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 //
 // basic_waitable_timer.hpp
 // ~~~~~~~~~~~~~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2023 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_BASIC_WAITABLE_TIMER_HPP
 #define BOOST_ASIO_BASIC_WAITABLE_TIMER_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <boost/asio/detail/config.hpp>
 #include <cstddef>
 #include <utility>
 #include <boost/asio/any_io_executor.hpp>
 #include <boost/asio/detail/chrono_time_traits.hpp>
 #include <boost/asio/detail/deadline_timer_service.hpp>
 #include <boost/asio/detail/handler_type_requirements.hpp>
 #include <boost/asio/detail/io_object_impl.hpp>
 #include <boost/asio/detail/non_const_lvalue.hpp>
 #include <boost/asio/detail/throw_error.hpp>
 #include <boost/asio/error.hpp>
 #include <boost/asio/wait_traits.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 
 #if !defined(BOOST_ASIO_BASIC_WAITABLE_TIMER_FWD_DECL)
 #define BOOST_ASIO_BASIC_WAITABLE_TIMER_FWD_DECL
 
 // Forward declaration with defaulted arguments.
 template <typename Clock,
     typename WaitTraits = boost::asio::wait_traits<Clock>,
     typename Executor = any_io_executor>
 class basic_waitable_timer;
 
 #endif // !defined(BOOST_ASIO_BASIC_WAITABLE_TIMER_FWD_DECL)
 
 /// Provides waitable timer functionality.
 /**
  * The basic_waitable_timer class template provides the ability to perform a
  * blocking or asynchronous wait for a timer to expire.
  *
  * A waitable timer is always in one of two states: "expired" or "not expired".
  * If the wait() or async_wait() function is called on an expired timer, the
  * wait operation will complete immediately.
  *
  * Most applications will use one of the boost::asio::steady_timer,
  * boost::asio::system_timer or boost::asio::high_resolution_timer typedefs.
  *
  * @note This waitable timer functionality is for use with the C++11 standard
  * library's @c &lt;chrono&gt; facility, or with the Boost.Chrono library.
  *
  * @par Thread Safety
  * @e Distinct @e objects: Safe.@n
  * @e Shared @e objects: Unsafe.
  *
  * @par Examples
  * Performing a blocking wait (C++11):
  * @code
  * // Construct a timer without setting an expiry time.
  * boost::asio::steady_timer timer(my_context);
  *
  * // Set an expiry time relative to now.
  * timer.expires_after(std::chrono::seconds(5));
  *
  * // Wait for the timer to expire.
  * timer.wait();
  * @endcode
  *
  * @par 
  * Performing an asynchronous wait (C++11):
  * @code
  * void handler(const boost::system::error_code& error)
  * {
  *   if (!error)
  *   {
  *     // Timer expired.
  *   }
  * }
  *
  * ...
  *
  * // Construct a timer with an absolute expiry time.
  * boost::asio::steady_timer timer(my_context,
  *     std::chrono::steady_clock::now() + std::chrono::seconds(60));
  *
  * // Start an asynchronous wait.
  * timer.async_wait(handler);
  * @endcode
  *
  * @par Changing an active waitable timer's expiry time
  *
  * Changing the expiry time of a timer while there are pending asynchronous
  * waits causes those wait operations to be cancelled. To ensure that the action
  * associated with the timer is performed only once, use something like this:
  * used:
  *
  * @code
  * void on_some_event()
  * {
  *   if (my_timer.expires_after(seconds(5)) > 0)
  *   {
  *     // We managed to cancel the timer. Start new asynchronous wait.
  *     my_timer.async_wait(on_timeout);
  *   }
  *   else
  *   {
  *     // Too late, timer has already expired!
  *   }
  * }
  *
  * void on_timeout(const boost::system::error_code& e)
  * {
  *   if (e != boost::asio::error::operation_aborted)
  *   {
  *     // Timer was not cancelled, take necessary action.
  *   }
  * }
  * @endcode
  *
  * @li The boost::asio::basic_waitable_timer::expires_after() function
  * cancels any pending asynchronous waits, and returns the number of
  * asynchronous waits that were cancelled. If it returns 0 then you were too
  * late and the wait handler has already been executed, or will soon be
  * executed. If it returns 1 then the wait handler was successfully cancelled.
  *
  * @li If a wait handler is cancelled, the boost::system::error_code passed to
  * it contains the value boost::asio::error::operation_aborted.
  */
 template <typename Clock, typename WaitTraits, typename Executor>
 class basic_waitable_timer
 {
 private:
   class initiate_async_wait;
 
 public:
   /// The type of the executor associated with the object.
   typedef Executor executor_type;
 
