Threads::Task< RT > Class Template Reference

#include <deal.II/base/thread_management.h>

Inheritance diagram for Threads::Task< RT >:

Classes
class	TaskData

Public Member Functions
	Task (const std::function< RT()> &function_object)
	Task ()=default
void	join () const
bool	joinable () const
internal::return_value< RT >::reference_type	return_value ()

Static Public Member Functions
static ::ExceptionBase &	ExcNoTask ()

Private Attributes
std::shared_ptr< TaskData >	task_data

Detailed Description

template<typename RT = void>
class Threads::Task< RT >

This class describes a task object, i.e., what one obtains by calling Threads::new_task(). The idea is that Threads::new_task() allows one to run a function whenever the C++ run-time system finds it convenient – typically, when there is an idle processor available. This can be used to run things in the background when there is no immediate need for the result, or if there are other things that could well be done in parallel. Whenever the result of that background task is needed, one can call either join() to just wait for the task to finish, or return_value() to obtain the value that was returned by the function that was run on that background task.

This class is conceptually similar to the std::future class that is returned by std::async (which is itself similar to what Threads::new_task() does). The principal conceptual difference is that one can only call std::future::get() once, whereas one can call Threads::Task::return_value() as many times as desired. It is, thus, comparable to the std::shared_future class. However, std::shared_future can not be used for types that can not be copied – a particular restriction for std::unique_ptr, for example.

Definition at line 999 of file thread_management.h.

Constructor & Destructor Documentation

Task() [1/2]

template<typename RT = void>

Threads::Task< RT >::Task ( const std::function< RT()> &  function_object )

inline

Construct a task object, given a function object to execute on the task, and then schedule this function for execution. However, when MultithreadInfo::n_threads() returns 1, i.e., if the deal.II runtime system has been configured to only use one thread, then just execute the given function object.

Postcondition

Using this constructor automatically makes the task object joinable().

Definition at line 1013 of file thread_management.h.

Task() [2/2]

template<typename RT = void>

Threads::Task< RT >::Task ( )

default

Default constructor. You can't do much with a task object constructed this way, except for assigning it a task object that holds data created by the Threads::new_task() functions.

Postcondition

Using this constructor leaves the object in an unjoinable state, i.e., joinable() will return false.

Member Function Documentation

join()

template<typename RT = void>

void Threads::Task< RT >::join ( ) const

inline

Join the task represented by this object, i.e. wait for it to finish.

A task can be joined multiple times (while the first join() operation may block until the task has completed running, all successive attempts to join will return immediately).

If the operation that was executed on the task with which this object was initialized throws an exception instead of returning regularly, then calling the current join() function will first wait for that task to finish, and then in turn throw the exception that the task operation had thrown originally. This allows for the propagation of exceptions from tasks executed on a separate thread to the calling thread.

(This behavior differs from that of std::future, where the std::future::wait() function only waits for completion of the operation, whereas the exception is propagated only once one calls std::future::get(). However, this is awkward when putting void functions onto separate tasks because these do not actually return anything; consequently, it is more natural to call std::task::wait() for such tasks than the std::task::get() function since the latter does not, actually, return anything that could be gotten.)

Precondition

You can't call this function if you have used the default constructor of this class and have not assigned a task object to it. In other words, the function joinable() must return true.

Definition at line 1184 of file thread_management.h.

joinable()

template<typename RT = void>

bool Threads::Task< RT >::joinable ( ) const

inline

Return whether the current object can be joined. You can join a task object once a task (typically created with Threads::new_task()) has actually been assigned to it. On the other hand, the function returns false if the object has been default constructed.

A task can be joined multiple times (while the first join() operation may block until the task has completed running, all successive attempts to join will return immediately). Consequently, if this function returns true, it will continue to return true until the task object it reports on is assigned to from another object.

Definition at line 1205 of file thread_management.h.

return_value()

template<typename RT = void>

internal::return_value< RT >::reference_type Threads::Task< RT >::return_value ( )

inline

Get the return value of the function of the task. Since it is only available once the thread finishes, this function internally also calls join(). You can call this function multiple times as long as the object refers to the same task, and expect to get the same return value every time. (With the exception of the case where the returned object has been moved; see below.)

Note

The function returns a non-const reference to the returned object, instead of the returned object. This allows writing code such as

  Threads::Task<int> t = Threads::new_task (...function returning an
int...); t.return_value() = 42;      // overwrite returned value int i =
t.return_value();   // i is now 42

You will rarely have a need to write such code. On the other hand, the function needs to return a writable (non-const) reference to support code such as this:

std::unique_ptr<int> create_int (const std::string &s) { ... }
 
void f()
{
  Threads::Task<std::unique_ptr<int>>
    t = Threads::new_task (&create_int, "42");
 
  std::unique_ptr<int> i = std::move(t.return_value());
  ...
}

Here, it is necessary to std::move the returned object (namely, the std::unique_ptr object) because std::unique_ptr objects can not be copied. In other words, to get the pointer out of the object returned from the task, it needs to be moved, and in order to be moved, the current function needs to return a writable (non-const) reference.

This function internally calls the join() member function. As a consequence, and as explained there, if the packaged task throws an exception that is then re-thrown by the join() function and consequently also the current function if you have not previously called join().

Precondition

You can't call this function if you have used the default constructor of this class and have not assigned a task object to it. In other words, the function joinable() must return true.

Definition at line 1261 of file thread_management.h.

Member Data Documentation

task_data

template<typename RT = void>

std::shared_ptr<TaskData> Threads::Task< RT >::task_data

private

A pointer to a descriptor of the object that described the task and its return value.

Definition at line 1502 of file thread_management.h.

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The documentation for this class was generated from the following file:

• include/deal.II/base/thread_management.h