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+//
+// prefer.hpp
+// ~~~~~~~~~~
+//
+// Copyright (c) 2003-2021 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// 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 ASIO_PREFER_HPP
+#define ASIO_PREFER_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include "asio/detail/type_traits.hpp"
+#include "asio/is_applicable_property.hpp"
+#include "asio/traits/prefer_free.hpp"
+#include "asio/traits/prefer_member.hpp"
+#include "asio/traits/require_free.hpp"
+#include "asio/traits/require_member.hpp"
+#include "asio/traits/static_require.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+#if defined(GENERATING_DOCUMENTATION)
+
+namespace asio {
+
+/// A customisation point that attempts to apply a property to an object.
+/**
+ * The name <tt>prefer</tt> denotes a customisation point object. The
+ * expression <tt>asio::prefer(E, P0, Pn...)</tt> for some subexpressions
+ * <tt>E</tt> and <tt>P0</tt>, and where <tt>Pn...</tt> represents <tt>N</tt>
+ * subexpressions (where <tt>N</tt> is 0 or more, and with types <tt>T =
+ * decay_t<decltype(E)></tt> and <tt>Prop0 = decay_t<decltype(P0)></tt>) is
+ * expression-equivalent to:
+ *
+ * @li If <tt>is_applicable_property_v<T, Prop0> && Prop0::is_preferable</tt> is
+ * not a well-formed constant expression with value <tt>true</tt>,
+ * <tt>asio::prefer(E, P0, Pn...)</tt> is ill-formed.
+ *
+ * @li Otherwise, <tt>E</tt> if <tt>N == 0</tt> and the expression
+ * <tt>Prop0::template static_query_v<T> == Prop0::value()</tt> is a
+ * well-formed constant expression with value <tt>true</tt>.
+ *
+ * @li Otherwise, <tt>(E).require(P0)</tt> if <tt>N == 0</tt> and the expression
+ * <tt>(E).require(P0)</tt> is a valid expression.
+ *
+ * @li Otherwise, <tt>require(E, P0)</tt> if <tt>N == 0</tt> and the expression
+ * <tt>require(E, P0)</tt> is a valid expression with overload resolution
+ * performed in a context that does not include the declaration of the
+ * <tt>require</tt> customization point object.
+ *
+ * @li Otherwise, <tt>(E).prefer(P0)</tt> if <tt>N == 0</tt> and the expression
+ * <tt>(E).prefer(P0)</tt> is a valid expression.
+ *
+ * @li Otherwise, <tt>prefer(E, P0)</tt> if <tt>N == 0</tt> and the expression
+ * <tt>prefer(E, P0)</tt> is a valid expression with overload resolution
+ * performed in a context that does not include the declaration of the
+ * <tt>prefer</tt> customization point object.
+ *
+ * @li Otherwise, <tt>E</tt> if <tt>N == 0</tt>.
+ *
+ * @li Otherwise,
+ * <tt>asio::prefer(asio::prefer(E, P0), Pn...)</tt>
+ * if <tt>N > 0</tt> and the expression
+ * <tt>asio::prefer(asio::prefer(E, P0), Pn...)</tt>
+ * is a valid expression.
+ *
+ * @li Otherwise, <tt>asio::prefer(E, P0, Pn...)</tt> is ill-formed.
+ */
+inline constexpr unspecified prefer = unspecified;
+
+/// A type trait that determines whether a @c prefer expression is well-formed.
+/**
+ * Class template @c can_prefer is a trait that is derived from
+ * @c true_type if the expression <tt>asio::prefer(std::declval<T>(),
+ * std::declval<Properties>()...)</tt> is well formed; otherwise @c false_type.
+ */
+template <typename T, typename... Properties>
+struct can_prefer :
+ integral_constant<bool, automatically_determined>
+{
+};
+
+/// A type trait that determines whether a @c prefer expression will not throw.
+/**
+ * Class template @c is_nothrow_prefer is a trait that is derived from
+ * @c true_type if the expression <tt>asio::prefer(std::declval<T>(),
+ * std::declval<Properties>()...)</tt> is @c noexcept; otherwise @c false_type.
+ */
+template <typename T, typename... Properties>
+struct is_nothrow_prefer :
+ integral_constant<bool, automatically_determined>
+{
+};
+
+/// A type trait that determines the result type of a @c prefer expression.
