feat():initial version

install/boost_1_75_0/include/boost/parameter/optional.hpp

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+// Copyright David Abrahams, Daniel Wallin 2003.
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_PARAMETER_OPTIONAL_HPP
+#define BOOST_PARAMETER_OPTIONAL_HPP
+
+#include <boost/parameter/aux_/use_default.hpp>
+
+namespace boost { namespace parameter {
+
+    // This metafunction can be used to describe the treatment of particular
+    // named parameters for the purposes of overload elimination with SFINAE,
+    // by placing specializations in the parameters<...> list.  In order for
+    // a treated function to participate in overload resolution:
+    //
+    //   - The actual argument type matched by every keyword tag
+    //     associated with a predicate must satisfy that predicate
+    //
+    //   - If a keyword k is specified without an optional<...> or
+    //     required<...> wrapper, it is treated as though
+    //     optional<k> were specified.
+    template <
+        typename Tag
+      , typename Predicate = ::boost::parameter::aux::use_default
+    >
+    struct optional
+    {
+        typedef Tag key_type;
+        typedef Predicate predicate;
+    };
+}}
+
+#include <boost/parameter/config.hpp>
+
+#if defined(BOOST_PARAMETER_CAN_USE_MP11)
+#include <boost/mp11/integral.hpp>
+#else
+#include <boost/mpl/bool.hpp>
+#endif
+
+namespace boost { namespace parameter { namespace aux {
+
+    template <typename T>
+    struct is_optional
+#if defined(BOOST_PARAMETER_CAN_USE_MP11)
+      : ::boost::mp11::mp_false
+#else
+      : ::boost::mpl::false_
+#endif
+    {
+    };
+
+    template <typename Tag, typename Predicate>
+    struct is_optional< ::boost::parameter::optional<Tag,Predicate> >
+#if defined(BOOST_PARAMETER_CAN_USE_MP11)
+      : ::boost::mp11::mp_true
+#else
+      : ::boost::mpl::true_
+#endif
+    {
+    };
+}}} // namespace boost::parameter::aux
+
+#endif  // include guard
+