time_unlimited.hpp

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/*!
 * \file
 * \brief Implementation of time-unlimited asynchronous one-time operation.
 *
 * \since
 * v.1.0.4
 */
 
#pragma once
 
#include <so_5_extra/async_op/details.hpp>
#include <so_5_extra/async_op/errors.hpp>
 
#include <so_5/details/invoke_noexcept_code.hpp>
 
#include <so_5/agent.hpp>
 
#include <so_5/outliving.hpp>
 
#include <vector>
 
namespace so_5 {
 
namespace extra {
 
namespace async_op {
 
namespace time_unlimited {
 
//! Enumeration for status of operation.
enum class status_t
    {
        //! Status of operation is unknown because the
        //! operation data has been moved to another proxy-object.
        unknown_moved_away,
        //! Operation is not activated yet.
        not_activated,
        //! Operation is activated.
        activated,
        //! Operation is completed.
        completed,
        //! Operation is cancelled.
        cancelled
    };
 
// Forward declarations.
// Necessary to declare friends for details::op_data_t.
 
template< typename Operation_Data >
class cancellation_point_t;
 
template< typename Operation_Data >
class definition_point_t;
 
namespace details
{
 
/*!
 * \brief A main class for implementation of time-unlimited
 * asynchronous one-time operation.
 *
 * This class contains an information about completion handlers for
 * async operation.
 *
 * Do not create objects of this class directly (unless you really know
 * what you are doing).
 * Use so_5::extra::async_op::time_unlimited::make() helper function for that.
 *
 * \note
 * Instances of that class should be created only as dynamically allocated
 * objects. It is because smart pointers are created inside the instance
 * (for example completed_on() creates intrusive_ptr_t for `this`). Because
 * of that this class has a protected destructor.
 *
 * \attention
 * This is not a thread-safe type. It is beter and safer to work with
 * an object of that type inside an agent for which that object is created.
 *
 * \since
 * v.1.0.4
 */
class op_data_t : protected ::so_5::atomic_refcounted_t
    {
        friend class ::so_5::intrusive_ptr_t<op_data_t>;
 
        template< typename Operation_Data >
        friend class ::so_5::extra::async_op::time_unlimited::cancellation_point_t;
        template< typename Operation_Data >
        friend class ::so_5::extra::async_op::time_unlimited::definition_point_t;
 
    private :
        //! Description of one subscription.
        struct subscription_data_t
            {
                //! Mbox from that a message is expected.
                ::so_5::mbox_t m_mbox;
 
                //! State for that a subscription should be created.
                /*!
                 * \attention
                 * Can't be nullptr.
                 */
                const ::so_5::state_t * m_state;
 
                //! Subscription type.
                /*!
                 * \note
                 * This is a subscription type. Not a type which will
                 * be passed to the event handler.
                 */
                std::type_index m_subscription_type;
 
                //! Event handler.
                ::so_5::event_handler_method_t m_handler;
 
                //! Initializing constructor.
                subscription_data_t(
                    so_5::mbox_t mbox,
                    const so_5::state_t & state,
                    std::type_index subscription_type,
                    so_5::event_handler_method_t handler )
                    :   m_mbox( std::move(mbox) )
                    ,   m_state( &state )
                    ,   m_subscription_type( std::move(subscription_type) )
                    ,   m_handler( std::move(handler) )
                    {}
            };
 
        //! Owner of async operation.
        ::so_5::outliving_reference_t<::so_5::agent_t> m_owner;
 
        //! The status of the async operation.
        status_t m_status = status_t::not_activated;
 
        //! Subscriptions which should be created on activation.
        std::vector< subscription_data_t > m_subscriptions;
 
    protected :
        // NOTE: constructor and destructor will be available only
        // for friends and for derived classes.
 