+/**
+ * Class template @c prefer_result is a trait that determines the result
+ * type of the expression <tt>asio::prefer(std::declval<T>(),
+ * std::declval<Properties>()...)</tt>.
+ */
+template <typename T, typename... Properties>
+struct prefer_result
+{
+ /// The result of the @c prefer expression.
+ typedef automatically_determined type;
+};
+
+} // namespace asio
+
+#else // defined(GENERATING_DOCUMENTATION)
+
+namespace asio_prefer_fn {
+
+using asio::conditional;
+using asio::decay;
+using asio::declval;
+using asio::enable_if;
+using asio::is_applicable_property;
+using asio::traits::prefer_free;
+using asio::traits::prefer_member;
+using asio::traits::require_free;
+using asio::traits::require_member;
+using asio::traits::static_require;
+
+void prefer();
+void require();
+
+enum overload_type
+{
+ identity,
+ call_require_member,
+ call_require_free,
+ call_prefer_member,
+ call_prefer_free,
+ two_props,
+ n_props,
+ ill_formed
+};
+
+template <typename Impl, typename T, typename Properties,
+ typename = void, typename = void, typename = void, typename = void,
+ typename = void, typename = void, typename = void>
+struct call_traits
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = ill_formed);
+ ASIO_STATIC_CONSTEXPR(bool, is_noexcept = false);
+ typedef void result_type;
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ static_require<T, Property>::is_valid
+ >::type>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
+ ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);
+
+#if defined(ASIO_HAS_MOVE)
+ typedef ASIO_MOVE_ARG(T) result_type;
+#else // defined(ASIO_HAS_MOVE)
+ typedef ASIO_MOVE_ARG(typename decay<T>::type) result_type;
+#endif // defined(ASIO_HAS_MOVE)
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ !static_require<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ require_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type> :
+ require_member<typename Impl::template proxy<T>::type, Property>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = call_require_member);
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ !static_require<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ require_free<T, Property>::is_valid
+ >::type> :
+ require_free<T, Property>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = call_require_free);
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ !static_require<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_free<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type> :
+ prefer_member<typename Impl::template proxy<T>::type, Property>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = call_prefer_member);
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ !static_require<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_free<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ prefer_free<T, Property>::is_valid
+ >::type> :
+ prefer_free<T, Property>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = call_prefer_free);
+};
+
+template <typename Impl, typename T, typename Property>
+struct call_traits<Impl, T, void(Property),
+ typename enable_if<
+ is_applicable_property<
+ typename decay<T>::type,
+ typename decay<Property>::type
+ >::value
+ >::type,
+ typename enable_if<
+ decay<Property>::type::is_preferable
+ >::type,
+ typename enable_if<
+ !static_require<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !require_free<T, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !prefer_member<typename Impl::template proxy<T>::type, Property>::is_valid
+ >::type,
+ typename enable_if<
+ !prefer_free<T, Property>::is_valid
+ >::type>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = identity);
+ ASIO_STATIC_CONSTEXPR(bool, is_noexcept = true);
+
+#if defined(ASIO_HAS_MOVE)
+ typedef ASIO_MOVE_ARG(T) result_type;
+#else // defined(ASIO_HAS_MOVE)
+ typedef ASIO_MOVE_ARG(typename decay<T>::type) result_type;
+#endif // defined(ASIO_HAS_MOVE)
+};
+
+template <typename Impl, typename T, typename P0, typename P1>
+struct call_traits<Impl, T, void(P0, P1),
+ typename enable_if<
+ call_traits<Impl, T, void(P0)>::overload != ill_formed
+ >::type,
+ typename enable_if<
+ call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1)
+ >::overload != ill_formed
+ >::type>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = two_props);
+
+ ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
+ (
+ call_traits<Impl, T, void(P0)>::is_noexcept
+ &&
+ call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1)
