diff --git a/src/Datafile.cpp b/src/Datafile.cpp
index f4d769c0..22fe00ef 100644
--- a/src/Datafile.cpp
+++ b/src/Datafile.cpp
@@ -77,7 +77,7 @@ std::string Datafile::lastLine ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-std::vector <std::string> Datafile::allLines ()
+const std::vector <std::string>& Datafile::allLines ()
 {
   if (! _lines_loaded)
     load_lines ();
diff --git a/src/Datafile.h b/src/Datafile.h
index 1bf638c4..f73c1d81 100644
--- a/src/Datafile.h
+++ b/src/Datafile.h
@@ -41,7 +41,7 @@ public:
   std::string name () const;
 
   std::string lastLine ();
-  std::vector <std::string> allLines ();
+  const std::vector <std::string>& allLines ();
 
   void addInterval (const Interval&);
   void deleteInterval (const Interval&);