        //! Initializing constructor.
        op_data_t(
            //! An agent which owns async operation.
            ::so_5::outliving_reference_t<::so_5::agent_t> owner )
            :   m_owner(owner)
            {}
 
        ~op_data_t() noexcept
            {}
 
        //! Reserve a capacity for vector with subscriptions' data.
        void
        reserve( std::size_t capacity )
            {
                m_subscriptions.reserve( capacity );
            }
 
        //! Add an operation completion handler.
        /*!
         * This method stores description of a competion handler. This
         * description will be used later in activate() method for subscription
         * to a completion message/signal.
         *
         * \tparam Operation_Data An actual type of operation data object.
         * In the most cases it will be op_data_t itself. But for debugging or
         * testing purposes it can also be a derived class. This type is
         * necessary to store the right smart pointer to actual operation
         * data object in event handler wrapper.
         *
         * \tparam Msg_Target A type of destination for message/signal about
         * the completion of the async operation.
         * It can be a const reference to so_5::mbox_t, so_5::agent_t,
         * so_5::adhoc_agent_definition_proxy_t.
         *
         * \tparam Event_Handler A type of handler for message/signal about
         * the completion of the async operation.
         */
        template<
                typename Operation_Data,
                typename Msg_Target,
                typename Event_Handler >
        void
        add_completion_handler(
            //! A smart pointer to the actual instance of operation data.
            ::so_5::intrusive_ptr_t< Operation_Data > actual_data,
            //! A destination for message about operation completion.
            Msg_Target && msg_target,
            //! A state in which message should be handled.
            const ::so_5::state_t & state,
            //! A message handler.
            Event_Handler && evt_handler )
            {
                ensure_not_activated();
 
                const auto mbox = ::so_5::extra::async_op::details::target_to_mbox(
                        msg_target );
 
                auto evt_handler_info =
                        ::so_5::preprocess_agent_event_handler(
                                mbox,
                                m_owner.get(),
                                std::forward<Event_Handler>(evt_handler) );
 
                ::so_5::event_handler_method_t actual_handler =
                    [self = std::move(actual_data),
                    user_handler = std::move(evt_handler_info.m_handler)](
                        ::so_5::message_ref_t & msg )
                    {
                        self->completed();
                        user_handler( msg );
                    };
 
                m_subscriptions.emplace_back(
                        mbox,
                        state,
                        evt_handler_info.m_msg_type,
                        std::move(actual_handler) );
            }
 
        /*!
         * \brief Performs all necessary activation actions.
         *
         * \note
         * It throws if:
         * - the operation is already activated;
         * - there is no defined completion handlers.
         */
        void
        activate()
            {
                ensure_not_activated();
 
                if( !m_subscriptions.empty() )
                    {
                        create_subscriptions();
                        m_status = status_t::activated;
                    }
                else
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::async_op::errors::rc_no_completion_handler,
                            "Operation can't be activated without any completion "
                            "handler" );
            }
 
        //! Cancel async operation.
        /*!
         * If an async operation is in progress then all subscriptions
         * will be destroyed and all information about completion handlers
         * will be erased.
         *
         * \note
         * It is safe to cancel an operation which wasn't activacted or
         * was already finished.
         * If cancel() is called before activate() then all information
         * about completion handlers created by previous calls to
         * add_completion_handler() will be lost.
         *
         * \attention
         * This method is marked as noexcept because event unsubscription
         * operations shouldn't throw. But if such operation throws then
         * there is no way to recover.
         */
        void
        cancel() noexcept
            {
                if( status_t::activated == m_status )
                    {
                        destroy_and_clear_subscriptions();
                        m_status = status_t::cancelled;
                    }
                else if( status_t::not_activated == m_status )
                    {
                        m_subscriptions.clear();
                    }
            }
 
        //! Get the current status of the operation.
        [[nodiscard]] status_t
        current_status() const noexcept
            {
                return m_status;
            }
 
        //! Is there any completion handler?
        [[nodiscard]] bool
        has_completion_handlers() const noexcept
            {
                return !m_subscriptions.empty();
            }
 
    private :
        //! Check if operation is activated and throw an exception if it is.
        void
        ensure_not_activated()
            {
                if( status_t::activated == m_status )
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::async_op::errors::rc_async_op_activated,
                            "Operation can't be performed when async_op is already "
                            "activated" );
            }
 