   /// Rebinds the timer type to another executor.
   template <typename Executor1>
   struct rebind_executor
   {
     /// The timer type when rebound to the specified executor.
     typedef basic_waitable_timer<Clock, WaitTraits, Executor1> other;
   };
 
   /// The clock type.
   typedef Clock clock_type;
 
   /// The duration type of the clock.
   typedef typename clock_type::duration duration;
 
   /// The time point type of the clock.
   typedef typename clock_type::time_point time_point;
 
   /// The wait traits type.
   typedef WaitTraits traits_type;
 
   /// Constructor.
   /**
    * This constructor creates a timer without setting an expiry time. The
    * expires_at() or expires_after() functions must be called to set an expiry
    * time before the timer can be waited on.
    *
    * @param ex The I/O executor that the timer will use, by default, to
    * dispatch handlers for any asynchronous operations performed on the timer.
    */
   explicit basic_waitable_timer(const executor_type& ex)
     : impl_(0, ex)
   {
   }
 
   /// Constructor.
   /**
    * This constructor creates a timer without setting an expiry time. The
    * expires_at() or expires_after() functions must be called to set an expiry
    * time before the timer can be waited on.
    *
    * @param context An execution context which provides the I/O executor that
    * the timer will use, by default, to dispatch handlers for any asynchronous
    * operations performed on the timer.
    */
   template <typename ExecutionContext>
   explicit basic_waitable_timer(ExecutionContext& context,
       constraint_t<
         is_convertible<ExecutionContext&, execution_context&>::value
       > = 0)
     : impl_(0, 0, context)
   {
   }
 
   /// Constructor to set a particular expiry time as an absolute time.
   /**
    * This constructor creates a timer and sets the expiry time.
    *
    * @param ex The I/O executor object that the timer will use, by default, to
    * dispatch handlers for any asynchronous operations performed on the timer.
    *
    * @param expiry_time The expiry time to be used for the timer, expressed
    * as an absolute time.
    */
   basic_waitable_timer(const executor_type& ex, const time_point& expiry_time)
     : impl_(0, ex)
   {
     boost::system::error_code ec;
     impl_.get_service().expires_at(impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_at");
   }
 
   /// Constructor to set a particular expiry time as an absolute time.
   /**
    * This constructor creates a timer and sets the expiry time.
    *
    * @param context An execution context which provides the I/O executor that
    * the timer will use, by default, to dispatch handlers for any asynchronous
    * operations performed on the timer.
    *
    * @param expiry_time The expiry time to be used for the timer, expressed
    * as an absolute time.
    */
   template <typename ExecutionContext>
   explicit basic_waitable_timer(ExecutionContext& context,
       const time_point& expiry_time,
       constraint_t<
         is_convertible<ExecutionContext&, execution_context&>::value
       > = 0)
     : impl_(0, 0, context)
   {
     boost::system::error_code ec;
     impl_.get_service().expires_at(impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_at");
   }
 
   /// Constructor to set a particular expiry time relative to now.
   /**
    * This constructor creates a timer and sets the expiry time.
    *
    * @param ex The I/O executor that the timer will use, by default, to
    * dispatch handlers for any asynchronous operations performed on the timer.
    *
    * @param expiry_time The expiry time to be used for the timer, relative to
    * now.
    */
   basic_waitable_timer(const executor_type& ex, const duration& expiry_time)
     : impl_(0, ex)
   {
     boost::system::error_code ec;
     impl_.get_service().expires_after(
         impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_after");
   }
 
   /// Constructor to set a particular expiry time relative to now.
   /**
    * This constructor creates a timer and sets the expiry time.
    *
    * @param context An execution context which provides the I/O executor that
    * the timer will use, by default, to dispatch handlers for any asynchronous
    * operations performed on the timer.
    *
    * @param expiry_time The expiry time to be used for the timer, relative to
    * now.
    */
   template <typename ExecutionContext>
   explicit basic_waitable_timer(ExecutionContext& context,
       const duration& expiry_time,
       constraint_t<
         is_convertible<ExecutionContext&, execution_context&>::value
       > = 0)
     : impl_(0, 0, context)
   {
     boost::system::error_code ec;
     impl_.get_service().expires_after(
         impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_after");
   }
 
   /// Move-construct a basic_waitable_timer from another.
   /**
    * This constructor moves a timer from one object to another.
    *
    * @param other The other basic_waitable_timer object from which the move will
    * occur.
    *
    * @note Following the move, the moved-from object is in the same state as if
    * constructed using the @c basic_waitable_timer(const executor_type&)
    * constructor.
    */
   basic_waitable_timer(basic_waitable_timer&& other)
     : impl_(std::move(other.impl_))
   {
   }
 
