[2701] Added the key_from_key.h header file.

src/lib/dhcpsrv/key_from_key.h

@@ -0,0 +1,69 @@
+// Copyright (C) 2013 Internet Systems Consortium, Inc. ("ISC")
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
+// OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#ifndef KEY_FROM_KEY_H
+#define KEY_FROM_KEY_H
+
+#include <functional>
+
+namespace isc {
+namespace dhcp {
+
+/// @brief Extractor class to extract key with another key.
+///
+/// This class solves the problem whereby the multi_index_container
+/// must access the values stored in object nested in other object.
+/// Assume that there is a multi_index_container that holds objects
+/// of the type A. Each object A contains the member object B.
+/// Each object B holds the member of the type C that is to be used
+/// for indexing objects A. Objects C can't be accessed directly
+/// from the object A as they are nested in the object B. In such case
+/// there is no easy way to point in the definition of the container
+/// that C should be used for indexing objects. The KeyFromKey class
+/// represents the cascaded key which can be used to extract the nested
+/// key value C from the object B.
+/// For some more examples of complex keys see:
+/// http://www.cs.brown.edu/~jwicks/boost/libs/multi_index/doc/index.html
+///
+/// @tparam KeyExtractor1 extractor used to access the object used as
+/// a key from the other object that holds it.
+/// @tparam KeyExtractor2 extractor used to access data in the object
+/// holding nested object used as a key.
+template<typename KeyExtractor1, typename KeyExtractor2>
+class KeyFromKey {
+public:
+    typedef typename KeyExtractor1::result_type result_type;
+
+    /// @brief Constructor.
+    KeyFromKey()
+        : key1_(KeyExtractor1()), key2_(KeyExtractor2()) { };
+
+    /// @brief Extract key with another key.
+    ///
+    /// @param arg the key value.
+    ///
+    /// @tparam key value type.
+    template<typename T>
+    result_type operator() (T& arg) const {
+        return (key1_(key2_(arg)));
+    }
+private:
+    KeyExtractor1 key1_; ///< key 1.
+    KeyExtractor2 key2_; ///< key 2.
+};
+
+} // end of isc::dhcp namespace
+} // end of isc namespace
+
+#endif // KEY_FROM_KEY_H