nic_router: overwritable reference wrapper

While references are const and always valid, this wrapper is like a reference
that is only always valid and can be overwritten.

Ref #2670

repos/os/src/server/nic_router/reference.h

@@ -0,0 +1,53 @@
+/*
+ * \brief  Reference that can be overwritten
+ * \author Martin Stein
+ * \date   2016-08-24
+ */
+
+/*
+ * Copyright (C) 2016-2017 Genode Labs GmbH
+ *
+ * This file is part of the Genode OS framework, which is distributed
+ * under the terms of the GNU Affero General Public License version 3.
+ */
+
+#ifndef _REFERENCE_H_
+#define _REFERENCE_H_
+
+namespace Net {
+
+	template <typename> class Reference;
+	template <typename> class Const_reference;
+}
+
+
+template <typename T>
+class Net::Reference
+{
+	private:
+
+		T *_obj;
+
+	public:
+
+		Reference(T &obj) : _obj(&obj) { }
+
+		T &operator () () const { return *_obj; }
+};
+
+
+template <typename T>
+class Net::Const_reference
+{
+	private:
+
+		T const *_obj;
+
+	public:
+
+		Const_reference(T const &obj) : _obj(&obj) { }
+
+		T const &operator () () const { return *_obj; }
+};
+
+#endif /* _REFERENCE_H_ */