adds empty() to [const_]string_proxy for more efficient checks for empty string

Author: Jonathan Marshall

inst/include/Rcpp/vector/const_string_proxy.h

@@ -93,6 +93,7 @@ namespace internal{
 		inline iterator begin() const { return CHAR( STRING_ELT( *parent, index ) ) ; }
 		inline iterator end() const { return begin() + size() ; }
 		inline int size() const { return strlen( begin() ) ; }
+		inline bool empty() const { return *begin() == '\0' ; }
 		inline reference operator[]( int n ){ return *( begin() + n ) ; }
 
 		bool operator==( const char* other){

inst/include/Rcpp/vector/string_proxy.h

@@ -158,6 +158,7 @@ namespace internal{
 		inline iterator begin() const { return CHAR( STRING_ELT( *parent, index ) ) ; }
 		inline iterator end() const { return begin() + size() ; }
 		inline int size() const { return strlen( begin() ) ; }
+		inline bool empty() const { return *begin() == '\0' ; }
 		inline reference operator[]( int n ){ return *( begin() + n ) ; }
 
 		template <typename UnaryOperator>