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include/asio/execution/occupancy.hpp

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+//
+// execution/occupancy.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2025 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_EXECUTION_OCCUPANCY_HPP
+#define ASIO_EXECUTION_OCCUPANCY_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/execution/executor.hpp"
+#include "asio/is_applicable_property.hpp"
+#include "asio/traits/query_static_constexpr_member.hpp"
+#include "asio/traits/static_query.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+
+#if defined(GENERATING_DOCUMENTATION)
+
+namespace execution {
+
+/// A property that gives an estimate of the number of execution agents that
+/// should occupy the associated execution context.
+struct occupancy_t
+{
+  /// The occupancy_t property applies to executors.
+  template <typename T>
+  static constexpr bool is_applicable_property_v = is_executor_v<T>;
+
+  /// The occupancy_t property cannot be required.
+  static constexpr bool is_requirable = false;
+
+  /// The occupancy_t property cannot be preferred.
+  static constexpr bool is_preferable = false;
+
+  /// The type returned by queries against an @c any_executor.
+  typedef std::size_t polymorphic_query_result_type;
+};
+
+/// A special value used for accessing the occupancy_t property.
+constexpr occupancy_t occupancy;
+
+} // namespace execution
+
+#else // defined(GENERATING_DOCUMENTATION)
+
+namespace execution {
+namespace detail {
+
+template <int I = 0>
+struct occupancy_t
+{
+#if defined(ASIO_HAS_VARIABLE_TEMPLATES)
+  template <typename T>
+  static constexpr bool is_applicable_property_v = is_executor<T>::value;
+#endif // defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+  static constexpr bool is_requirable = false;
+  static constexpr bool is_preferable = false;
+  typedef std::size_t polymorphic_query_result_type;
+
+  constexpr occupancy_t()
+  {
+  }
+
+  template <typename T>
+  struct static_proxy
+  {
+#if defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
+    struct type
+    {
+      template <typename P>
+      static constexpr auto query(P&& p)
+        noexcept(
+          noexcept(
+            conditional_t<true, T, P>::query(static_cast<P&&>(p))
+          )
+        )
+        -> decltype(
+          conditional_t<true, T, P>::query(static_cast<P&&>(p))
+        )
+      {
+        return T::query(static_cast<P&&>(p));
+      }
+    };
+#else // defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
+    typedef T type;
+#endif // defined(ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT)
+  };
+
+  template <typename T>
+  struct query_static_constexpr_member :
+    traits::query_static_constexpr_member<
+      typename static_proxy<T>::type, occupancy_t> {};
+
+#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
+  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+  template <typename T>
+  static constexpr typename query_static_constexpr_member<T>::result_type
+  static_query()
+    noexcept(query_static_constexpr_member<T>::is_noexcept)
+  {
+    return query_static_constexpr_member<T>::value();
+  }
+
+  template <typename E, typename T = decltype(occupancy_t::static_query<E>())>
+  static constexpr const T static_query_v = occupancy_t::static_query<E>();
+#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
+       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+};
+
+#if defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
+  && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+template <int I> template <typename E, typename T>
+const T occupancy_t<I>::static_query_v;
+#endif // defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
+       //   && defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+
+} // namespace detail
+
+typedef detail::occupancy_t<> occupancy_t;
+
+ASIO_INLINE_VARIABLE constexpr occupancy_t occupancy;
+
+} // namespace execution
+
+#if !defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+template <typename T>
+struct is_applicable_property<T, execution::occupancy_t>
+  : integral_constant<bool, execution::is_executor<T>::value>
+{
+};
+
+#endif // !defined(ASIO_HAS_VARIABLE_TEMPLATES)
+
+namespace traits {
+
+#if !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
+  || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+
+template <typename T>
+struct static_query<T, execution::occupancy_t,
+  enable_if_t<
+    execution::detail::occupancy_t<0>::
+      query_static_constexpr_member<T>::is_valid
+  >>
+{
+  static constexpr bool is_valid = true;
+  static constexpr bool is_noexcept = true;
+
+  typedef typename execution::detail::occupancy_t<0>::
+    query_static_constexpr_member<T>::result_type result_type;
+
+  static constexpr result_type value()
+  {
+    return execution::detail::occupancy_t<0>::
+      query_static_constexpr_member<T>::value();
+  }
+};
+
+#endif // !defined(ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
+       //   || !defined(ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
+
+} // namespace traits
+
+#endif // defined(GENERATING_DOCUMENTATION)
+
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_EXECUTION_OCCUPANCY_HPP