From b086e2d20cf3e5d0241d37e63f6a0f9c79c683c8 Mon Sep 17 00:00:00 2001
From: artemp <artem@mapnik.org>
Date: Tue, 7 May 2013 16:12:57 +0100
Subject: [PATCH] + small optimization - pass and return const ref

---
 include/mapnik/expression_evaluator.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/mapnik/expression_evaluator.hpp b/include/mapnik/expression_evaluator.hpp
index 5da143a1d..6d231aa0e 100644
--- a/include/mapnik/expression_evaluator.hpp
+++ b/include/mapnik/expression_evaluator.hpp
@@ -47,7 +47,7 @@ struct evaluate : boost::static_visitor<T1>
     explicit evaluate(feature_type const& f)
         : feature_(f) {}
 
-    value_type operator() (value_type x) const
+    value_type const& operator() (value_type const& x) const
     {
         return x;
     }