377 lines
13 KiB
C++
377 lines
13 KiB
C++
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//
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// io_context_strand.hpp
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// ~~~~~~~~~~~~~~~~~~~~~
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//
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// Copyright (c) 2003-2021 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 ASIO_IO_CONTEXT_STRAND_HPP
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#define ASIO_IO_CONTEXT_STRAND_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 "asio/detail/config.hpp"
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#if !defined(ASIO_NO_EXTENSIONS) \
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&& !defined(ASIO_NO_TS_EXECUTORS)
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#include "asio/async_result.hpp"
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#include "asio/detail/handler_type_requirements.hpp"
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#include "asio/detail/strand_service.hpp"
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#include "asio/detail/wrapped_handler.hpp"
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#include "asio/io_context.hpp"
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#include "asio/detail/push_options.hpp"
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namespace asio {
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/// Provides serialised handler execution.
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/**
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* The io_context::strand class provides the ability to post and dispatch
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* handlers with the guarantee that none of those handlers will execute
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* concurrently.
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*
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* @par Order of handler invocation
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* Given:
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*
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* @li a strand object @c s
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*
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* @li an object @c a meeting completion handler requirements
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*
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* @li an object @c a1 which is an arbitrary copy of @c a made by the
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* implementation
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*
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* @li an object @c b meeting completion handler requirements
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*
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* @li an object @c b1 which is an arbitrary copy of @c b made by the
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* implementation
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*
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* if any of the following conditions are true:
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*
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* @li @c s.post(a) happens-before @c s.post(b)
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*
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* @li @c s.post(a) happens-before @c s.dispatch(b), where the latter is
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* performed outside the strand
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*
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* @li @c s.dispatch(a) happens-before @c s.post(b), where the former is
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* performed outside the strand
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*
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* @li @c s.dispatch(a) happens-before @c s.dispatch(b), where both are
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* performed outside the strand
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*
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* then @c asio_handler_invoke(a1, &a1) happens-before
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* @c asio_handler_invoke(b1, &b1).
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*
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* Note that in the following case:
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* @code async_op_1(..., s.wrap(a));
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* async_op_2(..., s.wrap(b)); @endcode
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* the completion of the first async operation will perform @c s.dispatch(a),
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* and the second will perform @c s.dispatch(b), but the order in which those
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* are performed is unspecified. That is, you cannot state whether one
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* happens-before the other. Therefore none of the above conditions are met and
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* no ordering guarantee is made.
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*
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* @note The implementation makes no guarantee that handlers posted or
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* dispatched through different @c strand objects will be invoked concurrently.
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*
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* @par Thread Safety
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* @e Distinct @e objects: Safe.@n
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* @e Shared @e objects: Safe.
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*
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* @par Concepts:
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* Dispatcher.
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*/
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class io_context::strand
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{
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public:
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/// Constructor.
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/**
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* Constructs the strand.
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*
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* @param io_context The io_context object that the strand will use to
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* dispatch handlers that are ready to be run.
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*/
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explicit strand(asio::io_context& io_context)
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: service_(asio::use_service<
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asio::detail::strand_service>(io_context))
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{
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service_.construct(impl_);
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}
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/// Destructor.
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/**
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* Destroys a strand.
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*
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* Handlers posted through the strand that have not yet been invoked will
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* still be dispatched in a way that meets the guarantee of non-concurrency.
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*/
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~strand()
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{
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}
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/// Obtain the underlying execution context.
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asio::io_context& context() const ASIO_NOEXCEPT
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{
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return service_.get_io_context();
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}
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/// Inform the strand that it has some outstanding work to do.
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/**
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* The strand delegates this call to its underlying io_context.
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*/
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void on_work_started() const ASIO_NOEXCEPT
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{
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context().get_executor().on_work_started();
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}
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/// Inform the strand that some work is no longer outstanding.
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/**
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* The strand delegates this call to its underlying io_context.
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*/
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void on_work_finished() const ASIO_NOEXCEPT
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{
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context().get_executor().on_work_finished();
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}
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/// Request the strand to invoke the given function object.
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/**
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* This function is used to ask the strand to execute the given function
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* object on its underlying io_context. The function object will be executed
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* inside this function if the strand is not otherwise busy and if the
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* underlying io_context's executor's @c dispatch() function is also able to
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* execute the function before returning.
