task_group Class

The task_group class represents a collection of parallel work which can be waited on or canceled.

Syntax

class task_group;

Members

Public Constructors

Name	Description
task_group	Overloaded. Constructs a new task_group object.
~task_group Destructor	Destroys a task_group object. You are expected to call the either the wait or run_and_wait method on the object prior to the destructor executing, unless the destructor is executing as the 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 task_group object (although such certainly qualifies this method to return true). It may be the case that the 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 task_group object, true will be returned as well.
run	Overloaded. Schedules a task on the task_group object. If a task_handle object is passed as a parameter to run, the caller is responsible for managing the lifetime of the task_handle object. The version of the method that takes a reference to a function object as a parameter involves heap allocation inside the runtime which may be perform less well than using the version that takes a reference to a task_handle object. The version which 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 task_group object for full cancellation support. The function then waits until all work on the task_group object has either completed or been canceled. 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.
wait	Waits until all work on the task_group object has either completed or been canceled.

Remarks

Unlike the heavily restricted structured_task_group class, the task_group class is much more general construct. It does not have any of the restrictions described by structured_task_group. task_group objects may safely be used across threads and utilized in free-form ways. The disadvantage of the task_group construct is that it may not perform as well as the structured_task_group construct for tasks which perform small amounts of work.

For more information, see Task Parallelism.

Inheritance Hierarchy

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 task_group object (although such certainly qualifies this method to return true). It may be the case that the 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 task_group object, true will be returned as well.

bool is_canceling();

Return Value

An indication of whether the 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 task_group object. If a task_handle object is passed as a parameter to run, the caller is responsible for managing the lifetime of the task_handle object. The version of the method that takes a reference to a function object as a parameter involves heap allocation inside the runtime which may be perform less well than using the version that takes a reference to a task_handle object. The version which takes the parameter _Placement causes the task to be biased towards executing at the location specified by that parameter.

template<
   typename _Function
>
void run(
   const _Function& _Func
);

template<
   typename _Function
>
void run(
   const _Function& _Func,
   location& _Placement
);

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

template<
   typename _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.

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

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

_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 task_group object.

Remarks

The runtime schedules the provided work function to run at a later time, which can be after the calling function returns. This method uses a task_handle object to hold a copy of the provided work function. Therefore, 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. In addition, make sure that the lifetime of any objects that you pass by pointer or by reference to the work function remain valid until the work function returns.

If the 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.

The method 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 task_group object for full cancellation support. The function then waits until all work on the task_group object has either completed or been canceled. 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.

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 body of 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 expression 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 task_group object may execute inline on the calling context.

If one or more of the tasks scheduled to this 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.

Upon return from the run_and_wait method on a task_group object, the runtime resets the object to a clean state where it can be reused. This includes the case where the task_group object was canceled.

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 task_group executes.

task_group

Constructs a new task_group object.

task_group();

task_group(
   cancellation_token _CancellationToken
);

Parameters

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

Remarks

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

~task_group

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

~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 task_group object has either completed or been 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 task_group object may execute inline on the calling context.

If one or more of the tasks scheduled to this 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.

Calling wait on a task_group object resets it to a clean state where it can be reused. This includes the case where the task_group object was canceled.

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 task_group executes.

See also

concurrency Namespace
structured_task_group Class
task_handle Class