        //! Perform operation completion procedure.
        /*!
         * All subscriptions will be destroyed. All information about
         * subscriptions will be deleted.
         *
         * Status will be changed to status_t::completed.
         */
        void
        completed() noexcept
            {
                destroy_and_clear_subscriptions();
                m_status = status_t::completed;
            }
 
        //! Subscribe agent for all subscriptions in the subscriptions' container.
        /*!
         * \note
         * If an exception is thrown during subscription then all previously
         * created subscriptions will be destroyed.
         */
        void
        create_subscriptions()
            {
                // All subscriptions must be destroyed if an exception
                // is thrown.
                std::size_t current_index = 0;
                try
                    {
                        for(; current_index != m_subscriptions.size(); ++current_index )
                            {
                                auto & sd = m_subscriptions[ current_index ];
                                m_owner.get().so_create_event_subscription(
                                        sd.m_mbox,
                                        sd.m_subscription_type,
                                        *(sd.m_state),
                                        sd.m_handler,
                                        ::so_5::thread_safety_t::unsafe,
                                        ::so_5::event_handler_kind_t::final_handler );
                            }
                    }
                catch(...)
                    {
                        // All created subscriptions should be dropped.
                        destroy_subscriptions_up_to( current_index );
 
                        throw;
                    }
            }
 
        //! Destroy all subscriptions and clean subscriptions' container.
        void
        destroy_and_clear_subscriptions() noexcept
            {
                destroy_subscriptions_up_to( m_subscriptions.size() );
                m_subscriptions.clear();
            }
 
        //! Destroy subscriptions in range [0..n).
        void
        destroy_subscriptions_up_to(
            //! Last index of subscription in m_subscriptions container
            //! which shouldn't be included.
            const std::size_t n ) noexcept
            {
                for( std::size_t i = 0; i != n; ++i )
                    {
                        const auto & sd = m_subscriptions[ i ];
                        m_owner.get().so_destroy_event_subscription(
                                sd.m_mbox,
                                sd.m_subscription_type,
                                *(sd.m_state) );
                    }
            }
    };
 
//
// op_shptr_t
//
/*!
 * \brief An alias for smart pointer to operation data.
 *
 * \since
 * v.1.0.4
 */
template< typename Operation_Data >
using op_shptr_t = ::so_5::intrusive_ptr_t< Operation_Data >;
 
} /* namespace details */
 
/*!
 * \brief An object that allows to cancel async operation.
 *
 * Usage example:
 * \code
 * namespace asyncop = so_5::extra::async_op::time_unlimited;
 * class demo : public so_5::agent_t {
 *  asyncop::cancellation_point_t<> cp_;
 *  ...
 *  void initiate_async_op() {
 *      auto op = asyncop::make<timeout>(*this);
 *      op.completed_on(...);
 *      cp_ = op.activate(...);
 *  }
 *  void on_interruption_signal(mhood_t<interrupt_activity> cmd) {
 *      // Operation should be cancelled.
 *      cp_.cancel();
 *      ...
 *  }
 * };
 * \endcode
 *
 * \note
 * This class is DefaultConstructible and Moveable, but not Copyable
 * and not CopyConstructible.
 *
 * \attention
 * Objects of this class are not thread safe. It means that cancellation
 * point should be used only by agent which created it. And the cancellation
 * point can't be used inside thread-safe event handlers of that agent.
 *
 * \tparam Operation_Data Type of actual operation data representation.
 * Please note that this template parameter is indended to be used for
 * debugging and testing purposes only.
 *
 * \since
 * v.1.0.4
 */
template< typename Operation_Data = details::op_data_t >
class cancellation_point_t
    {
    private :
        template<typename Op_Data> friend class definition_point_t;
 
