Shield( SEXP t_) : t(PROTECT(t_)){}
+        Shield( SEXP t_) : t(Rcpp_protect(t_)){}
         ~Shield(){
-            UNPROTECT(1) ;    
-        }
-        
-        template <typename U>
-        inline operator U() const  ;
-        
-        inline operator SEXP() const { 
-            return t ; 
+            if( t != R_NilValue ) UNPROTECT(1) ;
         }
-        
-    private:  
-        // not defined on purpose
+
+        operator SEXP() const { return t; }
+        SEXP t ;
+
+    private:
         Shield( const Shield& ) ;
         Shield& operator=( const Shield& ) ;
-    
-        SEXP t ;
     } ;
-    
+
+
+
 }
 
 #endif