A single_assignment messaging block is a multi-target, multi-source, ordered propagator_block capable of storing a single, write-once message. More...

#include <agents.h>

Inheritance diagram for Concurrency::single_assignment< _Type >:

Public Member Functions
	single_assignment ()
	Constructs a `single_assignment` messaging block. More...

	single_assignment (filter_method const &_Filter)
	Constructs a `single_assignment` messaging block. More...

	~single_assignment ()
	Destroys the `single_assignment` messaging block. More...

bool	has_value () const
	Checks whether this `single_assignment` messaging block has been initialized with a value yet. More...

_Type const &	value ()
	Gets a reference to the current payload of the message being stored in the `single_assignment` messaging block. More...

Public Member Functions inherited from Concurrency::propagator_block< multi_link_registry< ITarget< _Type > >, multi_link_registry< ISource< _Type > > >
	propagator_block ()
	Constructs a `propagator_block` object. More...

virtual	~propagator_block ()
	Destroys a `propagator_block` object. More...

virtual message_status	propagate (_Inout_opt_ message< _Source_type > _PMessage, _Inout_opt_ ISource< _Source_type > _PSource)
	Asynchronously passes a message from a source block to this target block. More...

virtual message_status	send (_Inout_ message< _Source_type > _PMessage, _Inout_ ISource< _Source_type > _PSource)
	Synchronously initiates a message to this block. Called by an `ISource` block. When this function completes, the message will already have propagated into the block. More...

Public Member Functions inherited from Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >
	source_block ()
	Constructs a `source_block` object. More...

virtual	~source_block ()
	Destroys the `source_block` object. More...

virtual void	link_target (_Inout_ ITarget< _Target_type > *_PTarget)
	Links a target block to this `source_block` object. More...

virtual void	unlink_target (_Inout_ ITarget< _Target_type > *_PTarget)
	Unlinks a target block from this `source_block` object. More...

virtual void	unlink_targets ()
	Unlinks all target blocks from this `source_block` object. More...

virtual message< _Target_type > *	accept (runtime_object_identity _MsgId, _Inout_ ITarget< _Target_type > *_PTarget)
	Accepts a message that was offered by this `source_block` object, transferring ownership to the caller. More...

virtual bool	reserve (runtime_object_identity _MsgId, _Inout_ ITarget< _Target_type > *_PTarget)
	Reserves a message previously offered by this `source_block` object. More...

virtual message< _Target_type > *	consume (runtime_object_identity _MsgId, _Inout_ ITarget< _Target_type > *_PTarget)
	Consumes a message previously offered by this `source_block` object and successfully reserved by the target, transferring ownership to the caller. More...

virtual void	release (runtime_object_identity _MsgId, _Inout_ ITarget< _Target_type > *_PTarget)
	Releases a previous successful message reservation. More...

virtual void	acquire_ref (_Inout_ ITarget< _Target_type > *)
	Acquires a reference count on this `source_block` object, to prevent deletion. More...

virtual void	release_ref (_Inout_ ITarget< _Target_type > *_PTarget)
	Releases a reference count on this `source_block` object. More...

Public Member Functions inherited from Concurrency::ISource< multi_link_registry< ITarget< _Type > >::type::type >
virtual	~ISource ()
	Destroys the `ISource` object. More...

virtual void	link_target (_Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, links a target block to this `ISource` block. More...

virtual void	unlink_target (_Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, unlinks a target block from this `ISource` block, if found to be previously linked. More...

virtual message< multi_link_registry< ITarget< _Type > >::type::type > *	accept (runtime_object_identity _MsgId, _Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, accepts a message that was offered by this `ISource` block, transferring ownership to the caller. More...

virtual bool	reserve (runtime_object_identity _MsgId, _Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, reserves a message previously offered by this `ISource` block. More...

virtual message< multi_link_registry< ITarget< _Type > >::type::type > *	consume (runtime_object_identity _MsgId, _Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, consumes a message previously offered by this `ISource` block and successfully reserved by the target, transferring ownership to the caller. More...

virtual void	release (runtime_object_identity _MsgId, _Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, releases a previous successful message reservation. More...

virtual void	acquire_ref (_Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, acquires a reference count on this `ISource` block, to prevent deletion. More...

virtual void	release_ref (_Inout_ ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PTarget)=0
	When overridden in a derived class, releases a reference count on this `ISource` block. More...