        //! Actual data for async op.
        /*!
         * \note
         * This can be a nullptr if the default constructor was used,
         * or if operation is already cancelled, or if the content of
         * the cancellation_point was moved to another object.
         */
        details::op_shptr_t< Operation_Data > m_op;
 
        //! Initializing constructor to be used by definition_point.
        cancellation_point_t(
            //! Actual data for async operation.
            //! Can't be null.
            details::op_shptr_t< Operation_Data > op )
            :   m_op( std::move(op) )
            {}
 
    public :
        cancellation_point_t() = default;
 
        cancellation_point_t( const cancellation_point_t & ) = delete;
        cancellation_point_t( cancellation_point_t && ) = default;
 
        cancellation_point_t &
        operator=( const cancellation_point_t & ) = delete;
 
        cancellation_point_t &
        operator=( cancellation_point_t && ) = default;
 
        //! Get the status of the operation.
        /*!
         * \note
         * The value status_t::unknown_moved_away can be returned if
         * the actual data of the async operation was moved to another object
         * (like another cancellation_point_t). Or after a call to
         * cleanup() method.
         */
        [[nodiscard]] status_t
        status() const noexcept
            {
                if( m_op)
                    return m_op->current_status();
                return status_t::unknown_moved_away;
            }
 
        //! Can the async operation be cancelled via this cancellation point?
        /*!
         * \return true if the cancellation_point holds actual async operation's
         * data and this async operation is not completed yet.
         */
        [[nodiscard]] bool
        is_cancellable() const noexcept
            {
                return m_op && status_t::activated == m_op->current_status();
            }
 
        //! An attempt to cancel the async operation.
        /*!
         * \note
         * Operation will be cancelled only if (true == is_cancellable()).
         *
         * It is safe to call cancel() if the operation is already
         * cancelled or completed. In that case the call to
         * cancel() will have no effect.
         */
        void
        cancel() noexcept
            {
                if( is_cancellable() )
                    {
                        m_op->cancel();
                    }
            }
 
        //! Throw out a reference to the async operation data.
        /*!
         * A cancellation_point holds a reference to the async operation
         * data. It means that the async operation data will be destroyed
         * only when the cancellation_point will be destroyed. For example,
         * in that case:
         * \code
         * namespace asyncop = so_5::extra::async_op::time_unlimited;
         * class demo : public so_5::agent_t {
         *  ...
         *  asyncop::cancellation_point_t<> cp_;
         *  ...
         *  void initiate_async_op() {
         *      cp_ = asyncop::make<timeout_msg>(*this)
         *              ...
         *              .activate(...);
         *      ...
         *  }
         *  ...
         *  void on_some_interruption() {
         *      // Cancel asyncop.
         *      cp_.cancel();
         *      // Async operation is cancelled, but the async operation
         *      // data is still in memory. The data will be deallocated
         *      // only when cp_ receives new value or when cp_ will be
         *      // destroyed (e.g. after destruction of demo agent).
         *  }
         * \endcode
         *
         * A call to cleanup() method removes the reference to the async
         * operation data. It means that if the operation is already
         * completed or cancelled, then the operation data
         * fill be deallocated.
         *
         * \note
         * If the operation is still in progress then a call to cleanup()
         * doesn't break the operation. You need to call cancel() manually
         * before calling cleanup() to cancel the operation.
         */
        void
        cleanup() noexcept
            {
                m_op.reset();
            }
    };
 
