single_assignment Class

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

Syntax

template<class T>
class single_assignment : public propagator_block<multi_link_registry<ITarget<T>>, multi_link_registry<ISource<T>>>;

Parameters

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

Members

Public Constructors

Name	Description
single_assignment	Overloaded. Constructs a single_assignment messaging block.
~single_assignment Destructor	Destroys the single_assignment messaging block.

Public Methods

Name	Description
has_value	Checks whether this single_assignment messaging block has been initialized with a value yet.
value	Gets a reference to the current payload of the message being stored in the single_assignment messaging block.

Protected Methods

Name	Description
accept_message	Accepts a message that was offered by this single_assignment messaging block, returning a copy of the message to the caller.
consume_message	Consumes a message previously offered by the single_assignment and reserved by the target, returning a copy of the message to the caller.
link_target_notification	A callback that notifies that a new target has been linked to this single_assignment messaging block.
propagate_message	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.
propagate_to_any_targets	Places the message _PMessage in this single_assignment messaging block and offers it to all of the linked targets.
release_message	Releases a previous message reservation. (Overrides source_block::release_message.)
reserve_message	Reserves a message previously offered by this single_assignment messaging block. (Overrides source_block::reserve_message.)
resume_propagation	Resumes propagation after a reservation has been released. (Overrides source_block::resume_propagation.)
send_message	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.

Remarks

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

For more information, see Asynchronous Message Blocks.

Inheritance Hierarchy

ISource

ITarget

source_block

propagator_block

single_assignment

Requirements

Header: agents.h

Namespace: concurrency

accept_message

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

virtual message<T>* accept_message(runtime_object_identity _MsgId);

Parameters

_MsgId
The runtime_object_identity of the offered message object.

Return Value

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

Remarks

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

consume_message

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

virtual message<T>* consume_message(runtime_object_identity _MsgId);

Parameters

_MsgId
The runtime_object_identity of the message object being consumed.

Return Value

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

Remarks

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

has_value

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

bool has_value() const;

Return Value

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

link_target_notification

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

virtual void link_target_notification(_Inout_ ITarget<T>* _PTarget);

Parameters

_PTarget
A pointer to the newly linked target.

propagate_message

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.

virtual message_status propagate_message(
    _Inout_ message<T>* _PMessage,
    _Inout_ ISource<T>* _PSource);

Parameters

_PMessage
A pointer to the message object.

_PSource
A pointer to the source block offering the message.

Return Value

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

propagate_to_any_targets

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

virtual void propagate_to_any_targets(_Inout_opt_ message<T>* _PMessage);

Parameters

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

release_message

Releases a previous message reservation.

virtual void release_message(runtime_object_identity _MsgId);

Parameters

_MsgId
The runtime_object_identity of the message object being released.

reserve_message

Reserves a message previously offered by this single_assignment messaging block.

virtual bool reserve_message(runtime_object_identity _MsgId);

Parameters

_MsgId
The runtime_object_identity of the message object being reserved.

Return Value

true if the message was successfully reserved, false otherwise.

Remarks

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

resume_propagation

Resumes propagation after a reservation has been released.

virtual void resume_propagation();

send_message

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.

virtual message_status send_message(
    _Inout_ message<T>* _PMessage,
    _Inout_ ISource<T>* _PSource);

Parameters

_PMessage
A pointer to the message object.

_PSource
A pointer to the source block offering the message.

Return Value

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

single_assignment

Constructs a single_assignment messaging block.

single_assignment();

single_assignment(
    filter_method const& _Filter);

single_assignment(
    Scheduler& _PScheduler);

single_assignment(
    Scheduler& _PScheduler,
    filter_method const& _Filter);

single_assignment(
    ScheduleGroup& _PScheduleGroup);

single_assignment(
    ScheduleGroup& _PScheduleGroup,
    filter_method const& _Filter);

Parameters

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

_PScheduler
The Scheduler object within which the propagation task for the single_assignment messaging block is scheduled.

_PScheduleGroup
The ScheduleGroup object within which the propagation task for the single_assignment messaging block is scheduled. The Scheduler object used is implied by the schedule group.

Remarks

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 (T const &) which is invoked by this single_assignment messaging block to determine whether or not it should accept an offered message.

~single_assignment

Destroys the single_assignment messaging block.

~single_assignment();

value

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

T const& value();

Return Value

The payload of the stored message.

Remarks

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

See also

concurrency Namespace
overwrite_buffer Class
unbounded_buffer Class