Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit 43d231b32d.

boost/asio/execution/context_as.hpp

//
// execution/context_as.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 BOOST_ASIO_EXECUTION_CONTEXT_AS_HPP
#define BOOST_ASIO_EXECUTION_CONTEXT_AS_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)
# pragma once
#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include <boost/asio/detail/config.hpp>
#include <boost/asio/detail/type_traits.hpp>
#include <boost/asio/execution/context.hpp>
#include <boost/asio/execution/executor.hpp>
#include <boost/asio/execution/scheduler.hpp>
#include <boost/asio/execution/sender.hpp>
#include <boost/asio/is_applicable_property.hpp>
#include <boost/asio/query.hpp>
#include <boost/asio/traits/query_static_constexpr_member.hpp>
#include <boost/asio/traits/static_query.hpp>

#include <boost/asio/detail/push_options.hpp>

namespace boost {
namespace asio {

#if defined(GENERATING_DOCUMENTATION)

namespace execution {

/// A property that is used to obtain the execution context that is associated
/// with an executor.
template <typename U>
struct context_as_t
{
  /// The context_as_t property applies to executors, senders, and schedulers.
  template <typename T>
  static constexpr bool is_applicable_property_v =
    is_executor_v<T> || is_sender_v<T> || is_scheduler_v<T>;

  /// The context_t property cannot be required.
  static constexpr bool is_requirable = false;

  /// The context_t property cannot be preferred.
  static constexpr bool is_preferable = false;

  /// The type returned by queries against an @c any_executor.
  typedef T polymorphic_query_result_type;
};

/// A special value used for accessing the context_as_t property.
template <typename U>
constexpr context_as_t context_as;

} // namespace execution

#else // defined(GENERATING_DOCUMENTATION)

namespace execution {

template <typename T>
struct context_as_t
{
#if defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)
  template <typename U>
  BOOST_ASIO_STATIC_CONSTEXPR(bool,
    is_applicable_property_v = (
      is_executor<U>::value
        || conditional<
            is_executor<U>::value,
            false_type,
            is_sender<U>
          >::type::value
        || conditional<
            is_executor<U>::value,
            false_type,
            is_scheduler<U>
          >::type::value));
#endif // defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_requirable = false);
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_preferable = false);

  typedef T polymorphic_query_result_type;

  BOOST_ASIO_CONSTEXPR context_as_t()
  {
  }

  BOOST_ASIO_CONSTEXPR context_as_t(context_t)
  {
  }

#if defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
  template <typename E>
  static BOOST_ASIO_CONSTEXPR
  typename context_t::query_static_constexpr_member<E>::result_type
  static_query()
    BOOST_ASIO_NOEXCEPT_IF((
      context_t::query_static_constexpr_member<E>::is_noexcept))
  {
    return context_t::query_static_constexpr_member<E>::value();
  }

  template <typename E, typename U = decltype(context_as_t::static_query<E>())>
  static BOOST_ASIO_CONSTEXPR const U static_query_v
    = context_as_t::static_query<E>();
#endif // defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

  template <typename Executor, typename U>
  friend BOOST_ASIO_CONSTEXPR U query(
      const Executor& ex, const context_as_t<U>&,
      typename enable_if<
        is_same<T, U>::value
      >::type* = 0,
      typename enable_if<
        can_query<const Executor&, const context_t&>::value
      >::type* = 0)
#if !defined(__clang__) // Clang crashes if noexcept is used here.
#if defined(BOOST_ASIO_MSVC) // Visual C++ wants the type to be qualified.
    BOOST_ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&, const context_t&>::value))
#else // defined(BOOST_ASIO_MSVC)
    BOOST_ASIO_NOEXCEPT_IF((
      is_nothrow_query<const Executor&, const context_t&>::value))
#endif // defined(BOOST_ASIO_MSVC)
#endif // !defined(__clang__)
  {
    return boost::asio::query(ex, context);
  }
};

#if defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  && defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)
template <typename T> template <typename E, typename U>
const U context_as_t<T>::static_query_v;
#endif // defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   && defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if (defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES) \
    && defined(BOOST_ASIO_HAS_CONSTEXPR)) \
  || defined(GENERATING_DOCUMENTATION)
template <typename T>
constexpr context_as_t<T> context_as{};
#endif // (defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)
       //     && defined(BOOST_ASIO_HAS_CONSTEXPR))
       //   || defined(GENERATING_DOCUMENTATION)

} // namespace execution

#if !defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

template <typename T, typename U>
struct is_applicable_property<T, execution::context_as_t<U> >
  : integral_constant<bool,
      execution::is_executor<T>::value
        || conditional<
            execution::is_executor<T>::value,
            false_type,
            execution::is_sender<T>
          >::type::value
        || conditional<
            execution::is_executor<T>::value,
            false_type,
            execution::is_scheduler<T>
          >::type::value>
{
};

#endif // !defined(BOOST_ASIO_HAS_VARIABLE_TEMPLATES)

namespace traits {

#if !defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT) \
  || !defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

template <typename T, typename U>
struct static_query<T, execution::context_as_t<U>,
  typename enable_if<
    static_query<T, execution::context_t>::is_valid
  >::type> : static_query<T, execution::context_t>
{
};

#endif // !defined(BOOST_ASIO_HAS_DEDUCED_STATIC_QUERY_TRAIT)
       //   || !defined(BOOST_ASIO_HAS_SFINAE_VARIABLE_TEMPLATES)

#if !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

template <typename T, typename U>
struct query_free<T, execution::context_as_t<U>,
    typename enable_if<
      can_query<const T&, const execution::context_t&>::value
    >::type>
{
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_valid = true);
  BOOST_ASIO_STATIC_CONSTEXPR(bool, is_noexcept =
    (is_nothrow_query<const T&, const execution::context_t&>::value));

  typedef U result_type;
};

#endif // !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_FREE_TRAIT)

} // namespace traits

#endif // defined(GENERATING_DOCUMENTATION)

} // namespace asio
} // namespace boost

#include <boost/asio/detail/pop_options.hpp>

#endif // BOOST_ASIO_EXECUTION_CONTEXT_AS_HPP