[exec.task]

# 33 Execution control library [[exec]](./#exec)

## 33.13 Coroutine utilities [[exec.coro.util]](exec.coro.util#exec.task)

### 33.13.6 execution​::​task [exec.task]

#### [33.13.6.1](#task.overview) task overview [[task.overview]](task.overview)

[1](#task.overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7171)

The task class template represents a sender that can
be used as the return type of coroutines[.](#task.overview-1.sentence-1)

The first template parameter T defines the type of the value
completion datum ([[exec.async.ops]](exec.async.ops "33.3 Asynchronous operations")) if T is not void[.](#task.overview-1.sentence-2)

Otherwise, there are no value completion datums[.](#task.overview-1.sentence-3)

Inside coroutines returning task<T, E> the operand ofco_return (if any) becomes the argument of set_value[.](#task.overview-1.sentence-4)

The second template parameter Environment is used to customize
the behavior of task[.](#task.overview-1.sentence-5)

#### [33.13.6.2](#task.class) Class template task [[task.class]](task.class)

namespace std::execution {template<class T, class Environment>class [task](#lib:task "33.13.6.2 Class template task [task.class]") {// [[task.state]](#task.state "33.13.6.4 Class template task​::​state")template<[receiver](exec.recv.concepts#concept:receiver "33.7.1 Receiver concepts [exec.recv.concepts]") Rcvr>class *state*; // *exposition only*public:using sender_concept = sender_t; using completion_signatures = *see below*; using allocator_type = *see below*; using scheduler_type = *see below*; using stop_source_type = *see below*; using stop_token_type = decltype(declval<stop_source_type>().get_token()); using error_types = *see below*; // [[task.promise]](#task.promise "33.13.6.5 Class task​::​promise_­type")class promise_type;

 task(task&&) noexcept; ~task(); template<[receiver](exec.recv.concepts#concept:receiver "33.7.1 Receiver concepts [exec.recv.concepts]") Rcvr>*state*<Rcvr> connect(Rcvr&& rcvr); private: coroutine_handle<promise_type> *handle*; // *exposition only*};}

[1](#task.class-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7216)

task<T, E> models [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") ([[exec.snd]](exec.snd "33.9 Senders"))
if T is void, a reference type, or a cv-unqualified
non-array object type and E is a class type[.](#task.class-1.sentence-1)

Otherwise a program that instantiates the definition of task<T, E> is ill-formed[.](#task.class-1.sentence-2)

[2](#task.class-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7223)

The nested types of task template specializations
are determined based on the Environment parameter:

- [(2.1)](#task.class-2.1)

  allocator_type is Environment​::​allocator_type if that [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") is valid and denotes a type,allocator<byte> otherwise[.](#task.class-2.1.sentence-1)

- [(2.2)](#task.class-2.2)

  scheduler_type is Environment​::​scheduler_type if that [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") is valid and denotes a type,task_scheduler otherwise[.](#task.class-2.2.sentence-1)

- [(2.3)](#task.class-2.3)

  stop_source_type is Environment​::​stop_source_type if that [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") is valid and denotes a type,inplace_stop_source otherwise[.](#task.class-2.3.sentence-1)

- [(2.4)](#task.class-2.4)

  error_types is Environment​::​error_types if
that [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") is valid and denotes a type,completion_signatures<set_error_t(exception_ptr)> otherwise[.](#task.class-2.4.sentence-1)

[3](#task.class-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7241)

A program is ill-formed if error_types is not a
specialization of completion_signatures<ErrorSigs...> orErrorSigs contains an element which is not of the formset_error_t(E) for some type E[.](#task.class-3.sentence-1)

[4](#task.class-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7247)

The type alias completion_signatures is a specialization
of execution​::​completion_signatures with the template
arguments (in unspecified order):

- [(4.1)](#task.class-4.1)

set_value_t() if T is void,
and set_value_t(T) otherwise;

- [(4.2)](#task.class-4.2)

template arguments of the specialization ofexecution​::​completion_signatures denoted by error_types;
and

- [(4.3)](#task.class-4.3)

set_stopped_t()[.](#task.class-4.sentence-1)

[5](#task.class-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7260)

allocator_type shall meet the *Cpp17Allocator* requirements[.](#task.class-5.sentence-1)

#### [33.13.6.3](#task.members) task members [[task.members]](task.members)

[ð](#lib:task,constructor)

`task(task&& other) noexcept;
`

[1](#task.members-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7271)

*Effects*: Initializes *handle* with exchange(other.*handle*,{})[.](#task.members-1.sentence-1)

[ð](#lib:task,destructor)

`~task();
`

[2](#task.members-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7282)

