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Every I/O executor type now has an associated default completion token
type. This is specified via the `default_completion_token_type` trait.
This trait may be used in asynchronous operation declarations as
follows:
template <
typename IoObject,
typename CompletionToken =
typename default_completion_token_type<
typename IoObject::executor_type
>::type
>
auto async_foo(
IoObject& io_object,
CompletionToken&& token =
typename default_completion_token_type<
typename IoObject::executor_type
>::type{}
);
If not specialised, this trait type is `void`, meaning no default
completion token type is available for the given I/O executor.
The `default_completion_token_type` trait is specialised for the
`use_awaitable` completion token so that it may be used as shown in the
following example:
auto socket = use_awaitable.as_default_on(tcp::socket(my_context));
// ...
co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.
In this example, type of the `socket` object is transformed from
`tcp::socket` to have an I/O executor with the default completion token
set to `use_awaitable`.
Alternatively, the socket type may be computed directly:
using tcp_socket = use_awaitable_t<>::as_default_on_t<tcp::socket>;
tcp_socket socket(my_context);
// ...
co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.
112 lines
3.5 KiB
C++
112 lines
3.5 KiB
C++
//
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// use_awaitable.hpp
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// ~~~~~~~~~~~~~~~~~
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//
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// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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#ifndef BOOST_ASIO_USE_AWAITABLE_HPP
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#define BOOST_ASIO_USE_AWAITABLE_HPP
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#if defined(_MSC_VER) && (_MSC_VER >= 1200)
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# pragma once
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#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
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#include <boost/asio/detail/config.hpp>
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#if defined(BOOST_ASIO_HAS_CO_AWAIT) || defined(GENERATING_DOCUMENTATION)
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#include <boost/asio/awaitable.hpp>
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#include <boost/asio/detail/push_options.hpp>
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namespace boost {
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namespace asio {
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/// A completion token that represents the currently executing coroutine.
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/**
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* The @c use_awaitable_t class, with its value @c use_awaitable, is used to
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* represent the currently executing coroutine. This completion token may be
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* passed as a handler to an asynchronous operation. For example:
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*
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* @code awaitable<void> my_coroutine()
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* {
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* std::size_t n = co_await my_socket.async_read_some(buffer, use_awaitable);
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* ...
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* } @endcode
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*
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* When used with co_await, the initiating function (@c async_read_some in the
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* above example) suspends the current coroutine. The coroutine is resumed when
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* the asynchronous operation completes, and the result of the operation is
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* returned.
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*/
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template <typename Executor = executor>
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struct use_awaitable_t
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{
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/// Default constructor.
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BOOST_ASIO_CONSTEXPR use_awaitable_t()
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{
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}
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/// Adapts an executor to add the @c use_awaitable_t completion token as the
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/// default.
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template <typename InnerExecutor>
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struct executor_with_default : InnerExecutor
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{
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/// Specify @c use_awaitable_t as the default completion token type.
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typedef use_awaitable_t default_completion_token_type;
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/// Construct the adapted executor from the inner executor type.
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executor_with_default(const InnerExecutor& ex) BOOST_ASIO_NOEXCEPT
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: InnerExecutor(ex)
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{
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}
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};
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/// Type alias to adapt an I/O object to use @c use_awaitable_t as its
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/// default completion token type.
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#if defined(BOOST_ASIO_HAS_ALIAS_TEMPLATES) \
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|| defined(GENERATING_DOCUMENTATION)
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template <typename T>
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using as_default_on_t = typename T::template rebind_executor<
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executor_with_default<typename T::executor_type> >::other;
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#endif // defined(BOOST_ASIO_HAS_ALIAS_TEMPLATES)
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// || defined(GENERATING_DOCUMENTATION)
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/// Function helper to adapt an I/O object to use @c use_awaitable_t as its
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/// default completion token type.
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template <typename T>
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static typename T::template rebind_executor<
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executor_with_default<typename T::executor_type>
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>::other
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as_default_on(BOOST_ASIO_MOVE_ARG(T) object)
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{
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return typename as_default_on_t<typename decay<T>::type>::type(
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BOOST_ASIO_MOVE_CAST(T)(object));
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}
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};
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/// A completion token object that represents the currently executing coroutine.
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/**
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* See the documentation for boost::asio::use_awaitable_t for a usage example.
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*/
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#if defined(BOOST_ASIO_HAS_CONSTEXPR) || defined(GENERATING_DOCUMENTATION)
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constexpr use_awaitable_t<> use_awaitable;
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#elif defined(BOOST_ASIO_MSVC)
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__declspec(selectany) use_awaitable_t<> use_awaitable;
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#endif
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} // namespace asio
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} // namespace boost
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#include <boost/asio/detail/pop_options.hpp>
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#include <boost/asio/impl/use_awaitable.hpp>
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#endif // defined(BOOST_ASIO_HAS_CO_AWAIT) || defined(GENERATING_DOCUMENTATION)
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#endif // BOOST_ASIO_USE_AWAITABLE_HPP
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