From 07e5475b6b47b173e101213fc9bc39c910c2def0 Mon Sep 17 00:00:00 2001
From: dellaert <dellaert@cc.gatech.edu>
Date: Tue, 25 Nov 2014 11:02:54 +0100
Subject: [PATCH] Making friends...

---
 gtsam_unstable/nonlinear/Expression.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gtsam_unstable/nonlinear/Expression.h b/gtsam_unstable/nonlinear/Expression.h
index 3b5026348..40b55c410 100644
--- a/gtsam_unstable/nonlinear/Expression.h
+++ b/gtsam_unstable/nonlinear/Expression.h
@@ -27,8 +27,13 @@
 #include <boost/range/adaptor/map.hpp>
 #include <boost/range/algorithm.hpp>
 
+class ExpressionFactorShallowTest;
+
 namespace gtsam {
 
+// Forward declare
+template<typename T> class ExpressionFactor;
+
 /**
  * Expression class that supports automatic differentiation
  */
@@ -204,6 +209,10 @@ private:
     return value;
   }
 
+  // be very selective on who can access these private methods:
+  friend class ExpressionFactor<T>;
+  friend class ::ExpressionFactorShallowTest;
+
 };
 
 // http://stackoverflow.com/questions/16260445/boost-bind-to-operator