Implement identity_hash (resolves HowardHinnant#4)

identity_hash is useful for wrappers that cache the hash keys of their wrapped objects.

identity_hash.h

@@ -0,0 +1,39 @@
+//      Copyright Rein Halbersma 2016
+//      Distributed under the [Boost Software License, Version 1.0](http://www.boost.org/users/license.html).
+//      (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef IDENTITY_HASH_H
+#define IDENTITY_HASH_H
+
+#include "endian.h"     // endian
+#include <cstddef>      // size_t
+
+// namespace acme is used to demonstrate example code.  It is not proposed.
+
+namespace acme {
+
+class identity_hash
+{
+        std::size_t m_state;
+public:
+        static constexpr xstd::endian endian = xstd::endian::native;
+        using result_type = std::size_t;
+
+        constexpr void operator()(void const* key, std::size_t /* len */) // Throws: Nothing.
+        {
+                // Pre-condition: callers must be of the form: identity_hash{}(&x, sizeof(x)).
+                // In C++1z, we could annotate the len parameter with [[maybe_unused]]
+                // and check the pre-condition in debug mode with assert(len == sizeof(result_type)).
+                // In C++14, this requires disabling unused-parameter warnings.
+                m_state = *static_cast<result_type const*>(key);
+        }
+
+        explicit constexpr operator result_type() const noexcept
+        {
+                return m_state;
+        }
+};
+
+}       // namespace acme
+
+#endif  // IDENTITY_HASH_H