Public Member Functions inherited from Concurrency::ITarget< multi_link_registry< ISource< _Type > >::type::source_type >
virtual	~ITarget ()
	Destroys the `ITarget` object. More...

virtual message_status	propagate (_Inout_opt_ message< multi_link_registry< ISource< _Type > >::type::source_type > _PMessage, _Inout_opt_ ISource< multi_link_registry< ISource< _Type > >::type::source_type > _PSource)=0
	When overridden in a derived class, asynchronously passes a message from a source block to this target block. More...

virtual message_status	send (_Inout_ message< multi_link_registry< ISource< _Type > >::type::source_type > _PMessage, _Inout_ ISource< multi_link_registry< ISource< _Type > >::type::source_type > _PSource)=0
	When overridden in a derived class, synchronously passes a message to the target block. More...

virtual bool	supports_anonymous_source ()
	When overridden in a derived class, returns true or false depending on whether the message block accepts messages offered by a source that is not linked to it. If the overridden method returns `true`, the target cannot postpone an offered message, as consumption of a postponed message at a later time requires the source to be identified in its sourse link registry. More...

Protected Member Functions
virtual message_status	propagate_message (_Inout_ message< _Type > _PMessage, _Inout_ ISource< _Type > _PSource)
	Asynchronously passes a message from an `ISource` block to this `single_assignment` messaging block. It is invoked by the `propagate` method, when called by a source block. More...

virtual message_status	send_message (_Inout_ message< _Type > _PMessage, _Inout_ ISource< _Type > _PSource)
	Synchronously passes a message from an `ISource` block to this `single_assignment` messaging block. It is invoked by the `send` method, when called by a source block. More...

virtual message< _Type > *	accept_message (runtime_object_identity _MsgId)
	Accepts a message that was offered by this `single_assignment` messaging block, returning a copy of the message to the caller. More...

virtual bool	reserve_message (runtime_object_identity _MsgId)
	Reserves a message previously offered by this `single_assignment` messaging block. More...

virtual message< _Type > *	consume_message (runtime_object_identity _MsgId)
	Consumes a message previously offered by the `single_assignment` and reserved by the target, returning a copy of the message to the caller. More...

virtual void	release_message (runtime_object_identity _MsgId)
	Releases a previous message reservation. More...

virtual void	resume_propagation ()
	Resumes propagation after a reservation has been released. More...

virtual void	link_target_notification (_Inout_ ITarget< _Type > *_PTarget)
	A callback that notifies that a new target has been linked to this `single_assignment` messaging block. More...

virtual void	propagate_to_any_targets (_Inout_opt_ message< _Type > *_PMessage)
	Places the `message` _PMessage in this `single_assignment` messaging block and offers it to all of the linked targets. More...

Protected Member Functions inherited from Concurrency::propagator_block< multi_link_registry< ITarget< _Type > >, multi_link_registry< ISource< _Type > > >
virtual message_status	propagate_message (_Inout_ message< _Source_type > _PMessage, _Inout_ ISource< _Source_type > _PSource)=0
	When overridden in a derived class, this method asynchronously passes a message from an `ISource` block to this `propagator_block` object. It is invoked by the `propagate` method, when called by a source block. More...

virtual message_status	send_message (_Inout_ message< _Source_type > , _Inout_ ISource< _Source_type > )
	When overridden in a derived class, this method synchronously passes a message from an `ISource` block to this `propagator_block` object. It is invoked by the `send` method, when called by a source block. More...

virtual void	link_source (_Inout_ ISource< _Source_type > *_PSource)
	Links a specified source block to this `propagator_block` object. More...

virtual void	unlink_source (_Inout_ ISource< _Source_type > *_PSource)
	Unlinks a specified source block from this `propagator_block` object. More...

virtual void	unlink_sources ()
	Unlinks all source blocks from this `propagator_block` object. More...

virtual void	process_input_messages (_Inout_ message< _Target_type > *_PMessage)
	Process input messages. This is only useful for propagator blocks, which derive from source_block More...

void	initialize_source_and_target (_Inout_opt_ Scheduler _PScheduler=NULL, _Inout_opt_ ScheduleGroup _PScheduleGroup=NULL)
	Initializes the base object. Specifically, the `message_processor` object needs to be initialized. More...

void	register_filter (filter_method const &_Filter)
	Registers a filter method that will be invoked on every received message. More...

void	decline_incoming_messages ()
	Indicates to the block that new messages should be declined. More...

void	remove_network_links ()
	Removes all the source and target network links from this `propagator_block` object. More...

