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structured_task_group Class

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The structured_task_group class represents a highly structured collection of parallel work. You can queue individual parallel tasks to a structured_task_group using task_handle objects, and wait for them to complete, or cancel the task group before they have finished executing, which will abort any tasks that have not begun execution.

Syntax

class structured_task_group;

Members

Public Constructors

Name Description
structured_task_group Overloaded. Constructs a new structured_task_group object.
~structured_task_group Destructor Destroys a structured_task_group object. You are expected to call either the wait or run_and_wait method on the object prior to the destructor executing, unless the destructor is executing as a result of stack unwinding due to an exception.

Public Methods

Name Description
cancel Makes a best effort attempt to cancel the sub-tree of work rooted at this task group. Every task scheduled on the task group will get canceled transitively if possible.
is_canceling Informs the caller whether or not the task group is currently in the midst of a cancellation. This does not necessarily indicate that the cancel method was called on the structured_task_group object (although such certainly qualifies this method to return true). It may be the case that the structured_task_group object is executing inline and a task group further up in the work tree was canceled. In cases such as these where the runtime can determine ahead of time that cancellation will flow through this structured_task_group object, true will be returned as well.
run Overloaded. Schedules a task on the structured_task_group object. The caller manages the lifetime of the task_handle object passed in the _Task_handle parameter. The version that takes the parameter _Placement causes the task to be biased towards executing at the location specified by that parameter.
run_and_wait Overloaded. Schedules a task to be run inline on the calling context with the assistance of the structured_task_group object for full cancellation support. If a task_handle object is passed as a parameter to run_and_wait, the caller is responsible for managing the lifetime of the task_handle object. The function then waits until all work on the structured_task_group object has either completed or been canceled.
wait Waits until all work on the structured_task_group has completed or is canceled.

Remarks

There are a number of severe restrictions placed on usage of a structured_task_group object in order to gain performance:

  • A single structured_task_group object cannot be used by multiple threads. All operations on a structured_task_group object must be performed by the thread that created the object. The two exceptions to this rule are the member functions cancel and is_canceling. The object may not be in the capture list of a lambda expression and be used within a task, unless the task is using one of the cancellation operations.

  • All tasks scheduled to a structured_task_group object are scheduled through the use of task_handle objects which you must explicitly manage the lifetime of.

  • Multiple groups may only be used in absolutely nested order. If two structured_task_group objects are declared, the second one being declared (the inner one) must destruct before any method except cancel or is_canceling is called on the first one (the outer one). This condition holds true in both the case of simply declaring multiple structured_task_group objects within the same or functionally nested scopes as well as the case of a task that was queued to the structured_task_group via the run or run_and_wait methods.

  • Unlike the general task_group class, all states in the structured_task_group class are final. After you have queued tasks to the group and waited for them to complete, you may not use the same group again.

For more information, see Task Parallelism.

Inheritance Hierarchy

structured_task_group

Requirements

Header: ppl.h

Namespace: concurrency

cancel

Makes a best effort attempt to cancel the sub-tree of work rooted at this task group. Every task scheduled on the task group will get canceled transitively if possible.

void cancel();

Remarks

For more information, see Cancellation.

is_canceling

Informs the caller whether or not the task group is currently in the midst of a cancellation. This does not necessarily indicate that the cancel method was called on the structured_task_group object (although such certainly qualifies this method to return true). It may be the case that the structured_task_group object is executing inline and a task group further up in the work tree was canceled. In cases such as these where the runtime can determine ahead of time that cancellation will flow through this structured_task_group object, true will be returned as well.

bool is_canceling();

Return Value

An indication of whether the structured_task_group object is in the midst of a cancellation (or is guaranteed to be shortly).

Remarks

For more information, see Cancellation.

run

Schedules a task on the structured_task_group object. The caller manages the lifetime of the task_handle object passed in the _Task_handle parameter. The version that takes the parameter _Placement causes the task to be biased towards executing at the location specified by that parameter.

template<class _Function>
void run(
    task_handle<_Function>& _Task_handle);

template<class _Function>
void run(
    task_handle<_Function>& _Task_handle,
    location& _Placement);

Parameters

_Function
The type of the function object that will be invoked to execute the body of the task handle.

