33 Thread support library [thread]
[futures] describes components that a C++ program can use to retrieve in one thread the
result (value or exception) from a function that has run in the same thread or another thread
. [
Note: These components are not restricted to multi-threaded programs but can be useful in
single-threaded programs as well
. —
end note ]
namespace std {
enum class future_errc {
broken_promise = implementation-defined,
future_already_retrieved = implementation-defined,
promise_already_satisfied = implementation-defined,
no_state = implementation-defined
};
enum class launch : unspecified {
async = unspecified,
deferred = unspecified,
implementation-defined
};
enum class future_status {
ready,
timeout,
deferred
};
template <> struct is_error_code_enum<future_errc> : public true_type { };
error_code make_error_code(future_errc e) noexcept;
error_condition make_error_condition(future_errc e) noexcept;
const error_category& future_category() noexcept;
class future_error;
template <class R> class promise;
template <class R> class promise<R&>;
template <> class promise<void>;
template <class R>
void swap(promise<R>& x, promise<R>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>;
template <class R> class future;
template <class R> class future<R&>;
template <> class future<void>;
template <class R> class shared_future;
template <class R> class shared_future<R&>;
template <> class shared_future<void>;
template <class> class packaged_task; template <class R, class... ArgTypes>
class packaged_task<R(ArgTypes...)>;
template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&) noexcept;
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>;
template <class F, class... Args>
future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
async(F&& f, Args&&... args);
template <class F, class... Args>
future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
async(launch policy, F&& f, Args&&... args);
}The
enum type
launch is a bitmask type (
[bitmask.types]) with
elements
launch::async and
launch::deferred.[
Note: Implementations can provide bitmasks to specify restrictions on task
interaction by functions launched by
async() applicable to a
corresponding subset of available launch policies
.Implementations can extend
the behavior of the first overload of
async() by adding their extensions
to the launch policy under the “as if” rule
. —
end note ]
The enum values of
future_errc are distinct and not zero
.const error_category& future_category() noexcept;
Returns: A reference to an object of a type derived from class
error_category. The object's
default_error_condition and equivalent virtual functions shall
behave as specified for the class
error_category.The object's
name
virtual function shall return a pointer to the string
"future".error_code make_error_code(future_errc e) noexcept;
Returns: error_code(static_cast<int>(e), future_category()). error_condition make_error_condition(future_errc e) noexcept;
Returns: error_condition(static_cast<int>(e), future_category()).
namespace std {
class future_error : public logic_error {
public:
explicit future_error(future_errc e);
const error_code& code() const noexcept;
const char* what() const noexcept;
private:
error_code ec_; };
}explicit future_error(future_errc e);
Effects: Constructs an object of class
future_error
and initializes
ec_ with
make_error_code(e). const error_code& code() const noexcept;
const char* what() const noexcept;
Returns: An
ntbs incorporating
code().message(). Many of the classes introduced in this subclause use some state to communicate results
.This
shared state consists of some state information and some (possibly not
yet evaluated)
result, which can be a (possibly void) value or an exception
.[
Note: Futures, promises, and tasks defined in this clause reference such shared state
. —
end note ]
[
Note: The result can be any kind of object including a function to compute that result,
as used by
async when
policy is
launch::deferred. —
end note ]
A
waiting function of an asynchronous return object is one
that potentially blocks to wait for the shared state to be made
ready
. The result of a shared state is set by
respective functions on the asynchronous provider
.[
Note: Such as promises or tasks
. —
end note ]
The means of setting the result of a shared state is specified
in the description of those classes and functions that create such a state object
. When an asynchronous return object or an asynchronous provider is said to release its
shared state, it means:
if the return object or provider holds the last reference to its shared state,
the shared state is destroyed; and
the return object or provider gives up its reference to its shared state; and
these actions will not block for the shared state to become ready, except that it
may block if all of the following are true: the shared state was created by a call to
std::async, the shared state is not yet ready, and this was the last reference
to the shared state
.