   /// Move-assign a basic_waitable_timer from another.
   /**
    * This assignment operator moves a timer from one object to another. Cancels
    * any outstanding asynchronous operations associated with the target object.
    *
    * @param other The other basic_waitable_timer object from which the move will
    * occur.
    *
    * @note Following the move, the moved-from object is in the same state as if
    * constructed using the @c basic_waitable_timer(const executor_type&)
    * constructor.
    */
   basic_waitable_timer& operator=(basic_waitable_timer&& other)
   {
     impl_ = std::move(other.impl_);
     return *this;
   }
 
   // All timers have access to each other's implementations.
   template <typename Clock1, typename WaitTraits1, typename Executor1>
   friend class basic_waitable_timer;
 
   /// Move-construct a basic_waitable_timer from another.
   /**
    * This constructor moves a timer from one object to another.
    *
    * @param other The other basic_waitable_timer object from which the move will
    * occur.
    *
    * @note Following the move, the moved-from object is in the same state as if
    * constructed using the @c basic_waitable_timer(const executor_type&)
    * constructor.
    */
   template <typename Executor1>
   basic_waitable_timer(
       basic_waitable_timer<Clock, WaitTraits, Executor1>&& other,
       constraint_t<
         is_convertible<Executor1, Executor>::value
       > = 0)
     : impl_(std::move(other.impl_))
   {
   }
 
   /// Move-assign a basic_waitable_timer from another.
   /**
    * This assignment operator moves a timer from one object to another. Cancels
    * any outstanding asynchronous operations associated with the target object.
    *
    * @param other The other basic_waitable_timer object from which the move will
    * occur.
    *
    * @note Following the move, the moved-from object is in the same state as if
    * constructed using the @c basic_waitable_timer(const executor_type&)
    * constructor.
    */
   template <typename Executor1>
   constraint_t<
     is_convertible<Executor1, Executor>::value,
     basic_waitable_timer&
   > operator=(basic_waitable_timer<Clock, WaitTraits, Executor1>&& other)
   {
     basic_waitable_timer tmp(std::move(other));
     impl_ = std::move(tmp.impl_);
     return *this;
   }
 
   /// Destroys the timer.
   /**
    * This function destroys the timer, cancelling any outstanding asynchronous
    * wait operations associated with the timer as if by calling @c cancel.
    */
   ~basic_waitable_timer()
   {
   }
 
   /// Get the executor associated with the object.
   const executor_type& get_executor() noexcept
   {
     return impl_.get_executor();
   }
 
   /// Cancel any asynchronous operations that are waiting on the timer.
   /**
    * This function forces the completion of any pending asynchronous wait
    * operations against the timer. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * Cancelling the timer does not change the expiry time.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @throws boost::system::system_error Thrown on failure.
    *
    * @note If the timer has already expired when cancel() is called, then the
    * handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t cancel()
   {
     boost::system::error_code ec;
     std::size_t s = impl_.get_service().cancel(impl_.get_implementation(), ec);
     boost::asio::detail::throw_error(ec, "cancel");
     return s;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use non-error_code overload.) Cancel any asynchronous
   /// operations that are waiting on the timer.
   /**
    * This function forces the completion of any pending asynchronous wait
    * operations against the timer. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * Cancelling the timer does not change the expiry time.
    *
    * @param ec Set to indicate what error occurred, if any.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @note If the timer has already expired when cancel() is called, then the
    * handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t cancel(boost::system::error_code& ec)
   {
     return impl_.get_service().cancel(impl_.get_implementation(), ec);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Cancels one asynchronous operation that is waiting on the timer.
   /**
    * This function forces the completion of one pending asynchronous wait
    * operation against the timer. Handlers are cancelled in FIFO order. The
    * handler for the cancelled operation will be invoked with the
    * boost::asio::error::operation_aborted error code.
    *
    * Cancelling the timer does not change the expiry time.
    *
    * @return The number of asynchronous operations that were cancelled. That is,
    * either 0 or 1.
    *
    * @throws boost::system::system_error Thrown on failure.
    *
    * @note If the timer has already expired when cancel_one() is called, then
    * the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t cancel_one()
   {
     boost::system::error_code ec;
     std::size_t s = impl_.get_service().cancel_one(
         impl_.get_implementation(), ec);
     boost::asio::detail::throw_error(ec, "cancel_one");
     return s;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use non-error_code overload.) Cancels one asynchronous
   /// operation that is waiting on the timer.
   /**
    * This function forces the completion of one pending asynchronous wait
    * operation against the timer. Handlers are cancelled in FIFO order. The
    * handler for the cancelled operation will be invoked with the
    * boost::asio::error::operation_aborted error code.
    *
    * Cancelling the timer does not change the expiry time.
    *
    * @param ec Set to indicate what error occurred, if any.
    *
    * @return The number of asynchronous operations that were cancelled. That is,
    * either 0 or 1.
    *
    * @note If the timer has already expired when cancel_one() is called, then
    * the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t cancel_one(boost::system::error_code& ec)
   {
     return impl_.get_service().cancel_one(impl_.get_implementation(), ec);
   }
 