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*
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* @param f The function object to be called. The executor will make
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* a copy of the handler object as required. The function signature of the
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* function object must be: @code void function(); @endcode
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*
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* @param a An allocator that may be used by the executor to allocate the
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* internal storage needed for function invocation.
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*/
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template <typename Function, typename Allocator>
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void dispatch(ASIO_MOVE_ARG(Function) f, const Allocator& a) const
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{
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typename decay<Function>::type tmp(ASIO_MOVE_CAST(Function)(f));
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service_.dispatch(impl_, tmp);
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(void)a;
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}
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#if !defined(ASIO_NO_DEPRECATED)
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/// (Deprecated: Use asio::dispatch().) Request the strand to invoke
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/// the given handler.
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/**
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* This function is used to ask the strand to execute the given handler.
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*
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* The strand object guarantees that handlers posted or dispatched through
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* the strand will not be executed concurrently. The handler may be executed
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* inside this function if the guarantee can be met. If this function is
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* called from within a handler that was posted or dispatched through the same
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* strand, then the new handler will be executed immediately.
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*
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* The strand's guarantee is in addition to the guarantee provided by the
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* underlying io_context. The io_context guarantees that the handler will only
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* be called in a thread in which the io_context's run member function is
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* currently being invoked.
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*
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* @param handler The handler to be called. The strand will make a copy of the
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* handler object as required. The function signature of the handler must be:
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* @code void handler(); @endcode
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*/
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template <typename LegacyCompletionHandler>
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ASIO_INITFN_AUTO_RESULT_TYPE(LegacyCompletionHandler, void ())
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dispatch(ASIO_MOVE_ARG(LegacyCompletionHandler) handler)
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{
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return async_initiate<LegacyCompletionHandler, void ()>(
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initiate_dispatch(), handler, this);
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}
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#endif // !defined(ASIO_NO_DEPRECATED)
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/// Request the strand to invoke the given function object.
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/**
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* This function is used to ask the executor to execute the given function
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* object. The function object will never be executed inside this function.
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* Instead, it will be scheduled to run by the underlying io_context.
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*
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* @param f The function object to be called. The executor will make
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* a copy of the handler object as required. The function signature of the
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* function object must be: @code void function(); @endcode
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*
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* @param a An allocator that may be used by the executor to allocate the
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* internal storage needed for function invocation.
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*/
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template <typename Function, typename Allocator>
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void post(ASIO_MOVE_ARG(Function) f, const Allocator& a) const
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{
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typename decay<Function>::type tmp(ASIO_MOVE_CAST(Function)(f));
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service_.post(impl_, tmp);
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(void)a;
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}
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#if !defined(ASIO_NO_DEPRECATED)
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/// (Deprecated: Use asio::post().) Request the strand to invoke the
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/// given handler and return immediately.
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/**
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* This function is used to ask the strand to execute the given handler, but
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* without allowing the strand to call the handler from inside this function.
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*
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* The strand object guarantees that handlers posted or dispatched through
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* the strand will not be executed concurrently. The strand's guarantee is in
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* addition to the guarantee provided by the underlying io_context. The
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* io_context guarantees that the handler will only be called in a thread in
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* which the io_context's run member function is currently being invoked.
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*
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* @param handler The handler to be called. The strand will make a copy of the
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* handler object as required. The function signature of the handler must be:
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* @code void handler(); @endcode
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*/
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template <typename LegacyCompletionHandler>
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ASIO_INITFN_AUTO_RESULT_TYPE(LegacyCompletionHandler, void ())
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post(ASIO_MOVE_ARG(LegacyCompletionHandler) handler)
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{
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return async_initiate<LegacyCompletionHandler, void ()>(
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initiate_post(), handler, this);
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}
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#endif // !defined(ASIO_NO_DEPRECATED)
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/// Request the strand to invoke the given function object.
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/**
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* This function is used to ask the executor to execute the given function
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* object. The function object will never be executed inside this function.
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* Instead, it will be scheduled to run by the underlying io_context.
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*
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* @param f The function object to be called. The executor will make
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* a copy of the handler object as required. The function signature of the
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* function object must be: @code void function(); @endcode
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*
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* @param a An allocator that may be used by the executor to allocate the
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* internal storage needed for function invocation.