When an asynchronous provider is said to make its shared state ready, it means:
When an asynchronous provider is said to abandon its shared state, it means:
A shared state is
ready only if it holds a value or an exception ready for
retrieval
.Waiting for a shared state to become ready may invoke code to compute the result on
the waiting thread if so specified in the description of the class or function that creates
the state object
.Calls to functions that successfully set the stored result of a shared
state synchronize
with (
[intro.multithread]) calls to functions
successfully detecting the ready state resulting from that setting
.The storage of the result
(whether normal or exceptional) into the shared state
synchronizes with (
[intro.multithread])
the successful return from a call to a waiting function on the shared state
.Some functions (e.g.,
promise::set_value_at_thread_exit) delay making
the shared state ready until the calling thread exits
.The destruction of
each of that thread's objects with thread storage duration (
[basic.stc.thread])
is sequenced before making that shared state ready
. [
Note: This explicitly specifies that the result of the shared state is
visible in the objects that reference this state in the sense of data race
avoidance (
[res.on.data.races])
.For example, concurrent accesses through
references returned by
shared_future::get() (
[futures.shared_future])
must either use read-only operations or provide additional synchronization
. —
end note ]
namespace std {
template <class R>
class promise {
public:
promise();
template <class Allocator>
promise(allocator_arg_t, const Allocator& a);
promise(promise&& rhs) noexcept;
promise(const promise& rhs) = delete;
~promise();
promise& operator=(promise&& rhs) noexcept;
promise& operator=(const promise& rhs) = delete;
void swap(promise& other) noexcept;
future<R> get_future();
void set_value(see below);
void set_exception(exception_ptr p);
void set_value_at_thread_exit(see below);
void set_exception_at_thread_exit(exception_ptr p);
};
template <class R>
void swap(promise<R>& x, promise<R>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>;
}The implementation shall provide the template
promise and two specializations,
promise<R&> and
promise<void>.These differ only in the argument type
of the member functions
set_value and
set_value_at_thread_exit,
as set out in their descriptions, below
.The
set_value,
set_exception,
set_value_at_thread_exit,
and
set_exception_at_thread_exit member functions behave as though
they acquire a single mutex associated with the promise object while updating the
promise object
.template <class R, class Alloc>
struct uses_allocator<promise<R>, Alloc>
: true_type { };
promise();
template <class Allocator>
promise(allocator_arg_t, const Allocator& a);
Effects: constructs a
promise object and a shared state
. The second
constructor uses the allocator
a to allocate memory for the shared
state
.promise(promise&& rhs) noexcept;
Effects: constructs a new
promise object and transfers ownership of the shared state
of
rhs (if any) to the newly-constructed object
. Postconditions: rhs has no shared state
. ~promise();
promise& operator=(promise&& rhs) noexcept;
Effects:
Abandons any shared state (
[futures.state]) and then as if
promise(std::move(rhs)).swap(*this). void swap(promise& other) noexcept;
Effects: Exchanges the shared state of
*this and
other. Postconditions: *this has the shared state (if any) that
other had
prior to the call to
swap. other has the shared state (if any) that
*this had prior to the call to
swap. future<R> get_future();
Returns: A
future<R> object with the same shared state as
*this. Throws: future_error if
*this has no shared state or if
get_future has already been called on a
promise with the same
shared state as
*this. Error conditions:future_already_retrieved if
get_future has already been called on
a
promise with the same shared state as
*this. no_state if
*this has no shared state
.
void promise::set_value(const R& r);
void promise::set_value(R&& r);
void promise<R&>::set_value(R& r);
void promise<void>::set_value();
Effects: Atomically stores the value
r in the shared state and
makes that state ready (
[futures.state])
. Throws:future_error if its shared state
already has a stored value or exception, or
for the first version, any exception thrown by the constructor selected to copy an object of R, or
for the second version, any exception thrown by the constructor selected to move an object of
R.
Error conditions:
promise_already_satisfied if its shared state
already has a stored value or exception
. no_state if
*this has no shared state
.
void set_exception(exception_ptr p);
Effects: Atomically stores the exception pointer
p in the shared state
and makes that state ready (
[futures.state])
. Throws: future_error if its shared state
already has a stored value or exception
. Error conditions:promise_already_satisfied if its shared state
already has a stored value or exception
. no_state if
*this has no shared state
.
void promise::set_value_at_thread_exit(const R& r);
void promise::set_value_at_thread_exit(R&& r);
void promise<R&>::set_value_at_thread_exit(R& r);
void promise<void>::set_value_at_thread_exit();
Effects: Stores the value
r in the shared state without making that
state ready immediately
. Schedules that 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 its shared state
already has a stored value or exception, or
for the first version, any exception thrown by the constructor selected to copy an object of R, or
for the second version, any exception thrown by the constructor selected to move an object of
R.
