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+/*
+    Copyright (C) 2014 Carl Hetherington <cth@carlh.net>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program; if not, write to the Free Software
+    Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+
+*/
+
+/** @file  src/ref.h
+ *  @brief Ref class.
+ */
+
+#ifndef LIBDCP_REF_H
+#define LIBDCP_REF_H
+
+#include "exceptions.h"
+#include "object.h"
+#include "util.h"
+#include <boost/shared_ptr.hpp>
+#include <string>
+
+namespace dcp {
+
+/** @class Ref
+ *  @brief A reference to an object which is identified by a universally-unique identifier (UUID).
+ *
+ *  This class is a `pointer' to a thing.  It will always know the
+ *  UUID of the thing, and it may have a shared_ptr to the C++ object
+ *  which represents the thing.
+ *
+ *  If the Ref does not have a shared_ptr it may be given one by
+ *  calling resolve() with a list of objects.  The shared_ptr will be
+ *  set up using any object on the list which has a matching ID.
+ */
+template<class T>
+class Ref
+{
+public:
+	/** Initialise a Ref with an ID but no shared_ptr */
+	Ref (std::string id)
+		: _id (id)
+	{}
+
+	/** Initialise a Ref with a shared_ptr to an object */
+	Ref (boost::shared_ptr<T> object)
+		: _id (object->id ())
+		, _object (object)
+	{}
+
+	/** Set the ID of this Ref */
+	void set_id (std::string id)
+	{
+		_id = id;
+	}
+
+	/** Look through a list of objects and copy a shared_ptr to any object
+	 *  which matches the ID of this one.
+	 */
+	void resolve (std::list<boost::shared_ptr<Object> > objects)
+	{
+		typename std::list<boost::shared_ptr<Object> >::iterator i = objects.begin();
+		while (i != objects.end() && !ids_equal ((*i)->id(), _id)) {
+			++i;
+		}
+
+		if (i != objects.end ()) {
+			_object = boost::dynamic_pointer_cast<T> (*i);
+		}
+	}
+
+	/** @return the ID of the thing that we are pointing to */
+	std::string id () const {
+		return _id;
+	}
+
+	/** @return a shared_ptr to the thing; an UnresolvedRefError is thrown
+	 *  if the shared_ptr is not known.
+	 */
+	boost::shared_ptr<T> object () const {
+		if (!_object) {
+			throw UnresolvedRefError (_id);
+		}
+		
+		return _object;
+	}
+
+	/** operator-> to access the shared_ptr; an UnresolvedRefError is thrown
+	 *  if the shared_ptr is not known.
+	 */
+	T * operator->() const {
+		if (!_object) {
+			throw UnresolvedRefError (_id);
+		}
+		
+		return _object.get ();
+	}
+
+	/** @return true if a shared_ptr is known for this Ref */
+	bool resolved () const {
+		return _object;
+	}
+
+private:
+	std::string _id;              ///< ID; will always be known
+	boost::shared_ptr<T> _object; ///< shared_ptr to the thing, may be null.
+};
+
+}
+
+#endif