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*/
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template <typename Function, typename Allocator>
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void defer(ASIO_MOVE_ARG(Function) f, const Allocator& a) const
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{
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typename decay<Function>::type tmp(ASIO_MOVE_CAST(Function)(f));
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service_.post(impl_, tmp);
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(void)a;
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}
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#if !defined(ASIO_NO_DEPRECATED)
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/// (Deprecated: Use asio::bind_executor().) Create a new handler that
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/// automatically dispatches the wrapped handler on the strand.
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/**
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* This function is used to create a new handler function object that, when
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* invoked, will automatically pass the wrapped handler to the strand's
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* dispatch function.
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*
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* @param handler The handler to be wrapped. The strand will make a copy of
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* the handler object as required. The function signature of the handler must
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* be: @code void handler(A1 a1, ... An an); @endcode
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*
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* @return A function object that, when invoked, passes the wrapped handler to
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* the strand's dispatch function. Given a function object with the signature:
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* @code R f(A1 a1, ... An an); @endcode
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* If this function object is passed to the wrap function like so:
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* @code strand.wrap(f); @endcode
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* then the return value is a function object with the signature
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* @code void g(A1 a1, ... An an); @endcode
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* that, when invoked, executes code equivalent to:
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* @code strand.dispatch(boost::bind(f, a1, ... an)); @endcode
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*/
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template <typename Handler>
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#if defined(GENERATING_DOCUMENTATION)
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unspecified
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#else
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detail::wrapped_handler<strand, Handler, detail::is_continuation_if_running>
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#endif
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wrap(Handler handler)
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{
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return detail::wrapped_handler<io_context::strand, Handler,
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detail::is_continuation_if_running>(*this, handler);
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}
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#endif // !defined(ASIO_NO_DEPRECATED)
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/// Determine whether the strand is running in the current thread.
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/**
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* @return @c true if the current thread is executing a handler that was
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* submitted to the strand using post(), dispatch() or wrap(). Otherwise
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* returns @c false.
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*/
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bool running_in_this_thread() const ASIO_NOEXCEPT
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{
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return service_.running_in_this_thread(impl_);
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}
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/// Compare two strands for equality.
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/**
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* Two strands are equal if they refer to the same ordered, non-concurrent
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* state.
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*/
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friend bool operator==(const strand& a, const strand& b) ASIO_NOEXCEPT
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{
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return a.impl_ == b.impl_;
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}
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/// Compare two strands for inequality.
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/**
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* Two strands are equal if they refer to the same ordered, non-concurrent
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* state.
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*/
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friend bool operator!=(const strand& a, const strand& b) ASIO_NOEXCEPT
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{
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return a.impl_ != b.impl_;
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}
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private:
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#if !defined(ASIO_NO_DEPRECATED)
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struct initiate_dispatch
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{
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template <typename LegacyCompletionHandler>
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void operator()(ASIO_MOVE_ARG(LegacyCompletionHandler) handler,
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strand* self) const
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{
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// If you get an error on the following line it means that your
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// handler does not meet the documented type requirements for a
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// LegacyCompletionHandler.
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ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
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LegacyCompletionHandler, handler) type_check;
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detail::non_const_lvalue<LegacyCompletionHandler> handler2(handler);
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self->service_.dispatch(self->impl_, handler2.value);
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}
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};
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struct initiate_post
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{
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template <typename LegacyCompletionHandler>
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void operator()(ASIO_MOVE_ARG(LegacyCompletionHandler) handler,
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strand* self) const
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{
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// If you get an error on the following line it means that your
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// handler does not meet the documented type requirements for a
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// LegacyCompletionHandler.
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ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
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LegacyCompletionHandler, handler) type_check;
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detail::non_const_lvalue<LegacyCompletionHandler> handler2(handler);
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self->service_.post(self->impl_, handler2.value);
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}
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};
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#endif // !defined(ASIO_NO_DEPRECATED)
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asio::detail::strand_service& service_;
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mutable asio::detail::strand_service::implementation_type impl_;
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};
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} // namespace asio
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#include "asio/detail/pop_options.hpp"
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#endif // !defined(ASIO_NO_EXTENSIONS)
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// && !defined(ASIO_NO_TS_EXECUTORS)
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#endif // ASIO_IO_CONTEXT_STRAND_HPP
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