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+// Copyright Daniel Wallin, David Abrahams 2005. Use, modification and
+// distribution is subject to 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_TAGGED_ARGUMENT_050328_HPP
+# define BOOST_PARAMETER_TAGGED_ARGUMENT_050328_HPP
+
+# include <boost/parameter/aux_/void.hpp>
+# include <boost/parameter/aux_/arg_list.hpp>
+# include <boost/parameter/aux_/result_of0.hpp>
+# include <boost/mpl/if.hpp>
+# include <boost/mpl/apply_wrap.hpp>
+# include <boost/mpl/and.hpp>
+# include <boost/mpl/not.hpp>
+# include <boost/type_traits/is_same.hpp>
+# include <boost/type_traits/is_convertible.hpp>
+# include <boost/type_traits/is_reference.hpp>
+
+namespace boost { namespace parameter { namespace aux {
+
+struct empty_arg_list;
+struct arg_list_tag;
+
+struct tagged_argument_base {};
+
+// Holds a reference to an argument of type Arg associated with
+// keyword Keyword
+    
+template <class Keyword, class Arg>
+struct tagged_argument : tagged_argument_base
+{
+    typedef Keyword key_type;
+    typedef Arg value_type;
+    typedef Arg& reference;
+
+    tagged_argument(reference x) : value(x) {}
+
+    // A metafunction class that, given a keyword and a default
+    // type, returns the appropriate result type for a keyword
+    // lookup given that default
+    struct binding
+    {
+        template <class KW, class Default, class Reference>
+        struct apply
+        {
+          typedef typename mpl::eval_if<
+                boost::is_same<KW, key_type>
+              , mpl::if_<Reference, reference, value_type>
+              , mpl::identity<Default>
+          >::type type;
+        };
+    };
+
+    // Comma operator to compose argument list without using parameters<>.
+    // Useful for argument lists with undetermined length.
+    template <class Keyword2, class Arg2>
+    arg_list<
+        tagged_argument<Keyword, Arg>
+      , arg_list<tagged_argument<Keyword2, Arg2> > 
+    >
+    operator,(tagged_argument<Keyword2, Arg2> x) const
+    {
+        return arg_list<
+            tagged_argument<Keyword, Arg>
+          , arg_list<tagged_argument<Keyword2, Arg2> > 
+        >(
+            *this
+          , arg_list<tagged_argument<Keyword2, Arg2> >(x, empty_arg_list())
+        );
+    }
+
+    reference operator[](keyword<Keyword> const&) const
+    {
+        return value;
+    }
+
+# if defined(BOOST_NO_FUNCTION_TEMPLATE_ORDERING) || BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x564))
+    template <class KW, class Default>
+    Default& get_with_default(default_<KW,Default> const& x, int) const
+    {
+        return x.value;
+    }
+
+    template <class Default>
+    reference get_with_default(default_<key_type,Default> const&, long) const
+    {
+        return value;
+    }
+
+    template <class KW, class Default>
+    typename mpl::apply_wrap3<binding, KW, Default&, mpl::true_>::type
+    operator[](default_<KW,Default> const& x) const
+    {
+        return get_with_default(x, 0L);
+    }
+
+    template <class KW, class F>
+    typename result_of0<F>::type 
+    get_with_lazy_default(lazy_default<KW,F> const& x, int) const
+    {
+        return x.compute_default();
+    }
+
+    template <class F>
+    reference get_with_lazy_default(lazy_default<key_type,F> const&, long) const
+    {
+        return value;
+    }
+
+    template <class KW, class F>
+    typename mpl::apply_wrap3<
+        binding,KW
+      , typename result_of0<F>::type
+      , mpl::true_
+    >::type
+    operator[](lazy_default<KW,F> const& x) const
+    {
+        return get_with_lazy_default(x, 0L);
+    }
+# else
+    template <class Default>
+    reference operator[](default_<key_type,Default> const& ) const
+    {
+        return value;
+    }
+
+    template <class F>
+    reference operator[](lazy_default<key_type,F> const& ) const
+    {
+        return value;
+    }
+
+    template <class KW, class Default>
+    Default& operator[](default_<KW,Default> const& x) const
+    {
+        return x.value;
+    }
+
+    template <class KW, class F>
+    typename result_of0<F>::type operator[](lazy_default<KW,F> const& x) const
+    {
+        return x.compute_default();
+    }
+
+    template <class ParameterRequirements>
+    static typename ParameterRequirements::has_default
+    satisfies(ParameterRequirements*);
+
+    template <class HasDefault, class Predicate>
+    static typename mpl::apply1<Predicate, value_type>::type
+    satisfies(
+        parameter_requirements<key_type,Predicate,HasDefault>*
+    );
+# endif
+
+    reference value;
+# if BOOST_WORKAROUND(BOOST_MSVC, BOOST_TESTED_AT(1310))
+    // warning suppression
+ private:
+    void operator=(tagged_argument const&);
+ public:    
+# endif
+    // MPL sequence support
+    typedef tagged_argument type;            // Convenience for users
+    typedef empty_arg_list tail_type;        // For the benefit of iterators
+    typedef arg_list_tag tag; // For dispatching to sequence intrinsics
+};
+
+// Defines a metafunction, is_tagged_argument, that identifies
+// tagged_argument specializations and their derived classes.
+template <class T>
+struct is_tagged_argument_aux
+  : is_convertible<T*,tagged_argument_base const*>
+{};
+
+template <class T>
+struct is_tagged_argument
+  : mpl::and_<
+        mpl::not_<is_reference<T> >
+      , is_tagged_argument_aux<T>
+    >
+{};
+
+}}} // namespace boost::parameter::aux
+
+#endif // BOOST_PARAMETER_TAGGED_ARGUMENT_050328_HPP
+