Protected Member Functions inherited from Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >
virtual void	link_target_notification (_Inout_ ITarget< _Target_type > *)
	A callback that notifies that a new target has been linked to this `source_block` object. More...

virtual void	unlink_target_notification (_Inout_ ITarget< _Target_type > *_PTarget)
	A callback that notifies that a target has been unlinked from this `source_block` object. More...

virtual void	propagate_output_messages ()
	Propagate messages to targets. More...

virtual void	propagate_to_any_targets (_Inout_opt_ message< _Target_type > *_PMessage)
	When overridden in a derived class, propagates the given message to any or all of the linked targets. This is the main propagation routine for message blocks. More...

void	initialize_source (_Inout_opt_ Scheduler _PScheduler=NULL, _Inout_opt_ ScheduleGroup _PScheduleGroup=NULL)
	Initializes the `message_propagator` within this `source_block`. More...

void	enable_batched_processing ()
	Enables batched processing for this block. More...

virtual void	sync_send (_Inout_opt_ message< _Target_type > *_Msg)
	Synchronously queues up messages and starts a propagation task, if this has not been done already. More...

virtual void	async_send (_Inout_opt_ message< _Target_type > *_Msg)
	Asynchronously queues up messages and starts a propagation task, if this has not been done already More...

void	wait_for_outstanding_async_sends ()
	Waits for all asynchronous propagations to complete. This propagator-specific spin wait is used in destructors of message blocks to make sure that all asynchronous propagations have time to finish before destroying the block. More...

void	remove_targets ()
	Removes all target links for this source block. This should be called from the destructor. More...

Protected Member Functions inherited from Concurrency::ISource< multi_link_registry< ITarget< _Type > >::type::type >
void	_Invoke_link_source (ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PLinkFrom)
	Links this source to a target. More...

void	_Invoke_unlink_source (ITarget< multi_link_registry< ITarget< _Type > >::type::type > *_PUnlinkFrom)
	Unlinks this source from a target. More...

Protected Member Functions inherited from Concurrency::ITarget< multi_link_registry< ISource< _Type > >::type::source_type >
virtual void	link_source (_Inout_ ISource< multi_link_registry< ISource< _Type > >::type::source_type > *_PSource)=0
	When overridden in a derived class, links a specified source block to this `ITarget` block. More...

virtual void	unlink_source (_Inout_ ISource< multi_link_registry< ISource< _Type > >::type::source_type > *_PSource)=0
	When overridden in a derived class, unlinks a specified source block from this `ITarget` block. More...

Private Member Functions
void	_Delete_stored_messages ()
	Deletes all messages currently stored in this message block. Should be called by the destructor to ensure any messages propagated in are cleaned up. More...

single_assignment const &	operator= (single_assignment const &)

	single_assignment (single_assignment const &)

Private Attributes
message< _Type > *	_M_pMessage

::Concurrency::details::_NonReentrantPPLLock	_M_propagationLock

volatile bool	_M_fIsInitialized

Additional Inherited Members
Public Types inherited from Concurrency::propagator_block< multi_link_registry< ITarget< _Type > >, multi_link_registry< ISource< _Type > > >
typedef multi_link_registry< ISource< _Type > >::type::source_type	_Source_type
	The type of the payload for the incoming message to this `propagator_block`. More...

typedef source_link_manager< multi_link_registry< ISource< _Type > > >	_SourceLinkManager
	The type of the `source_link_manager` this `propagator_block`. More...

typedef _SourceLinkManager::iterator	source_iterator
	The type of the iterator for the `source_link_manager` for this `propagator_block`. More...

Public Types inherited from Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >
typedef multi_link_registry< ITarget< _Type > >::type::type	_Target_type
	The payload type of messages handled by this `source_block`. More...

typedef multi_link_registry< ITarget< _Type > >::iterator	target_iterator
	The iterator to walk the connected targets. More...