+ >::is_noexcept
+ ));
+
+ typedef typename decay<
+ typename call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1)
+ >::result_type
+ >::type result_type;
+};
+
+template <typename Impl, typename T, typename P0,
+ typename P1, typename ASIO_ELLIPSIS PN>
+struct call_traits<Impl, T, void(P0, P1, PN ASIO_ELLIPSIS),
+ typename enable_if<
+ call_traits<Impl, T, void(P0)>::overload != ill_formed
+ >::type,
+ typename enable_if<
+ call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1, PN ASIO_ELLIPSIS)
+ >::overload != ill_formed
+ >::type>
+{
+ ASIO_STATIC_CONSTEXPR(overload_type, overload = n_props);
+
+ ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
+ (
+ call_traits<Impl, T, void(P0)>::is_noexcept
+ &&
+ call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1, PN ASIO_ELLIPSIS)
+ >::is_noexcept
+ ));
+
+ typedef typename decay<
+ typename call_traits<
+ Impl,
+ typename call_traits<Impl, T, void(P0)>::result_type,
+ void(P1, PN ASIO_ELLIPSIS)
+ >::result_type
+ >::type result_type;
+};
+
+struct impl
+{
+ template <typename T>
+ struct proxy
+ {
+#if defined(ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT) \
+ && defined(ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
+ struct type
+ {
+ template <typename P>
+ auto require(ASIO_MOVE_ARG(P) p)
+ noexcept(
+ noexcept(
+ declval<typename conditional<true, T, P>::type>().require(
+ ASIO_MOVE_CAST(P)(p))
+ )
+ )
+ -> decltype(
+ declval<typename conditional<true, T, P>::type>().require(
+ ASIO_MOVE_CAST(P)(p))
+ );
+
+ template <typename P>
+ auto prefer(ASIO_MOVE_ARG(P) p)
+ noexcept(
+ noexcept(
+ declval<typename conditional<true, T, P>::type>().prefer(
+ ASIO_MOVE_CAST(P)(p))
+ )
+ )
+ -> decltype(
+ declval<typename conditional<true, T, P>::type>().prefer(
+ ASIO_MOVE_CAST(P)(p))
+ );
+ };
+#else // defined(ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
+ // && defined(ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
+ typedef T type;
+#endif // defined(ASIO_HAS_DEDUCED_REQUIRE_MEMBER_TRAIT)
+ // && defined(ASIO_HAS_DEDUCED_PREFER_MEMBER_TRAIT)
+ };
+
+ template <typename T, typename Property>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(Property)>::overload == identity,
+ typename call_traits<impl, T, void(Property)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(Property)) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(Property)>::is_noexcept))
+ {
+ return ASIO_MOVE_CAST(T)(t);
+ }
+
+ template <typename T, typename Property>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(Property)>::overload == call_require_member,
+ typename call_traits<impl, T, void(Property)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(Property) p) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(Property)>::is_noexcept))
+ {
+ return ASIO_MOVE_CAST(T)(t).require(
+ ASIO_MOVE_CAST(Property)(p));
+ }
+
+ template <typename T, typename Property>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(Property)>::overload == call_require_free,
+ typename call_traits<impl, T, void(Property)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(Property) p) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(Property)>::is_noexcept))
+ {
+ return require(
+ ASIO_MOVE_CAST(T)(t),
+ ASIO_MOVE_CAST(Property)(p));
+ }
+
+ template <typename T, typename Property>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(Property)>::overload == call_prefer_member,
+ typename call_traits<impl, T, void(Property)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(Property) p) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(Property)>::is_noexcept))
+ {
+ return ASIO_MOVE_CAST(T)(t).prefer(
+ ASIO_MOVE_CAST(Property)(p));
+ }
+
+ template <typename T, typename Property>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(Property)>::overload == call_prefer_free,
+ typename call_traits<impl, T, void(Property)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(Property) p) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(Property)>::is_noexcept))
+ {
+ return prefer(
+ ASIO_MOVE_CAST(T)(t),
+ ASIO_MOVE_CAST(Property)(p));
+ }
+
+ template <typename T, typename P0, typename P1>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T, void(P0, P1)>::overload == two_props,
+ typename call_traits<impl, T, void(P0, P1)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(P0) p0,
+ ASIO_MOVE_ARG(P1) p1) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(P0, P1)>::is_noexcept))
+ {
+ return (*this)(
+ (*this)(
+ ASIO_MOVE_CAST(T)(t),
