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 //
 // io_context_strand.hpp
 // ~~~~~~~~~~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2024 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_IO_CONTEXT_STRAND_HPP
 #define BOOST_ASIO_IO_CONTEXT_STRAND_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS) \
   && !defined(BOOST_ASIO_NO_TS_EXECUTORS)
 
 #include <boost/asio/async_result.hpp>
 #include <boost/asio/detail/handler_type_requirements.hpp>
 #include <boost/asio/detail/strand_service.hpp>
 #include <boost/asio/detail/wrapped_handler.hpp>
 #include <boost/asio/io_context.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 
 /// Provides serialised handler execution.
 /**
  * The io_context::strand class provides the ability to post and dispatch
  * handlers with the guarantee that none of those handlers will execute
  * concurrently.
  *
  * @par Order of handler invocation
  * Given:
  *
  * @li a strand object @c s
  *
  * @li an object @c a meeting completion handler requirements
  *
  * @li an object @c a1 which is an arbitrary copy of @c a made by the
  * implementation
  *
  * @li an object @c b meeting completion handler requirements
  *
  * @li an object @c b1 which is an arbitrary copy of @c b made by the
  * implementation
  *
  * if any of the following conditions are true:
  *
  * @li @c s.post(a) happens-before @c s.post(b)
  *
  * @li @c s.post(a) happens-before @c s.dispatch(b), where the latter is
  * performed outside the strand
  *
  * @li @c s.dispatch(a) happens-before @c s.post(b), where the former is
  * performed outside the strand
  *
  * @li @c s.dispatch(a) happens-before @c s.dispatch(b), where both are
  * performed outside the strand
  *
  * then @c a() happens-before @c b()
  *
  * Note that in the following case:
  * @code async_op_1(..., s.wrap(a));
  * async_op_2(..., s.wrap(b)); @endcode
  * the completion of the first async operation will perform @c s.dispatch(a),
  * and the second will perform @c s.dispatch(b), but the order in which those
  * are performed is unspecified. That is, you cannot state whether one
  * happens-before the other. Therefore none of the above conditions are met and
  * no ordering guarantee is made.
  *
  * @note The implementation makes no guarantee that handlers posted or
  * dispatched through different @c strand objects will be invoked concurrently.
  *
  * @par Thread Safety
  * @e Distinct @e objects: Safe.@n
  * @e Shared @e objects: Safe.
  *
  * @par Concepts:
  * Dispatcher.
  */
 class io_context::strand
 {
 private:
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   struct initiate_dispatch;
   struct initiate_post;
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
 public:
   /// Constructor.
   /**
    * Constructs the strand.
    *
    * @param io_context The io_context object that the strand will use to
    * dispatch handlers that are ready to be run.
    */
   explicit strand(boost::asio::io_context& io_context)
     : service_(boost::asio::use_service<
         boost::asio::detail::strand_service>(io_context))
   {
     service_.construct(impl_);
   }
 
   /// Copy constructor.
   /**
    * Creates a copy such that both strand objects share the same underlying
    * state.
    */
   strand(const strand& other) noexcept
     : service_(other.service_),
       impl_(other.impl_)
   {
   }
 
   /// Destructor.
   /**
    * Destroys a strand.
    *
    * Handlers posted through the strand that have not yet been invoked will
    * still be dispatched in a way that meets the guarantee of non-concurrency.
    */
   ~strand()
   {
   }
 
   /// Obtain the underlying execution context.
   boost::asio::io_context& context() const noexcept
   {
     return service_.get_io_context();
   }
 
   /// Inform the strand that it has some outstanding work to do.
   /**
    * The strand delegates this call to its underlying io_context.
    */
   void on_work_started() const noexcept
   {
     context().get_executor().on_work_started();
   }
 
   /// Inform the strand that some work is no longer outstanding.
   /**
    * The strand delegates this call to its underlying io_context.
    */
   void on_work_finished() const noexcept
   {
     context().get_executor().on_work_finished();
   }
 