Public Types inherited from Concurrency::ISource< multi_link_registry< ITarget< _Type > >::type::type >
typedef multi_link_registry< ITarget< _Type > >::type::type	source_type
	A type alias for _Type . More...

Public Types inherited from Concurrency::ITarget< multi_link_registry< ISource< _Type > >::type::source_type >
typedef multi_link_registry< ISource< _Type > >::type::source_type	type
	A type alias for _Type . More...

typedef std::tr1::function< bool(multi_link_registry< ISource< _Type > >::type::source_typeconst &)>	filter_method
	The signature of any method used by the block that returns a `bool` value to determine whether an offered message should be accepted. More...

Protected Attributes inherited from Concurrency::propagator_block< multi_link_registry< ITarget< _Type > >, multi_link_registry< ISource< _Type > > >
_SourceLinkManager	_M_connectedSources
	The container for all the sources connected to this block. More...

filter_method *	_M_pFilter
	The filter function which determines whether offered messages should be accepted. More...

volatile bool	_M_fDeclineMessages
	A `bool` that is set to indicate that all messages should be declined in preparation for deleting the block More...

Protected Attributes inherited from Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >
ITarget< _Target_type > *	_M_pReservedFor
	Connected target that is holding a reservation More...

runtime_object_identity	_M_reservedId
	Reserved message ID More...

multi_link_registry< ITarget< _Type > >	_M_connectedTargets
	Connected targets More...

_MessageProcessorType	_M_messageProcessor
	Processor used for asynchronous message handling More...

Detailed Description

template<class _Type>
class Concurrency::single_assignment< _Type >

A single_assignment messaging block is a multi-target, multi-source, ordered propagator_block capable of storing a single, write-once message.

Template Parameters

_Type The payload type of the message stored and propagated by the buffer.

A single_assignment messaging block propagates out copies of its message to each target.

For more information, see Asynchronous Message Blocks.

See also: overwrite_buffer Class, unbounded_buffer Class

Constructor & Destructor Documentation

template<class _Type>

Concurrency::single_assignment< _Type >::single_assignment ( )

inline

Constructs a single_assignment messaging block.

The runtime uses the default scheduler if you do not specify the _PScheduler or _PScheduleGroup parameters.

The type filter_method is a functor with signature bool (_Type const &) which is invoked by this single_assignment messaging block to determine whether or not it should accept an offered message.

See also: Scheduler Class, ScheduleGroup Class

                         :
         _M_fIsInitialized(false), _M_pMessage(NULL)
     {
         initialize_source_and_target();
     }

template<class _Type>

Concurrency::single_assignment< _Type >::single_assignment ( filter_method const & _Filter )

inline

Constructs a single_assignment messaging block.

Parameters

_Filter A filter function which determines whether offered messages should be accepted.

The runtime uses the default scheduler if you do not specify the _PScheduler or _PScheduleGroup parameters.

The type filter_method is a functor with signature bool (_Type const &) which is invoked by this single_assignment messaging block to determine whether or not it should accept an offered message.

See also: Scheduler Class, ScheduleGroup Class

                                                     :
         _M_fIsInitialized(false), _M_pMessage(NULL)
     {
         initialize_source_and_target();
         register_filter(_Filter);
     }

template<class _Type>

Concurrency::single_assignment< _Type >::~single_assignment ( )

inline

Destroys the single_assignment messaging block.

     {
         // Remove all links
         remove_network_links();
 
         // Clean up any messages left in this message block
         _Delete_stored_messages();
     }

template<class _Type>

Concurrency::single_assignment< _Type >::single_assignment ( single_assignment< _Type > const & )

private

Member Function Documentation

template<class _Type>

void Concurrency::single_assignment< _Type >::_Delete_stored_messages ( )

inlineprivate

Deletes all messages currently stored in this message block. Should be called by the destructor to ensure any messages propagated in are cleaned up.