/*!
 * \brief An interface for definition of async operation.
 *
 * Object of this type is usually created by make() function and
 * is used for definition of async operation. Completion and timeout
 * handlers are set for async operation by using definition_point object.
 *
 * Then an user calls activate() method and the definition_point transfers
 * the async operation data into cancellation_point object. It means that
 * after a call to activate() method the definition_point object should
 * not be used. Because it doesn't hold any async operation anymore.
 *
 * A simple usage without storing the cancellation_point:
 * \code
 * namespace asyncop = so_5::extra::async_op::time_unlimited;
 * class demo : public so_5::agent_t {
 *  ...
 *  void initiate_async_op() {
 *      // Create a definition_point for a new async operation...
 *      asyncop::make(*this)
 *          // ...then set up completion handler(s)...
 *          .completed_on(
 *              *this,
 *              so_default_state(),
 *              &demo::on_first_completion_msg )
 *          .completed_on(
 *              some_external_mbox_,
 *              some_user_defined_state_,
 *              [this](mhood_t<another_completion_msg> cmd) {...})
 *          // ...and now we can activate the operation.
 *          .activate();
 *      ...
 *  }
 * };
 * \endcode
 * \note
 * There is no need to hold definition_point object after activation
 * of the async operation. This object can be safely discarded.
 *
 * A more complex example using cancellation_point for
 * cancelling the operation.
 * \code
 * namespace asyncop = so_5::extra::async_op::time_unlimited;
 * class demo : public so_5::agent_t {
 *  ...
 *  // Cancellation point for the async operation.
 *  asyncop::cancellation_point_t<> cp_;
 *  ...
 *  void initiate_async_op() {
 *      // Create a definition_point for a new async operation
 *      // and store the cancellation point after activation.
 *      cp_ = asyncop::make(*this)
 *          // ...then set up completion handler(s)...
 *          .completed_on(
 *              *this,
 *              so_default_state(),
 *              &demo::on_first_completion_msg )
 *          .completed_on(
 *              some_external_mbox_,
 *              some_user_defined_state_,
 *              [this](mhood_t<another_completion_msg> cmd) {...})
 *          // ...and now we can activate the operation.
 *          .activate();
 *      ...
 *  }
 *  ...
 *  void on_abortion_signal(mhood_t<abort_signal>) {
 *      // The current async operation should be cancelled.
 *      cp_.cancel();
 *  }
 * };
 * \endcode
 *
 * \note
 * There are two forms of activate() method. The first one doesn't receive
 * any arguments. It was shown in the examples above. The second one receives
 * the lambda-function as argument. It can be used when some addtional
 * actions should be performed during the activation of the async operation.
 * For example:
 * \code
 * void initiate_async_op() {
 *  asyncop::make(*this)
 *  .completed_on(...)
 *  .completed_on(...)
 *  .activate([this] {
 *          // Several messages must be sent at the start of async op.
 *          so_5::send<first_initial_msg>(some_target, ...);
 *          so_5::send<second_initial_msg>(some_target, ...);
 *          ...
 *      });
 * }
 * \endcode
 *
 * \note
 * This class is Moveable, but not DefaultConstructible nor Copyable.
 *
 * \attention
 * Objects of this class are not thread safe. It means that a definition
 * point should be used only by agent which created it. And the definition
 * point can't be used inside thread-safe event handlers of that agent.
 *
 * \since
 * v.1.0.4
 */
template<
    typename Operation_Data = details::op_data_t >
class definition_point_t
    {
        //! Actual operation data.
        /*!
         * \note
         * This pointer can be nullptr after activation or after the
         * content of the object is moved away.
         */
        details::op_shptr_t< Operation_Data > m_op;
 
        //! Checks that the definition_point owns async operation data.
        void
        ensure_not_empty() const
            {
                if( !m_op )
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::async_op::errors::rc_empty_definition_point_object,
                            "an attempt to use empty definition_point object" );
            }
 
    public :
        //! Initializing constructor.
        definition_point_t(
            //! The owner of the async operation.
            ::so_5::outliving_reference_t< ::so_5::agent_t > owner )
            :   m_op( new Operation_Data( owner ) )
            {}
 
        ~definition_point_t()
            {
                // If operation data is still here then it means that
                // there wasn't call to `activate()` and we should cancel
                // all described handlers.
                // This will lead to deallocation of operation data.
                if( this->m_op )
                    this->m_op->cancel();
            }
 
        definition_point_t( const definition_point_t & ) = delete;
        definition_point_t &
        operator=( const definition_point_t & ) = delete;
 
        definition_point_t( definition_point_t && ) = default;
        definition_point_t &
        operator=( definition_point_t && ) = default;
 