   /// Request the strand to invoke the given function object.
   /**
    * This function is used to ask the strand to execute the given function
    * object on its underlying io_context. The function object will be executed
    * inside this function if the strand is not otherwise busy and if the
    * underlying io_context's executor's @c dispatch() function is also able to
    * execute the function before returning.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename Allocator>
   void dispatch(Function&& f, const Allocator& a) const
   {
     decay_t<Function> tmp(static_cast<Function&&>(f));
     service_.dispatch(impl_, tmp);
     (void)a;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use boost::asio::dispatch().) Request the strand to invoke
   /// the given handler.
   /**
    * This function is used to ask the strand to execute the given handler.
    *
    * The strand object guarantees that handlers posted or dispatched through
    * the strand will not be executed concurrently. The handler may be executed
    * inside this function if the guarantee can be met. If this function is
    * called from within a handler that was posted or dispatched through the same
    * strand, then the new handler will be executed immediately.
    *
    * The strand's guarantee is in addition to the guarantee provided by the
    * underlying io_context. The io_context guarantees that the handler will only
    * be called in a thread in which the io_context's run member function is
    * currently being invoked.
    *
    * @param handler The handler to be called. The strand will make a copy of the
    * handler object as required. The function signature of the handler must be:
    * @code void handler(); @endcode
    */
   template <typename LegacyCompletionHandler>
   auto dispatch(LegacyCompletionHandler&& handler)
     -> decltype(
       async_initiate<LegacyCompletionHandler, void ()>(
         declval<initiate_dispatch>(), handler, this))
   {
     return async_initiate<LegacyCompletionHandler, void ()>(
         initiate_dispatch(), handler, this);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Request the strand to invoke the given function object.
   /**
    * This function is used to ask the executor to execute the given function
    * object. The function object will never be executed inside this function.
    * Instead, it will be scheduled to run by the underlying io_context.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename Allocator>
   void post(Function&& f, const Allocator& a) const
   {
     decay_t<Function> tmp(static_cast<Function&&>(f));
     service_.post(impl_, tmp);
     (void)a;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use boost::asio::post().) Request the strand to invoke the
   /// given handler and return immediately.
   /**
    * This function is used to ask the strand to execute the given handler, but
    * without allowing the strand to call the handler from inside this function.
    *
    * The strand object guarantees that handlers posted or dispatched through
    * the strand will not be executed concurrently. The strand's guarantee is in
    * addition to the guarantee provided by the underlying io_context. The
    * io_context guarantees that the handler will only be called in a thread in
    * which the io_context's run member function is currently being invoked.
    *
    * @param handler The handler to be called. The strand will make a copy of the
    * handler object as required. The function signature of the handler must be:
    * @code void handler(); @endcode
    */
   template <typename LegacyCompletionHandler>
   auto post(LegacyCompletionHandler&& handler)
     -> decltype(
       async_initiate<LegacyCompletionHandler, void ()>(
         declval<initiate_post>(), handler, this))
   {
     return async_initiate<LegacyCompletionHandler, void ()>(
         initiate_post(), handler, this);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Request the strand to invoke the given function object.
   /**
    * This function is used to ask the executor to execute the given function
    * object. The function object will never be executed inside this function.
    * Instead, it will be scheduled to run by the underlying io_context.
    *
    * @param f The function object to be called. The executor will make
    * a copy of the handler object as required. The function signature of the
    * function object must be: @code void function(); @endcode
    *
    * @param a An allocator that may be used by the executor to allocate the
    * internal storage needed for function invocation.
    */
   template <typename Function, typename Allocator>
   void defer(Function&& f, const Allocator& a) const
   {
     decay_t<Function> tmp(static_cast<Function&&>(f));
     service_.post(impl_, tmp);
     (void)a;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use boost::asio::bind_executor().) Create a new handler that
   /// automatically dispatches the wrapped handler on the strand.
   /**
    * This function is used to create a new handler function object that, when
    * invoked, will automatically pass the wrapped handler to the strand's
    * dispatch function.
    *
    * @param handler The handler to be wrapped. The strand will make a copy of
    * the handler object as required. The function signature of the handler must
    * be: @code void handler(A1 a1, ... An an); @endcode
    *
    * @return A function object that, when invoked, passes the wrapped handler to
    * the strand's dispatch function. Given a function object with the signature:
    * @code R f(A1 a1, ... An an); @endcode
    * If this function object is passed to the wrap function like so:
    * @code strand.wrap(f); @endcode
    * then the return value is a function object with the signature
    * @code void g(A1 a1, ... An an); @endcode
    * that, when invoked, executes code equivalent to:
    * @code strand.dispatch(boost::bind(f, a1, ... an)); @endcode
    */
   template <typename Handler>
 #if defined(GENERATING_DOCUMENTATION)
   unspecified
 #else
   detail::wrapped_handler<strand, Handler, detail::is_continuation_if_running>
 #endif
   wrap(Handler handler)
   {
     return detail::wrapped_handler<io_context::strand, Handler,
         detail::is_continuation_if_running>(*this, handler);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Determine whether the strand is running in the current thread.
   /**
    * @return @c true if the current thread is executing a handler that was
    * submitted to the strand using post(), dispatch() or wrap(). Otherwise
    * returns @c false.
    */
   bool running_in_this_thread() const noexcept
   {
     return service_.running_in_this_thread(impl_);
   }
 
   /// Compare two strands for equality.
   /**
    * Two strands are equal if they refer to the same ordered, non-concurrent
    * state.
    */
   friend bool operator==(const strand& a, const strand& b) noexcept
   {
     return a.impl_ == b.impl_;
   }
 
   /// Compare two strands for inequality.
   /**
    * Two strands are equal if they refer to the same ordered, non-concurrent
    * state.
    */
   friend bool operator!=(const strand& a, const strand& b) noexcept
   {
     return a.impl_ != b.impl_;
   }
 
 private:
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   struct initiate_dispatch
   {
     template <typename LegacyCompletionHandler>
     void operator()(LegacyCompletionHandler&& handler,
         strand* self) const
     {
       // If you get an error on the following line it means that your
       // handler does not meet the documented type requirements for a
       // LegacyCompletionHandler.
       BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
           LegacyCompletionHandler, handler) type_check;
 
       detail::non_const_lvalue<LegacyCompletionHandler> handler2(handler);
       self->service_.dispatch(self->impl_, handler2.value);
     }
   };
 
   struct initiate_post
   {
     template <typename LegacyCompletionHandler>
     void operator()(LegacyCompletionHandler&& handler,
         strand* self) const
     {
       // If you get an error on the following line it means that your
       // handler does not meet the documented type requirements for a
       // LegacyCompletionHandler.
       BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
           LegacyCompletionHandler, handler) type_check;
 
       detail::non_const_lvalue<LegacyCompletionHandler> handler2(handler);
       self->service_.post(self->impl_, handler2.value);
     }
   };
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   boost::asio::detail::strand_service& service_;
   mutable boost::asio::detail::strand_service::implementation_type impl_;
 };
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
        //   && !defined(BOOST_ASIO_NO_TS_EXECUTORS)
 
 #endif // BOOST_ASIO_IO_CONTEXT_STRAND_HPP

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