Rearrange sources

include/xrpl/basics/comparators.h

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+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012, 2013 Ripple Labs Inc.
+
+    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 RIPPLE_BASICS_COMPARATORS_H_INCLUDED
+#define RIPPLE_BASICS_COMPARATORS_H_INCLUDED
+
+#include <functional>
+
+namespace ripple {
+
+#ifdef _MSC_VER
+
+/*
+ * MSVC 2019 version 16.9.0 added [[nodiscard]] to the std comparison
+ * operator() functions. boost::bimap checks that the comparitor is a
+ * BinaryFunction, in part by calling the function and ignoring the value.
+ * These two things don't play well together. These wrapper classes simply
+ * strip [[nodiscard]] from operator() for use in boost::bimap.
+ *
+ * See also:
+ *  https://www.boost.org/doc/libs/1_75_0/libs/bimap/doc/html/boost_bimap/the_tutorial/controlling_collection_types.html
+ */
+
+template <class T = void>
+struct less
+{
+    using result_type = bool;
+
+    constexpr bool
+    operator()(const T& left, const T& right) const
+    {
+        return std::less<T>()(left, right);
+    }
+};
+
+template <class T = void>
+struct equal_to
+{
+    using result_type = bool;
+
+    constexpr bool
+    operator()(const T& left, const T& right) const
+    {
+        return std::equal_to<T>()(left, right);
+    }
+};
+
+#else
+
+template <class T = void>
+using less = std::less<T>;
+
+template <class T = void>
+using equal_to = std::equal_to<T>;
+
+#endif
+
+}  // namespace ripple
+
+#endif