Error conditions:
promise_already_satisfied if its shared state
already has a stored value or exception
. no_state if
*this has no shared state
.
void set_exception_at_thread_exit(exception_ptr p);
Effects: Stores the exception pointer
p in the shared state without
making that state ready immediately
. Schedules that 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:
promise_already_satisfied if its shared state
already has a stored value or exception
. no_state if
*this has no shared state
.
template <class R>
void swap(promise<R>& x, promise<R>& y) noexcept;
Effects: As if by
x.swap(y). The class template
future defines a type for asynchronous return objects which
do not share their shared state with other asynchronous return objects
.A default-constructed
future object has no
shared state
.A
future object with shared state can be created by
functions on asynchronous providers (
[futures.state]) or by the move constructor
and shares its shared state with
the original asynchronous provider
.The result (value or exception) of
a
future object
can be
set by
calling a respective function on an
object that shares the same
shared state
.[
Note: Member functions of
future do not synchronize with themselves or with
member functions of
shared_future. —
end note ]
The effect of calling any member function other than the destructor, the
move-assignment operator,
share, or
valid on a
future object for which
valid() == false
is undefined
.[
Note: It is valid to move from a future object for which
valid() == false. —
end note ]
[
Note: Implementations are encouraged to detect this case and throw an object of type
future_error with an error condition of
future_errc::no_state. —
end note ]
namespace std {
template <class R>
class future {
public:
future() noexcept;
future(future&&) noexcept;
future(const future& rhs) = delete;
~future();
future& operator=(const future& rhs) = delete;
future& operator=(future&&) noexcept;
shared_future<R> share() noexcept;
see below get();
bool valid() const noexcept;
void wait() const;
template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
};
}The implementation shall provide the template
future and two specializations,
future<R&> and
future<void>.These differ only in the return type and return
value of the member function
get, as set out in its description, below
.future() noexcept;
Effects: Constructs an
empty
future object that does not refer to a
shared state
. Postconditions: valid() == false. future(future&& rhs) noexcept;
Effects: Move constructs a
future object that refers to the shared
state that
was originally referred to by
rhs (if any)
. Postconditions:
valid() returns the same value as
rhs.valid() prior to the
constructor invocation
.
~future();
future& operator=(future&& rhs) noexcept;
Effects:
move assigns the contents of
rhs to
*this.
Postconditions:
valid() returns the same value as
rhs.valid() prior to the
assignment
.
shared_future<R> share() noexcept;
Returns: shared_future<R>(std::move(*this)). Postconditions: valid() == false. R future::get();
R& future<R&>::get();
void future<void>::get();
[
Note: As described above, the template and its two required specializations differ only in
the return type and return value of the member function
get. —
end note ]
Returns:
future::get() returns the value
v stored in the object's shared state as
std::move(v). future<R&>::get() returns the reference stored as value in the object's shared state
. future<void>::get() returns nothing
.
Throws: the stored exception, if an exception was stored in the shared state
. Postconditions: valid() == false. bool valid() const noexcept;
Returns: true only if
*this refers to a shared state
. void wait() const;
Effects:
Blocks until the shared state is ready
. template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
Effects:
None if the shared state contains a deferred function (
[futures.async]),
otherwise
blocks until the shared state is ready or until
the relative timeout (
[thread.req.timing]) specified by
rel_time has expired
. Returns:future_status::deferred if the shared state contains a deferred
function
. future_status::ready if the shared state is ready
. future_status::timeout if the function is returning because the
relative timeout (
[thread.req.timing])
specified by
rel_time has expired
.
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
Effects:
None if the shared state contains a deferred function (
[futures.async]),
otherwise
blocks until the shared state is ready or until
the absolute timeout (
[thread.req.timing]) specified by
abs_time has expired
. Returns:future_status::deferred if the shared state contains a deferred
function
. future_status::ready if the shared state is ready
. future_status::timeout if the function is returning because the
absolute timeout (
[thread.req.timing])
specified by
abs_time has expired
.
