diff --git a/MovePlugin.hh b/MovePlugin.hh
index 3bb7ee4c03ef6274d7cf0713d402c5295c6a72b4..39030b5be60cc7815c69215fc6f8f2c024da1723 100644
--- a/MovePlugin.hh
+++ b/MovePlugin.hh
@@ -102,10 +102,10 @@ public:
     void nodeVisibilityChanged(int);
 
     // PickingInterface
-    void addPickMode( const std::string _mode );
-    void addHiddenPickMode( const std::string _mode );
-    void setPickModeMouseTracking (const std::string _mode, bool _mouseTracking);
-    void setPickModeToolbar (const std::string _mode, QToolBar * _toolbar);
+    void addPickMode( const std::string& _mode );
+    void addHiddenPickMode( const std::string& _mode );
+    void setPickModeMouseTracking (const std::string& _mode, bool _mouseTracking);
+    void setPickModeToolbar (const std::string& _mode, QToolBar * _toolbar);
 
     // ContextMenuInterface
     void addContextMenuItem(QAction* _action , ContextMenuType _type);