     {
         // Input messages for this message block are in the base-class input buffer
         // All messages in that buffer are guaranteed to have moved to the output
         // buffer because the destructor first waits for all async sends to finish
         // before reaching this point
 
         // The messages for a single_assignment are deleted at the end when
         // single_assignment is deleted.
         delete _M_pMessage;
     }

template<class _Type>

virtual message<_Type>* Concurrency::single_assignment< _Type >::accept_message ( runtime_object_identity _MsgId )

inlineprotectedvirtual

Accepts a message that was offered by this single_assignment messaging block, returning a copy of the message to the caller.

Parameters

_MsgId The runtime_object_identity of the offered message object.

Returns: A pointer to the message object that the caller now has ownership of.

The single_assignment messaging block returns copies of the message to its targets, rather than transferring ownership of the currently held message.

Implements Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >.

     {
         // This check is to prevent spoofing and verify that the propagated message is
         // the one that is accepted at the end.
         if (_M_pMessage == NULL || _MsgId != _M_pMessage->msg_id())
         {
             return NULL;
         }
 
         //
         // Instead of returning the internal message, we return a copy of the
         // message passed in.
         //
         // Because we are returning a copy, the accept routine for a single_assignment
         // does not need to grab the internal lock.
         //
         return (new message<_Type>(_M_pMessage->payload));
     }

template<class _Type>

virtual message<_Type>* Concurrency::single_assignment< _Type >::consume_message ( runtime_object_identity _MsgId )

inlineprotectedvirtual

Consumes a message previously offered by the single_assignment and reserved by the target, returning a copy of the message to the caller.

Parameters

_MsgId The runtime_object_identity of the message object being consumed.

Returns: A pointer to the message object that the caller now has ownership of.

Similar to accept, but is always preceded by a call to reserve.

Implements Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >.

     {
         _CONCRT_ASSERT(_M_fIsInitialized);
 
         return accept_message(_MsgId);
     }

template<class _Type>

bool Concurrency::single_assignment< _Type >::has_value ( ) const

inline

Checks whether this single_assignment messaging block has been initialized with a value yet.

Returns: true if the block has received a value, false otherwise.

     {
         return (_M_pMessage != NULL);
     }

template<class _Type>

virtual void Concurrency::single_assignment< _Type >::link_target_notification ( _Inout_ ITarget< _Type > * _PTarget )

inlineprotectedvirtual

A callback that notifies that a new target has been linked to this single_assignment messaging block.

Parameters

_PTarget A pointer to the newly linked target.

     {
         // If there is a message available already, propagate it.
 
         if (_M_pMessage != NULL)
         {
             _PTarget->propagate(_M_pMessage, this);
         }
     }

template<class _Type>

single_assignment const& Concurrency::single_assignment< _Type >::operator= ( single_assignment< _Type > const & )

private

template<class _Type>

virtual message_status Concurrency::single_assignment< _Type >::propagate_message	(	_Inout_ message< _Type > *	_PMessage,
		_Inout_ ISource< _Type > *	_PSource
	)

inlineprotectedvirtual

Asynchronously passes a message from an ISource block to this single_assignment messaging block. It is invoked by the propagate method, when called by a source block.

Parameters

_PMessage	A pointer to the `message` object.
_PSource	A pointer to the source block offering the message.

Returns: A message_status indication of what the target decided to do with the message.

     {
         // It is important that calls to propagate do *not* take the same lock on the
         // internal structure that is used by Consume and the LWT.  Doing so could
         // result in a deadlock with the Consume call.
 
         message_status _Result = accepted;
 
         // single_assignment messaging block can be initialized only once
         if (_M_fIsInitialized)
         {
             return declined;
         }
 
         {
             _NR_lock _Lock(_M_propagationLock);
 
             if (_M_fIsInitialized)
             {
                 _Result = declined;
             }
             else
             {
                 _PMessage = _PSource->accept(_PMessage->msg_id(), this);
 
                 // Set initialized flag only if we have a message
                 if (_PMessage != NULL)
                 {
                     _M_fIsInitialized = true;
                 }
                 else
                 {
                     _Result = missed;
                 }
             }
         }
 
         //
         // If message was accepted, set the member variables for
         // this block and start the asynchronous propagation task
         //
         if (_Result == accepted)
         {
             async_send(_PMessage);
         }
 
         return _Result;
     }

template<class _Type>

virtual void Concurrency::single_assignment< _Type >::propagate_to_any_targets ( _Inout_opt_ message< _Type > * _PMessage )

inlineprotectedvirtual

Places the message _PMessage in this single_assignment messaging block and offers it to all of the linked targets.