        /*!
         * \brief Reserve a space for storage of completion handlers.
         *
         * Usage example:
         * \code
         * namespace asyncop = so_5::extra::async_op::time_unlimited;
         * auto op = asyncop::make(some_agent);
         * // Reserve space for four completion handlers.
         * op.reserve_completion_handlers_capacity(4);
         * op.completed_on(...);
         * op.completed_on(...);
         * op.completed_on(...);
         * op.completed_on(...);
         * op.activate();
         * \endcode
         */
        definition_point_t &
        reserve_completion_handlers_capacity(
            //! A required capacity.
            std::size_t capacity ) &
            {
                ensure_not_empty();
 
                m_op->reserve( capacity );
 
                return *this;
            }
 
        //! Just a proxy for actual version of %reserve_completion_handlers_capacity.
        template< typename... Args >
        auto
        reserve_completion_handlers_capacity( Args && ...args ) &&
            {
                return std::move(this->reserve_completion_handlers_capacity(
                            std::forward<Args>(args)... ));
            }
 
        /*!
         * \brief Checks if the async operation can be activated.
         *
         * The operation can be activated if the definition_point still
         * holds the operation data (e.g. operation is not activated yet) and
         * there is at least one completion handler for the operation.
         */
        [[nodiscard]] bool
        is_activable() const noexcept
            {
                // Operation is activable if we still hold the operation data.
                return m_op && m_op->has_completion_handlers();
            }
 
        /*!
         * \brief Add a completion handler for the async operation.
         *
         * Usage example:
         * \code
         * namespace asyncop = so_5::extra::async_op::time_unlimited;
         * class demo : public so_5::agent_t {
         *  ...
         *  void initiate_async_op() {
         *          asyncop::make<timeout>(*this)
         *              .completed_on(
         *                  *this,
         *                  so_default_state(),
         *                  &demo::some_event )
         *              .completed_on(
         *                  some_mbox_,
         *                  some_agent_state_,
         *                  [this](mhood_t<some_msg> cmd) {...})
         *              ...
         *              .activate(...);
         *  }
         * };
         * \endcode
         *
         * \note
         * The completion handler will be stored inside async operation
         * data. Actual subscription for it will be made during activation
         * of the async operation.
         *
         * \tparam Msg_Target It can be a mbox, or a reference to an agent.
         * In the case if Msg_Target if a reference to an agent the agent's
         * direct mbox will be used as message source.
         *
         * \tparam Event_Handler Type of actual handler for message/signal.
         * It can be a pointer to agent's method or lambda (or another
         * type of functional object).
         */
        template<
                typename Msg_Target,
                typename Event_Handler >
        definition_point_t &
        completed_on(
            //! A source from which a completion message is expected.
            //! It \a msg_target is a reference to an agent then
            //! the agent's direct mbox will be used.
            Msg_Target && msg_target,
            //! A state for which that completion handler will be subscribed.
            const ::so_5::state_t & state,
            //! The completion handler itself.
            Event_Handler && evt_handler ) &
            {
                ensure_not_empty();
 
                this->m_op->add_completion_handler(
                        this->m_op,
                        std::forward<Msg_Target>(msg_target),
                        state,
                        std::forward<Event_Handler>(evt_handler) );
 
                return *this;
            }
 
        //! Just a proxy for the main version of %completed_on.
        template< typename... Args >
        definition_point_t &&
        completed_on( Args && ...args ) &&
            {
                return std::move(this->completed_on(std::forward<Args>(args)...));
            }
 
