Use =0 for new functions, use default for destructor

include/guisan/listmodel.hpp

@@ -76,7 +76,7 @@ namespace gcn
         /**
          * Destructor.
          */
-        virtual ~ListModel() { }
+        virtual ~ListModel() = default;
 
         /**
          * Gets the number of elements in the list.
@@ -92,6 +92,12 @@ namespace gcn
          * @return An element as a string at the a certain index.
          */
         virtual std::string getElementAt(int i) = 0;
+
+        // Add a new element
+        virtual void add(const std::string& str) = 0;
+
+        // Clear all elements
+        virtual void clear_elements() = 0;
     };
 }