Parameters

_PMessage A pointer to a message that this single_assignment messaging block has taken ownership of.

     {
         // Initialized flag should have been set by the propagate function using interlocked operation.
         _CONCRT_ASSERT(_M_fIsInitialized);
 
         // Move the message to the internal storage
 
         _CONCRT_ASSERT(_M_pMessage == NULL);
         _M_pMessage = _PMessage;
 
         for (target_iterator _Iter = _M_connectedTargets.begin(); *_Iter != NULL; ++_Iter)
         {
             // Single assignment can propagate its message out
             // to any number of Targets
 
             ITarget<_Type> * _PTarget = *_Iter;
             _PTarget->propagate(_PMessage, this);
         }
     }

template<class _Type>

virtual void Concurrency::single_assignment< _Type >::release_message ( runtime_object_identity _MsgId )

inlineprotectedvirtual

Releases a previous message reservation.

Parameters

_MsgId The runtime_object_identity of the message object being released.

Implements Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >.

     {
         _CONCRT_ASSERT(_M_fIsInitialized);
 
         if (_M_pMessage == NULL || _M_pMessage->msg_id() != _MsgId)
         {
             throw message_not_found();
         }
     }

template<class _Type>

virtual bool Concurrency::single_assignment< _Type >::reserve_message ( runtime_object_identity _MsgId )

inlineprotectedvirtual

Reserves a message previously offered by this single_assignment messaging block.

Parameters

_MsgId The runtime_object_identity of the message object being reserved.

Returns: true if the message was successfully reserved, false otherwise.

After reserve is called, if it returns true, either consume or release must be called to either take or release ownership of the message.

Implements Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >.

     {
         if (_M_pMessage == NULL)
         {
             return false;
         }
 
         if (_M_pMessage->msg_id() != _MsgId)
         {
             throw message_not_found();
         }
 
         return true;
     }

template<class _Type>

virtual void Concurrency::single_assignment< _Type >::resume_propagation ( )

inlineprotectedvirtual

Resumes propagation after a reservation has been released.

Implements Concurrency::source_block< multi_link_registry< ITarget< _Type > >, _MessageProcessorType >.

     {
         // Because reservation doesn't stop propagation, we don't
         // need to do anything on resume after consume/release.
     }

template<class _Type>

virtual message_status Concurrency::single_assignment< _Type >::send_message	(	_Inout_ message< _Type > *	_PMessage,
		_Inout_ ISource< _Type > *	_PSource
	)

inlineprotectedvirtual

Synchronously passes a message from an ISource block to this single_assignment messaging block. It is invoked by the send method, when called by a source block.

Parameters

_PMessage	A pointer to the `message` object.
_PSource	A pointer to the source block offering the message.

Returns: A message_status indication of what the target decided to do with the message.

     {
         message_status _Result = accepted;
 
         // single_assignment messaging block can be initialized only once
         if (_M_fIsInitialized)
         {
             return declined;
         }
 
         {
             _NR_lock _Lock(_M_propagationLock);
 
             if (_M_fIsInitialized)
             {
                 _Result = declined;
             }
             else
             {
                 _PMessage = _PSource->accept(_PMessage->msg_id(), this);
 
                 // Set initialized flag only if we have a message
                 if (_PMessage != NULL)
                 {
                     _M_fIsInitialized = true;
                 }
                 else
                 {
                     _Result = missed;
                 }
             }
         }
 
         //
         // If message was accepted, set the member variables for
         // this block and start the asynchronous propagation task
         //
         if (_Result == accepted)
         {
             sync_send(_PMessage);
         }
 
         return _Result;
     }

template<class _Type>

_Type const& Concurrency::single_assignment< _Type >::value ( )

inline

Gets a reference to the current payload of the message being stored in the single_assignment messaging block.

Returns: The payload of the stored message.

This method will wait until a message arrives if no message is currently stored in the single_assignment messaging block.

     {
         if (_M_pMessage == NULL)
         {
             receive<_Type>(this);
         }
         _CONCRT_ASSERT(_M_pMessage != NULL);
 
         return _M_pMessage->payload;
     }