*Effects*: Equivalent to:if (*handle*)*handle*.destroy();

[ð](#lib:connect,task)

`template<[receiver](exec.recv.concepts#concept:receiver "33.7.1 Receiver concepts [exec.recv.concepts]") Rcvr>
  state<Rcvr> connect(Rcvr&& recv);
`

[3](#task.members-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7297)

*Preconditions*: bool(*handle*) is true[.](#task.members-3.sentence-1)

[4](#task.members-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7301)

*Effects*: Equivalent to:return *state*<Rcvr>(exchange(*handle*, {}), std::forward<Rcvr>(recv));

#### [33.13.6.4](#task.state) Class template task​::​*state* [[task.state]](task.state)

namespace std::execution {template<class T, class Environment>template<[receiver](exec.recv.concepts#concept:receiver "33.7.1 Receiver concepts [exec.recv.concepts]") Rcvr>class task<T, Environment>::*state* { // *exposition only*public:using operation_state_concept = operation_state_t; template<class R>*state*(coroutine_handle<promise_type> h, R&& rr); ~*state*(); void start() & noexcept; private:using *own-env-t* = *see below*; // *exposition only* coroutine_handle<promise_type> *handle*; // *exposition only* remove_cvref_t<Rcvr> *rcvr*; // *exposition only**own-env-t* *own-env*; // *exposition only* Environment *environment*; // *exposition only*};}

[1](#task.state-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7336)

The type *own-env-t* is Environment​::​template env_type<decltype(get_env(​declval​<Rcvr>(​)))​> if that[*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]") is valid and denotes a type, env<> otherwise[.](#task.state-1.sentence-1)

[ð](#lib:task::state,constructor)

`template<class R>
  state(coroutine_handle<promise_type> h, R&& rr);
`

[2](#task.state-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7347)

*Effects*: Initializes

- [(2.1)](#task.state-2.1)

  *handle* with std​::​move(h);

- [(2.2)](#task.state-2.2)

  *rcvr* with std​::​forward<R>(rr);

- [(2.3)](#task.state-2.3)

  *own-env* with *own-env-t*(get_env(*rcvr*)) if that expression
is valid and *own-env-t*() otherwise[.](#task.state-2.3.sentence-1)
  If neither of these expressions is valid, the program is ill-formed[.](#task.state-2.3.sentence-2)

- [(2.4)](#task.state-2.4)

  *environment* withEnvironment(*own-env*) if that expression is
valid, otherwise Environment(​get_env(*rcvr*)) if this expression is valid, otherwise Environment()[.](#task.state-2.4.sentence-1)
  If neither of these expressions is valid, the program is ill-formed[.](#task.state-2.4.sentence-2)

[ð](#lib:task::state,destructor)

`~state();
`

[3](#task.state-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7370)

*Effects*: Equivalent to:if (*handle*)*handle*.destroy();

[ð](#lib:start,task::state)

`void start() & noexcept;
`

[4](#task.state-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7384)

*Effects*: Let *prom* be the object *handle*.promise()[.](#task.state-4.sentence-1)

Associates *STATE*(*prom*), *RCVR*(*prom*), and *SCHED*(*prom*) with *this as follows:

- [(4.1)](#task.state-4.1)

  *STATE*(*prom*) is *this[.](#task.state-4.1.sentence-1)

- [(4.2)](#task.state-4.2)

  *RCVR*(*prom*) is *rcvr*[.](#task.state-4.2.sentence-1)

- [(4.3)](#task.state-4.3)

  *SCHED*(*prom*) is the object initialized
with scheduler_type(get_scheduler(get_env(*rcvr*))) if that expression is valid and scheduler_type() otherwise[.](#task.state-4.3.sentence-1)
  If neither of these expressions is valid, the program is ill-formed[.](#task.state-4.3.sentence-2)

Let *st* be get_stop_token(get_env(*rcvr*))[.](#task.state-4.sentence-3)

Initializes *prom*.*token* and*prom*.*source* such that

- [(4.4)](#task.state-4.4)

*prom*.*token*.stop_requested() returns*st*.stop_requested();

- [(4.5)](#task.state-4.5)

*prom*.*token*.stop_possible() returns*st*.stop_possible(); and

- [(4.6)](#task.state-4.6)

for types Fn and Init such that both[invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]")<Fn> and[constructible_from](concept.constructible#concept:constructible_from "18.4.11 Concept constructible_­from [concept.constructible]")<Fn, Init> are modeled,stop_token_type​::​callback_type<Fn> models[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]")<Fn,​ stop_token_type, Init>[.](#task.state-4.sentence-4)

