Importing null (no-op) property map from SOC/2007.

[SVN r52510]

include/boost/graph/property_maps/null_property_map.hpp

@@ -0,0 +1,38 @@
+// (C) Copyright Andrew Sutton 2007
+//
+// Use, modification and distribution are subject to the
+// Boost Software License, Version 1.0 (See accompanying file
+// LICENSE_1_0.txt or http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_GRAPH_NULL_PROPERTY_HPP
+#define BOOST_GRAPH_NULL_PROPERTY_HPP
+
+#include <boost/property_map.hpp>
+
+namespace boost
+{
+    // A null property is somewhat like the inverse of the constant
+    // property map except that instead of returning a single value,
+    // this eats any writes and cannot be read from.
+
+    template <typename Key, typename Value>
+    struct null_property_map
+    {
+        typedef Key key_type;
+        typedef Value value_type;
+        typedef void reference;
+        typedef boost::writable_property_map_tag category;
+    };
+
+    // The null_property_map<K,V> only has a put() function.
+    template <typename K, typename V>
+    void put(null_property_map<K,V>& pm, const K& key, const V& value)
+    { }
+
+    // A helper function for intantiating null property maps.
+    template <typename Key, typename Value>
+    inline null_property_map<Key, Value> make_null_property()
+    { return null_property_map<Key, Value>(); }
+}
+
+#endif