The class template
shared_future defines a type for asynchronous return objects
which may share their shared state with other asynchronous return
objects
.A default-constructed
shared_future
object has no shared state
.A
shared_future object with
shared state can
be created
by conversion from a
future object and shares its shared state with the
original asynchronous provider (
[futures.state]) of the shared state
.The result (value or exception) of a
shared_future object
can be set by
calling a respective function on an
object that shares the same shared state
.[
Note: Member functions of
shared_future do not synchronize with themselves,
but they synchronize with the shared state
. —
end note ]
The effect of calling any member function other than the destructor,
the move-assignment operator, the copy-assignment operator, or
valid() on a
shared_future object for which
valid() == false is undefined
.[
Note: It is valid to copy or move from a
shared_future
object for which
valid() is
false. —
end note ]
[
Note: Implementations are encouraged to detect this case and throw an object of type
future_error with an error condition of
future_errc::no_state. —
end note ]
namespace std {
template <class R>
class shared_future {
public:
shared_future() noexcept;
shared_future(const shared_future& rhs) noexcept;
shared_future(future<R>&&) noexcept;
shared_future(shared_future&& rhs) noexcept;
~shared_future();
shared_future& operator=(const shared_future& rhs) noexcept;
shared_future& operator=(shared_future&& rhs) noexcept;
see below get() const;
bool valid() const noexcept;
void wait() const;
template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
};
}The implementation shall provide the template
shared_future and two
specializations,
shared_future<R&> and
shared_future<void>.These
differ only in the return type and return value of the member function
get, as
set out in its description, below
.shared_future() noexcept;
Effects: Constructs an
empty shared_future object that does not refer to a
shared state
. Postconditions: valid() == false. shared_future(const shared_future& rhs) noexcept;
Effects: Constructs a
shared_future object that refers to the same
shared state as
rhs (if any)
. Postconditions: valid() returns the same value as
rhs.valid(). shared_future(future<R>&& rhs) noexcept;
shared_future(shared_future&& rhs) noexcept;
Effects: Move constructs a
shared_future object that refers to the
shared state that was originally referred to by
rhs (if any)
. Postconditions:
valid() returns the same value as
rhs.valid() returned prior to
the constructor invocation
.
~shared_future();
shared_future& operator=(shared_future&& rhs) noexcept;
Effects:
move assigns the contents of
rhs to
*this.
Postconditions:
valid() returns the same value as
rhs.valid() returned prior to
the assignment
.
shared_future& operator=(const shared_future& rhs) noexcept;
Effects:
assigns the contents of
rhs to
*this.[
Note: As a result,
*this refers to the same shared state as
rhs
(if any)
. —
end note ]
Postconditions: valid() == rhs.valid(). const R& shared_future::get() const;
R& shared_future<R&>::get() const;
void shared_future<void>::get() const;
[
Note: As described above, the template and its two required specializations differ only in
the return type and return value of the member function
get. —
end note ]
[
Note: Access to a value object stored in the shared state is
unsynchronized, so programmers should apply only those operations on
R that do not
introduce a data race (
[intro.multithread])
. —
end note ]
Effects: wait()s until the shared state is ready, then retrieves the
value stored in the shared state
. Returns:
shared_future::get() returns a const reference to the value stored in the object's
shared state
. [
Note: Access through that reference after the shared state has been
destroyed produces undefined behavior; this can be avoided by not storing the reference in any
storage with a greater lifetime than the
shared_future object that returned the
reference
. —
end note ]
shared_future<R&>::get() returns the reference stored as value in the object's
shared state
. shared_future<void>::get() returns nothing
.
Throws: the stored exception, if an exception was stored in the shared state
. bool valid() const noexcept;
Returns: true only if
*this refers to a shared state
. void wait() const;
Effects:
Blocks until the shared state is ready
. template <class Rep, class Period>
future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;
Effects:
None if the shared state contains a deferred function (
[futures.async]),
otherwise
blocks until the shared state is ready or until
the relative timeout (
[thread.req.timing]) specified by
rel_time has expired
. Returns:future_status::deferred if the shared state contains a deferred
function
. future_status::ready if the shared state is ready
. future_status::timeout if the function is returning because the
relative timeout (
[thread.req.timing])
specified by
rel_time has expired
.
template <class Clock, class Duration>
future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;
Effects:
None if the shared state contains a deferred function (
[futures.async]),
otherwise
blocks until the shared state is ready or until the
absolute timeout (
[thread.req.timing]) specified by
abs_time has expired
. Returns:future_status::deferred if the shared state contains a deferred
function
. future_status::ready if the shared state is ready
. future_status::timeout if the function is returning because the
absolute timeout (
[thread.req.timing])
specified by
abs_time has expired
.