_Task_handle
A handle to the work being scheduled. Note that the caller has responsibility for the lifetime of this object. The runtime will continue to expect it to live until either the wait or run_and_wait method has been called on this structured_task_group object.

_Placement
A reference to the location where the task represented by the _Task_handle parameter should execute.

Remarks

The runtime creates a copy of the work function that you pass to this method. Any state changes that occur in a function object that you pass to this method will not appear in your copy of that function object.

If the structured_task_group destructs as the result of stack unwinding from an exception, you do not need to guarantee that a call has been made to either the wait or run_and_wait method. In this case, the destructor will appropriately cancel and wait for the task represented by the _Task_handle parameter to complete.

Throws an invalid_multiple_scheduling exception if the task handle given by the _Task_handle parameter has already been scheduled onto a task group object via the run method and there has been no intervening call to either the wait or run_and_wait method on that task group.

run_and_wait

Schedules a task to be run inline on the calling context with the assistance of the structured_task_group object for full cancellation support. If a task_handle object is passed as a parameter to run_and_wait, the caller is responsible for managing the lifetime of the task_handle object. The function then waits until all work on the structured_task_group object has either completed or been canceled.

template<class _Function>
task_group_status run_and_wait(task_handle<_Function>& _Task_handle);

template<class _Function>
task_group_status run_and_wait(const _Function& _Func);

Parameters

_Function
The type of the function object that will be invoked to execute the task.

_Task_handle
A handle to the task which will be run inline on the calling context. Note that the caller has responsibility for the lifetime of this object. The runtime will continue to expect it to live until the run_and_wait method finishes execution.

_Func
A function which will be called to invoke the body of the work. This may be a lambda or other object which supports a version of the function call operator with the signature void operator()().

Return Value

An indication of whether the wait was satisfied or the task group was canceled, due to either an explicit cancel operation or an exception being thrown from one of its tasks. For more information, see task_group_status

Remarks

Note that one or more of the tasks scheduled to this structured_task_group object may execute inline on the calling context.

If one or more of the tasks scheduled to this structured_task_group object throws an exception, the runtime will select one such exception of its choosing and propagate it out of the call to the run_and_wait method.

After this function returns, the structured_task_group object is considered in a final state and should not be used. Note that utilization after the run_and_wait method returns will result in undefined behavior.

In the non-exceptional path of execution, you have a mandate to call either this method or the wait method before the destructor of the structured_task_group executes.

structured_task_group

Constructs a new structured_task_group object.

structured_task_group();

structured_task_group(cancellation_token _CancellationToken);

Parameters

_CancellationToken
A cancellation token to associate with this structured task group. The structured task group will be canceled when the token is canceled.

Remarks

The constructor that takes a cancellation token creates a structured_task_group that will be canceled when the source associated with the token is canceled. Providing an explicit cancellation token also isolates this structured task group from participating in an implicit cancellation from a parent group with a different token or no token.

~structured_task_group

Destroys a structured_task_group object. You are expected to call either the wait or run_and_wait method on the object prior to the destructor executing, unless the destructor is executing as a result of stack unwinding due to an exception.

~structured_task_group();

Remarks

If the destructor runs as the result of normal execution (for example, not stack unwinding due to an exception) and neither the wait nor run_and_wait methods have been called, the destructor may throw a missing_wait exception.

wait

Waits until all work on the structured_task_group has completed or is canceled.

task_group_status wait();

Return Value

An indication of whether the wait was satisfied or the task group was canceled, due to either an explicit cancel operation or an exception being thrown from one of its tasks. For more information, see task_group_status

Remarks

Note that one or more of the tasks scheduled to this structured_task_group object may execute inline on the calling context.

If one or more of the tasks scheduled to this structured_task_group object throws an exception, the runtime will select one such exception of its choosing and propagate it out of the call to the wait method.

After this function returns, the structured_task_group object is considered in a final state and should not be used. Note that utilization after the wait method returns will result in undefined behavior.

In the non-exceptional path of execution, you have a mandate to call either this method or the run_and_wait method before the destructor of the structured_task_group executes.

See also

concurrency Namespace
task_group Class
task_handle Class