32 Concurrency support library [thread]

32.10 Futures [futures]

32.10.10 Class template packaged_task [futures.task]

32.10.10.2 Member functions [futures.task.members]

packaged_task() noexcept;

Effects: The object has no shared state and no stored task.

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template<class F>
  explicit packaged_task(F&& f);

Constraints: remove_cvref_t<F> is not the same type as packaged_task<R(ArgTypes...)>.

Mandates: is_invocable_r_v<R, decay_t<F>&, ArgTypes...> is true.

Effects: Constructs a new packaged_task object with a stored task of type decay_t<F> and a shared state.

Initializes the object's stored task with std::forward<F>(f).

Throws: Any exceptions thrown by the copy or move constructor of f, or bad_alloc if memory for the internal data structures cannot be allocated.

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template<class F> packaged_task(F) -> packaged_task<see below>;

Constraints: &F::operator() is well-formed when treated as an unevaluated operand ([expr.context]) and either

(6.1)
F::operator() is a non-static member function and decltype(&F::operator()) is either of the form R(G::*)(A...) cv & $_{o p t}$ noexcept $_{o p t}$ or of the form R(*)(G, A...) noexcept $_{o p t}$ for a type G, or
(6.2)
F::operator() is a static member function and decltype(&F::operator()) is of the form R(*)(A...) noexcept $_{o p t}$ .

Remarks: The deduced type is packaged_task<R(A...)>.

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packaged_task(packaged_task&& rhs) noexcept;

Effects: Transfers ownership of rhs's shared state to *this, leaving rhs with no shared state.

Moves the stored task from rhs to *this.

Postconditions: rhs has no shared state.

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packaged_task& operator=(packaged_task&& rhs) noexcept;

Effects:

(10.1)
Releases any shared state ([futures.state]);
(10.2)
calls packaged_task(std::move(rhs)).swap(*this).

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~packaged_task();

Effects: Abandons any shared state ([futures.state]).

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void swap(packaged_task& other) noexcept;

Effects: Exchanges the shared states and stored tasks of *this and other.

Postconditions: *this has the same shared state and stored task (if any) as other prior to the call to swap.

other has the same shared state and stored task (if any) as *this prior to the call to swap.

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bool valid() const noexcept;

Returns: true only if *this has a shared state.

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future<R> get_future();

Synchronization: Calls to this function do not introduce data races ([intro.multithread]) with calls to operator() or make_ready_at_thread_exit.

[Note 1:

Such calls need not synchronize with each other.

— end note]

Returns: A future object that shares the same shared state as *this.

Throws: A future_error object if an error occurs.

Error conditions:

(18.1)
future_already_retrieved if get_future has already been called on a packaged_task object with the same shared state as *this.
(18.2)
no_state if *this has no shared state.

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void operator()(ArgTypes... args);

Effects: As if by INVOKE<R>(f, t

_{1}

, t

_{2}

, …, t

_{N}

) ([func.require]), where f is the stored task of *this and t

_{1}

, t

_{2}

, …, t

_{N}

are the values in args....

If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored.

The shared state of *this is made ready, and any threads blocked in a function waiting for the shared state of *this to become ready are unblocked.

Throws: A future_error exception object if there is no shared state or the stored task has already been invoked.

Error conditions:

(21.1)
promise_already_satisfied if the stored task has already been invoked.
(21.2)
no_state if *this has no shared state.

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void make_ready_at_thread_exit(ArgTypes... args);

Effects: As if by INVOKE<R>(f, t

_{1}

, t

_{2}

, …, t

_{N}

) ([func.require]), where f is the stored task and t

_{1}

, t

_{2}

, …, t

_{N}

are the values in args....

If the task returns normally, the return value is stored as the asynchronous result in the shared state of *this, otherwise the exception thrown by the task is stored.

In either case, this is done without making that state ready ([futures.state]) immediately.

Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

Throws: future_error if an error condition occurs.

Error conditions:

(24.1)
promise_already_satisfied if the stored task has already been invoked.
(24.2)
no_state if *this has no shared state.

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void reset();

Effects: As if *this = packaged_task(std::move(f)), where f is the task stored in *this.

[Note 2:

This constructs a new shared state for *this.

The old state is abandoned ([futures.state]).

— end note]

Throws:

(26.1)
bad_alloc if memory for the new shared state cannot be allocated.
(26.2)
Any exception thrown by the move constructor of the task stored in the shared state.
(26.3)
future_error with an error condition of no_state if *this has no shared state.