The function template
async provides a mechanism to launch a function potentially
in a new thread and provides the result of the function in a
future object with which
it shares a shared state
.template <class F, class... Args>
future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
async(F&& f, Args&&... args);
template <class F, class... Args>
future<invoke_result_t<decay_t<F>, decay_t<Args>...>>
async(launch policy, F&& f, Args&&... args);
Requires: F and each
Ti in
Args shall
satisfy the
MoveConstructible requirements, and
INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
shall be a valid expression
. Effects:
The first function
behaves the same as a call to the second function with a
policy argument of
launch::async | launch::deferred
and the same arguments for
F and
Args. The second function creates a shared state that is associated with
the returned
future object
.The further behavior
of the second function depends on the policy argument as follows (if
more than one of these conditions applies, the implementation may choose any of
the corresponding policies):
If
launch::async is set in
policy, calls
INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
(
[func.require],
[thread.thread.constr])
as if in a new thread of execution represented by a
thread object
with the calls to
DECAY_COPY() being evaluated in the thread that called
async.Any return value
is stored as the result in the
shared state
.Any exception propagated from
the execution of
INVOKE(DECAY_COPY(std::forward<F>(f)),
DECAY_COPY(std::forward<Args>(args))...)
is stored as the exceptional result in the shared state
.The
thread object is
stored in the shared state
and affects the behavior of any asynchronous return objects that
reference that state
.If
launch::deferred is set in
policy,
stores
DECAY_COPY(std::forward<F>(f)) and
DECAY_COPY(std::forward<Args>(args))...
in the shared state
.Invocation of the deferred function evaluates
INVOKE(std::move(g), std::move(xyz)) where
g is the stored value of
DECAY_COPY(std::forward<F>(f)) and
xyz is the stored copy of
DECAY_COPY(std::forward<Args>(args))....Any return value is stored
as the result in the shared state
.Any exception propagated
from the execution
of the deferred function
is stored as the exceptional result
in the shared state
.The shared state is not
made ready until the function has completed
.The first call to a
non-timed waiting function (
[futures.state])
on an asynchronous return object referring to
this shared state shall invoke the
deferred function in the thread that called the waiting function
.Once evaluation of
INVOKE(std::move(g), std::move(xyz)) begins, the function is no longer
considered deferred
.[
Note: If this policy is specified together with other policies, such as when using a
policy value of
launch::async | launch::deferred, implementations should defer
invocation or the selection of the policy when no more concurrency can be effectively
exploited
. —
end note ]
If no value is set in the launch policy, or a value is set that is neither specified
in this International Standard nor by the implementation, the behavior is undefined
.
Returns: An object of type
future<invoke_result_t<decay_t<F>, decay_t<Args>...>> that refers
to the shared state created by this call to
async. [
Note: If a future obtained from
async is moved outside the local scope,
other code that uses the future must be aware that the future's destructor may
block for the shared state to become ready
. —
end note ]
Synchronization:
Regardless of the provided
policy argument,
-
[
Note: This statement applies even when the corresponding
future object is moved to
another thread
. —
end note ]
; and
the completion of the function
f is sequenced before (
[intro.multithread]) the
shared state is made ready
.[
Note: f might not be called at all,
so its completion might never happen
. —
end note ]
If the implementation chooses the
launch::async policy,
a call to a waiting function on an asynchronous return
object that shares the shared state created by this
async call shall
block until the associated thread has completed, as if joined, or else time
out (
[thread.thread.member]);
the associated thread completion
synchronizes with (
[intro.multithread])
the return from
the first function
that successfully detects the ready status of the shared state or
with the return from the last
function that releases the shared state, whichever
happens first
.
