--- a/SConstruct	Fri May 25 12:27:40 2007 +0200
+++ b/SConstruct	Fri May 25 14:13:23 2007 +0200
@@ -61,6 +61,7 @@
 ])
 core.add_inst_headers([
     'system-wall-clock-ms.h',
+    'empty.h',
     'callback.h',
     'ptr.h',
     'object.h',

--- a/src/core/callback.h	Fri May 25 12:27:40 2007 +0200
+++ b/src/core/callback.h	Fri May 25 14:13:23 2007 +0200
@@ -24,6 +24,7 @@
 
 #include "ptr.h"
 #include "fatal-error.h"
+#include "empty.h"
 
 namespace ns3 {
 
@@ -56,8 +57,6 @@
  * and relies on a reference list rather than autoPtr to hold
  * the pointer.
  */
-class empty {};
-
 template <typename T>
 struct CallbackTraits;
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/core/empty.h	Fri May 25 14:13:23 2007 +0200
@@ -0,0 +1,8 @@
+#ifndef EMPTY_H
+#define EMPTY_H
+
+namespace ns3 {
+class empty {};
+}
+
+#endif /* EMPTY_H */