+ ASIO_MOVE_CAST(P0)(p0)),
+ ASIO_MOVE_CAST(P1)(p1));
+ }
+
+ template <typename T, typename P0, typename P1,
+ typename ASIO_ELLIPSIS PN>
+ ASIO_NODISCARD ASIO_CONSTEXPR typename enable_if<
+ call_traits<impl, T,
+ void(P0, P1, PN ASIO_ELLIPSIS)>::overload == n_props,
+ typename call_traits<impl, T,
+ void(P0, P1, PN ASIO_ELLIPSIS)>::result_type
+ >::type
+ operator()(
+ ASIO_MOVE_ARG(T) t,
+ ASIO_MOVE_ARG(P0) p0,
+ ASIO_MOVE_ARG(P1) p1,
+ ASIO_MOVE_ARG(PN) ASIO_ELLIPSIS pn) const
+ ASIO_NOEXCEPT_IF((
+ call_traits<impl, T, void(P0, P1, PN ASIO_ELLIPSIS)>::is_noexcept))
+ {
+ return (*this)(
+ (*this)(
+ ASIO_MOVE_CAST(T)(t),
+ ASIO_MOVE_CAST(P0)(p0)),
+ ASIO_MOVE_CAST(P1)(p1),
+ ASIO_MOVE_CAST(PN)(pn) ASIO_ELLIPSIS);
+ }
+};
+
+template <typename T = impl>
+struct static_instance
+{
+ static const T instance;
+};
+
+template <typename T>
+const T static_instance<T>::instance = {};
+
+} // namespace asio_prefer_fn
+namespace asio {
+namespace {
+
+static ASIO_CONSTEXPR const asio_prefer_fn::impl&
+ prefer = asio_prefer_fn::static_instance<>::instance;
+
+} // namespace
+
+typedef asio_prefer_fn::impl prefer_t;
+
+#if defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename... Properties>
+struct can_prefer :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(Properties...)>::overload
+ != asio_prefer_fn::ill_formed>
+{
+};
+
+#else // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename P0 = void,
+ typename P1 = void, typename P2 = void>
+struct can_prefer :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1, P2)>::overload
+ != asio_prefer_fn::ill_formed>
+{
+};
+
+template <typename T, typename P0, typename P1>
+struct can_prefer<T, P0, P1> :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1)>::overload
+ != asio_prefer_fn::ill_formed>
+{
+};
+
+template <typename T, typename P0>
+struct can_prefer<T, P0> :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0)>::overload
+ != asio_prefer_fn::ill_formed>
+{
+};
+
+template <typename T>
+struct can_prefer<T> :
+ false_type
+{
+};
+
+#endif // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+template <typename T, typename ASIO_ELLIPSIS Properties>
+constexpr bool can_prefer_v
+ = can_prefer<T, Properties ASIO_ELLIPSIS>::value;
+
+#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+#if defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename... Properties>
+struct is_nothrow_prefer :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(Properties...)>::is_noexcept>
+{
+};
+
+#else // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename P0 = void,
+ typename P1 = void, typename P2 = void>
+struct is_nothrow_prefer :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1, P2)>::is_noexcept>
+{
+};
+
+template <typename T, typename P0, typename P1>
+struct is_nothrow_prefer<T, P0, P1> :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1)>::is_noexcept>
+{
+};
+
+template <typename T, typename P0>
+struct is_nothrow_prefer<T, P0> :
+ integral_constant<bool,
+ asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0)>::is_noexcept>
+{
+};
+
+template <typename T>
+struct is_nothrow_prefer<T> :
+ false_type
+{
+};
+
+#endif // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+template <typename T, typename ASIO_ELLIPSIS Properties>
+constexpr bool is_nothrow_prefer_v
+ = is_nothrow_prefer<T, Properties ASIO_ELLIPSIS>::value;
+
+#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+#if defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename... Properties>
+struct prefer_result
+{
+ typedef typename asio_prefer_fn::call_traits<
+ prefer_t, T, void(Properties...)>::result_type type;
+};
+
+#else // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+template <typename T, typename P0 = void,
+ typename P1 = void, typename P2 = void>
+struct prefer_result
+{
+ typedef typename asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1, P2)>::result_type type;
+};
+
+template <typename T, typename P0, typename P1>
+struct prefer_result<T, P0, P1>
+{
+ typedef typename asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0, P1)>::result_type type;
+};
+
+template <typename T, typename P0>
+struct prefer_result<T, P0>
+{
+ typedef typename asio_prefer_fn::call_traits<
+ prefer_t, T, void(P0)>::result_type type;
+};
+
+template <typename T>
+struct prefer_result<T>
+{
+};
+
+#endif // defined(ASIO_HAS_VARIADIC_TEMPLATES)
+
+} // namespace asio
+
+#endif // defined(GENERATING_DOCUMENTATION)
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_PREFER_HPP