Throws: system_error if
policy == launch::async and the
implementation is unable to start a new thread, or
std::bad_alloc if memory for the internal data structures
could not be allocated
. Error conditions:
resource_unavailable_try_again — if
policy == launch::async and the system is unable to start a new thread
.
[
Example:
int work1(int value);
int work2(int value);
int work(int value) {
auto handle = std::async([=]{ return work2(value); });
int tmp = work1(value);
return tmp + handle.get(); }[
Note: Line #1 might not result in concurrency because
the
async call uses the default policy, which may use
launch::deferred, in which case the lambda might not be invoked until the
get() call; in that case,
work1 and
work2 are called on the
same thread and there is no concurrency
. —
end note ]
—
end example ]
The class template
packaged_task defines a type for wrapping a function or
callable object so that the return value of the function or callable object is stored in
a future when it is invoked
.When the
packaged_task object is invoked, its stored task is invoked and the
result (whether normal or exceptional) stored in the shared state
.Any futures that
share the shared state will then be able to access the stored result
.
namespace std {
template<class> class packaged_task;
template<class R, class... ArgTypes>
class packaged_task<R(ArgTypes...)> {
public:
packaged_task() noexcept;
template <class F>
explicit packaged_task(F&& f);
~packaged_task();
packaged_task(const packaged_task&) = delete;
packaged_task& operator=(const packaged_task&) = delete;
packaged_task(packaged_task&& rhs) noexcept;
packaged_task& operator=(packaged_task&& rhs) noexcept;
void swap(packaged_task& other) noexcept;
bool valid() const noexcept;
future<R> get_future();
void operator()(ArgTypes... );
void make_ready_at_thread_exit(ArgTypes...);
void reset();
};
template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>;
}packaged_task() noexcept;
Effects: Constructs a
packaged_task object with no shared state and no stored task
. template <class F>
packaged_task(F&& f);
Requires:
INVOKE<R>(f, t1, t2, ..., tN), where
t1, t2, ..., tN are values
of the corresponding types in
ArgTypes..., shall be a valid expression
. Invoking
a copy of
f shall behave the same as invoking
f.Remarks:
This constructor shall not participate in overload resolution if
decay_t<F>
is the same type as
packaged_task<R(ArgTypes...)>. Effects: Constructs a new
packaged_task object with a shared state and
initializes the object's stored task with
std::forward<F>(f). Throws:
Any exceptions thrown by the copy or move constructor of
f.For the first version,
bad_alloc if memory for the internal data structures could not be allocated
.For the second version,
any exceptions thrown by
allocator_traits<Allocator>::template
rebind_traits<unspecified>::allocate.
packaged_task(packaged_task&& rhs) noexcept;
Effects: Constructs a new
packaged_task object and 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
. packaged_task& operator=(packaged_task&& rhs) noexcept;
Effects:
calls
packaged_task(std::move(rhs)).swap(*this).
~packaged_task();
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. bool valid() const noexcept;
Returns: true only if
*this has a shared state
. future<R> get_future();
Returns: A
future object that shares the same shared state as
*this. Throws: a
future_error object if an error occurs
. Error conditions:
future_already_retrieved if
get_future has already been called on
a
packaged_task object with the same shared state as
*this. no_state if
*this has no shared state
.
void operator()(ArgTypes... args);
Effects: As if by
INVOKE<R>(f, t1, t2, ..., tN),
where
f is the
stored task of
*this and
t1, t2, ..., tN 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:
promise_already_satisfied if
the stored task has already been invoked
. no_state if
*this has no shared state
.
void make_ready_at_thread_exit(ArgTypes... args);
Effects: As if by
INVOKE<R>(f, t1, t2, ..., tN),
where
f is the stored task and
t1, t2, ..., tN 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 shall be 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:
promise_already_satisfied if the
stored task has already been invoked
. no_state if
*this has no shared state
.
void reset();
Effects: As if
*this = packaged_task(std::move(f)), where
f is the task stored in
*this. [
Note: This constructs a new shared state for
*this. —
end note ]
Throws:
bad_alloc if memory for the new shared state could not be allocated
. any exception thrown by the move constructor of the task stored in the shared
state
.future_error with an error condition of
no_state if
*this
has no shared state
.
template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
Effects: As if by
x.swap(y). template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>
: true_type { };