   /// (Deprecated: Use expiry().) Get the timer's expiry time as an absolute
   /// time.
   /**
    * This function may be used to obtain the timer's current expiry time.
    * Whether the timer has expired or not does not affect this value.
    */
   time_point expires_at() const
   {
     return impl_.get_service().expires_at(impl_.get_implementation());
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Get the timer's expiry time as an absolute time.
   /**
    * This function may be used to obtain the timer's current expiry time.
    * Whether the timer has expired or not does not affect this value.
    */
   time_point expiry() const
   {
     return impl_.get_service().expiry(impl_.get_implementation());
   }
 
   /// Set the timer's expiry time as an absolute time.
   /**
    * This function sets the expiry time. Any pending asynchronous wait
    * operations will be cancelled. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * @param expiry_time The expiry time to be used for the timer.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @throws boost::system::system_error Thrown on failure.
    *
    * @note If the timer has already expired when expires_at() is called, then
    * the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t expires_at(const time_point& expiry_time)
   {
     boost::system::error_code ec;
     std::size_t s = impl_.get_service().expires_at(
         impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_at");
     return s;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use non-error_code overload.) Set the timer's expiry time as
   /// an absolute time.
   /**
    * This function sets the expiry time. Any pending asynchronous wait
    * operations will be cancelled. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * @param expiry_time The expiry time to be used for the timer.
    *
    * @param ec Set to indicate what error occurred, if any.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @note If the timer has already expired when expires_at() is called, then
    * the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t expires_at(const time_point& expiry_time,
       boost::system::error_code& ec)
   {
     return impl_.get_service().expires_at(
         impl_.get_implementation(), expiry_time, ec);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Set the timer's expiry time relative to now.
   /**
    * This function sets the expiry time. Any pending asynchronous wait
    * operations will be cancelled. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * @param expiry_time The expiry time to be used for the timer.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @throws boost::system::system_error Thrown on failure.
    *
    * @note If the timer has already expired when expires_after() is called,
    * then the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t expires_after(const duration& expiry_time)
   {
     boost::system::error_code ec;
     std::size_t s = impl_.get_service().expires_after(
         impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_after");
     return s;
   }
 
 #if !defined(BOOST_ASIO_NO_DEPRECATED)
   /// (Deprecated: Use expiry().) Get the timer's expiry time relative to now.
   /**
    * This function may be used to obtain the timer's current expiry time.
    * Whether the timer has expired or not does not affect this value.
    */
   duration expires_from_now() const
   {
     return impl_.get_service().expires_from_now(impl_.get_implementation());
   }
 
   /// (Deprecated: Use expires_after().) Set the timer's expiry time relative
   /// to now.
   /**
    * This function sets the expiry time. Any pending asynchronous wait
    * operations will be cancelled. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * @param expiry_time The expiry time to be used for the timer.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @throws boost::system::system_error Thrown on failure.
    *
    * @note If the timer has already expired when expires_from_now() is called,
    * then the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t expires_from_now(const duration& expiry_time)
   {
     boost::system::error_code ec;
     std::size_t s = impl_.get_service().expires_from_now(
         impl_.get_implementation(), expiry_time, ec);
     boost::asio::detail::throw_error(ec, "expires_from_now");
     return s;
   }
 
