Rearrange sources (#4997)

include/xrpl/beast/utility/maybe_const.h

@@ -0,0 +1,44 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of Beast: https://github.com/vinniefalco/Beast
+    Copyright 2013, Vinnie Falco <vinnie.falco@gmail.com>
+
+    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 THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR 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 BEAST_UTILITY_MAYBE_CONST_H_INCLUDED
+#define BEAST_UTILITY_MAYBE_CONST_H_INCLUDED
+
+#include <type_traits>
+
+namespace beast {
+
+/** Makes T const or non const depending on a bool. */
+template <bool IsConst, class T>
+struct maybe_const
+{
+    explicit maybe_const() = default;
+    using type = typename std::conditional<
+        IsConst,
+        typename std::remove_const<T>::type const,
+        typename std::remove_const<T>::type>::type;
+};
+
+/** Alias for omitting `typename`. */
+template <bool IsConst, class T>
+using maybe_const_t = typename maybe_const<IsConst, T>::type;
+
+}  // namespace beast
+
+#endif