        //! Activate async operation with addition starting action.
        /*!
         * This method performs two steps:
         *
         * 1. Activates the async operation.
         * 2. Calls \a action lambda-function (or functional object).
         *
         * It an exception is thrown from \a action then the activated
         * async operation will be cancelled automatically.
         *
         * Usage example:
         * \code
         * namespace asyncop = so_5::extra::async_op::time_unlimited;
         * class demo : public so_5::agent_t {
         * public:
         *  ...
         *  virtual void so_evt_start() override {
         *      // Set up operation completion handlers...
         *      asyncop::make(*this)
         *          .completed_on(...)
         *          .completed_on(...)
         *          .completed_on(...)
         *          // And now the operation can be activated.
         *          .activate([this] {
         *              ... // Some initial actions like sending a message...
         *          });
         *      ...
         *  }
         * };
         * \endcode
         *
         * \attention
         * It throws if `!is_activable()`.
         *
         * \attention
         * If an exception is thrown during the activation procedure then
         * the definition_point will become empty. It means that after an
         * exception from activate() the definition_point object should not
         * be used.
         *
         * \tparam Activation_Action A type of lambda-function or functional
         * object to be excecuted on activation of operation.
         */
        template< typename Activation_Action >
        cancellation_point_t< Operation_Data >
        activate(
            //! A lambda-function or functional object which will be called
            //! after creation of all necessary subscriptions and switching
            //! to activated status.
            Activation_Action && action ) &
            {
                if( !this->is_activable() )
                    SO_5_THROW_EXCEPTION(
                            ::so_5::extra::async_op::errors::rc_op_cant_be_activated,
                            "definition_point_t doesn't hold operation data anymore." );
                auto op = std::move(m_op);
 
                op->activate();
 
                ::so_5::details::do_with_rollback_on_exception(
                    [&] { action(); },
                    [&] { op->cancel(); } );
 
                return { std::move(op) };
            }
 
        //! Activate async operation.
        /*!
         * Usage example:
         * \code
         * namespace asyncop = so_5::extra::async_op::time_unlimited;
         * class demo : public so_5::agent_t {
         * public:
         *  ...
         *  virtual void so_evt_start() override {
         *      // Create an async operation...
         *      asyncop::make(*this)
         *          // Then set up completion handlers...
         *          ->completed_on(...)
         *          .completed_on(...)
         *          .completed_on(...)
         *          // And now the operation can be activated.
         *          .activate();
         *      ...
         *  }
         * };
         * \endcode
         *
         * \attention
         * It throws if `!is_activable()`.
         *
         * \attention
         * If an exception is thrown during the activation procedure then
         * the definition_point will become empty. It means that after an
         * exception from activate() the definition_point object should not
         * be used.
         */
        cancellation_point_t< Operation_Data >
        activate() &
            {
                return this->activate( []{/* Nothing to do*/} );
            }
 
        //! Just a proxy for actual %activate() methods.
        template< typename... Args >
        auto
        activate( Args && ...args ) &&
            {
                return this->activate( std::forward<Args>(args)... );
            }
    };
 
//
// make
//
/*!
 * \brief Helper function for creation an instance of async operation.
 *
 * Instead of creation of op_data_t's instances directly by hand this
 * factory function should be used:
 * \code
 * namespace asyncop = so_5::extra::async_op::time_unlimited;
 * class demo : public so_5::agent_t {
 * public:
 *  ...
 *  virtual void so_evt_start() override {
 *      // Create an async operation...
 *      asyncop::make(*this)
 *          // ...set up operation completion handlers...
 *          .completed_on(...)
 *          .completed_on(...)
 *          .completed_on(...)
 *          // And now the operation can be activated.
 *          .activate();
 *      ...
 *  }
 * };
 * \endcode
 *
 * \since
 * v.1.0.4
 */
[[nodiscard]] inline definition_point_t<details::op_data_t>
make(
    //! Agent for that this async operation will be created.
    ::so_5::agent_t & owner )
    {
        return { ::so_5::outliving_mutable(owner) };
    }
 
} /* namespace time_unlimited */
 
} /* namespace async_op */
 
} /* namespace extra */
 
} /* namespace so_5 */