   /// (Deprecated: Use expires_after().) Set the timer's expiry time relative
   /// to now.
   /**
    * This function sets the expiry time. Any pending asynchronous wait
    * operations will be cancelled. The handler for each cancelled operation will
    * be invoked with the boost::asio::error::operation_aborted error code.
    *
    * @param expiry_time The expiry time to be used for the timer.
    *
    * @param ec Set to indicate what error occurred, if any.
    *
    * @return The number of asynchronous operations that were cancelled.
    *
    * @note If the timer has already expired when expires_from_now() is called,
    * then the handlers for asynchronous wait operations will:
    *
    * @li have already been invoked; or
    *
    * @li have been queued for invocation in the near future.
    *
    * These handlers can no longer be cancelled, and therefore are passed an
    * error code that indicates the successful completion of the wait operation.
    */
   std::size_t expires_from_now(const duration& expiry_time,
       boost::system::error_code& ec)
   {
     return impl_.get_service().expires_from_now(
         impl_.get_implementation(), expiry_time, ec);
   }
 #endif // !defined(BOOST_ASIO_NO_DEPRECATED)
 
   /// Perform a blocking wait on the timer.
   /**
    * This function is used to wait for the timer to expire. This function
    * blocks and does not return until the timer has expired.
    *
    * @throws boost::system::system_error Thrown on failure.
    */
   void wait()
   {
     boost::system::error_code ec;
     impl_.get_service().wait(impl_.get_implementation(), ec);
     boost::asio::detail::throw_error(ec, "wait");
   }
 
   /// Perform a blocking wait on the timer.
   /**
    * This function is used to wait for the timer to expire. This function
    * blocks and does not return until the timer has expired.
    *
    * @param ec Set to indicate what error occurred, if any.
    */
   void wait(boost::system::error_code& ec)
   {
     impl_.get_service().wait(impl_.get_implementation(), ec);
   }
 
   /// Start an asynchronous wait on the timer.
   /**
    * This function may be used to initiate an asynchronous wait against the
    * timer. It is an initiating function for an @ref asynchronous_operation,
    * and always returns immediately.
    *
    * For each call to async_wait(), the completion handler will be called
    * exactly once. The completion handler will be called when:
    *
    * @li The timer has expired.
    *
    * @li The timer was cancelled, in which case the handler is passed the error
    * code boost::asio::error::operation_aborted.
    *
    * @param token The @ref completion_token that will be used to produce a
    * completion handler, which will be called when the timer expires. Potential
    * completion tokens include @ref use_future, @ref use_awaitable, @ref
    * yield_context, or a function object with the correct completion signature.
    * The function signature of the completion handler must be:
    * @code void handler(
    *   const boost::system::error_code& error // Result of operation.
    * ); @endcode
    * Regardless of whether the asynchronous operation completes immediately or
    * not, the completion handler will not be invoked from within this function.
    * On immediate completion, invocation of the handler will be performed in a
    * manner equivalent to using boost::asio::post().
    *
    * @par Completion Signature
    * @code void(boost::system::error_code) @endcode
    *
    * @par Per-Operation Cancellation
    * This asynchronous operation supports cancellation for the following
    * boost::asio::cancellation_type values:
    *
    * @li @c cancellation_type::terminal
    *
    * @li @c cancellation_type::partial
    *
    * @li @c cancellation_type::total
    */
   template <
       BOOST_ASIO_COMPLETION_TOKEN_FOR(void (boost::system::error_code))
         WaitToken = default_completion_token_t<executor_type>>
   auto async_wait(
       WaitToken&& token = default_completion_token_t<executor_type>())
     -> decltype(
       async_initiate<WaitToken, void (boost::system::error_code)>(
         declval<initiate_async_wait>(), token))
   {
     return async_initiate<WaitToken, void (boost::system::error_code)>(
         initiate_async_wait(this), token);
   }
 
 private:
   // Disallow copying and assignment.
   basic_waitable_timer(const basic_waitable_timer&) = delete;
   basic_waitable_timer& operator=(const basic_waitable_timer&) = delete;
 
   class initiate_async_wait
   {
   public:
     typedef Executor executor_type;
 
     explicit initiate_async_wait(basic_waitable_timer* self)
       : self_(self)
     {
     }
 
     const executor_type& get_executor() const noexcept
     {
       return self_->get_executor();
     }
 
     template <typename WaitHandler>
     void operator()(WaitHandler&& handler) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a WaitHandler.
       BOOST_ASIO_WAIT_HANDLER_CHECK(WaitHandler, handler) type_check;
 
       detail::non_const_lvalue<WaitHandler> handler2(handler);
       self_->impl_.get_service().async_wait(
           self_->impl_.get_implementation(),
           handler2.value, self_->impl_.get_executor());
     }
 
   private:
     basic_waitable_timer* self_;
   };
 
   detail::io_object_impl<
     detail::deadline_timer_service<
       detail::chrono_time_traits<Clock, WaitTraits>>,
     executor_type > impl_;
 };
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #endif // BOOST_ASIO_BASIC_WAITABLE_TIMER_HPP

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