After that invokes *handle*.resume()[.](#task.state-4.sentence-5)

#### [33.13.6.5](#task.promise) Class task​::​promise_type [[task.promise]](task.promise)

namespace std::execution {template<class T, class Environment>class task<T, Environment>::promise_type {public:template<class... Args> promise_type(const Args&... args);

 task get_return_object() noexcept; auto initial_suspend() noexcept; auto final_suspend() noexcept; void uncaught_exception();
 coroutine_handle<> unhandled_stopped(); void return_void(); // present only if is_void_v<T> is truetemplate<class V>void return_value(V&& value); // present only if is_void_v<T> is falsetemplate<class E>*unspecified* yield_value(with_error<E> error); template<class A>auto await_transform(A&& a); template<class Sch>auto await_transform(change_coroutine_scheduler<Sch> sch); *unspecified* get_env() const noexcept; template<class... Args>void* operator new(size_t size, Args&&... args); void operator delete(void* pointer, size_t size) noexcept; private:using *error-variant* = *see below*; // *exposition only* allocator_type *alloc*; // *exposition only* stop_source_type *source*; // *exposition only* stop_token_type *token*; // *exposition only* optional<T> *result*; // *exposition only*; present only if is_void_v<T> is false*error-variant* *errors*; // *exposition only*};}

[1](#task.promise-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7466)

Let *prom* be an object of promise_type and let *tsk* be the task object
created by *prom*.get_return_object()[.](#task.promise-1.sentence-1)

The description below
refers to objects *STATE*(*prom*),*RCVR*(*prom*),
and *SCHED*(*prom*) associated with *tsk* during evaluation of task​::​*state*<Rcvr>​::​start for some receiver Rcvr[.](#task.promise-1.sentence-2)

[2](#task.promise-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7478)

*error-variant* is a variant<monostate,
remove_cvref_t<E>...>, with duplicate types removed, where E... are the parameter types of the template arguments of the specialization ofexecution​::​completion_signatures denoted byerror_types[.](#task.promise-2.sentence-1)

[ð](#lib:task::promise_type,constructor)

`template<class... Args>
  promise_type(const Args&... args);
`

[3](#task.promise-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7491)

*Mandates*: The first parameter of type allocator_arg_t (if any) is not
the last parameter[.](#task.promise-3.sentence-1)

[4](#task.promise-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7496)

*Effects*: If Args contains an element of type allocator_arg_t then *alloc* is initialized with the corresponding next
element of args[.](#task.promise-4.sentence-1)

Otherwise, *alloc* is initialized with allocator_type()[.](#task.promise-4.sentence-2)

[ð](#lib:get_return_object,task::promise_type)

`task get_return_object() noexcept;
`

[5](#task.promise-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7509)

*Returns*: A task object whose member *handle* iscoroutine_handle<promise_type>​::​​from_promise​(*this)[.](#task.promise-5.sentence-1)

[ð](#lib:initial_suspend,task::promise_type)

`auto initial_suspend() noexcept;
`

[6](#task.promise-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7520)

*Returns*: An awaitable object of unspecified type ([[expr.await]](expr.await "7.6.2.4 Await")) whose
member functions arrange for

- [(6.1)](#task.promise-6.1)

the calling coroutine to be suspended,

- [(6.2)](#task.promise-6.2)

the coroutine to be resumed on an execution agent of the
execution resource associated with *SCHED*(*this)[.](#task.promise-6.sentence-1)

[ð](#lib:final_suspend,task::promise_type)

`auto final_suspend() noexcept;
`

[7](#task.promise-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7536)

*Returns*: An awaitable object of unspecified type ([[expr.await]](expr.await "7.6.2.4 Await")) whose
member functions arrange for the completion of the asynchronous
operation associated with *STATE*(*this) by invoking:

- [(7.1)](#task.promise-7.1)

set_error(std​::​move(*RCVR*(*this)), std​::​move(e)) if *errors*.index() is greater than zero ande is the value held by *errors*, otherwise

- [(7.2)](#task.promise-7.2)

set_value(std​::​move(*RCVR*(*this))) if is_void<T> is true,
and otherwise

- [(7.3)](#task.promise-7.3)

set_value(std​::​move(*RCVR*(*this)), **result*)[.](#task.promise-7.sentence-1)

[ð](#lib:yield_value,task::promise_type)

`template<class Err>
  auto yield_value(with_error<Err> err);
`

[8](#task.promise-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7560)

*Mandates*: std​::​move(err.error) is convertible to exactly one of theset_error_t argument types of error_types[.](#task.promise-8.sentence-1)

Let *Cerr* be that type[.](#task.promise-8.sentence-2)

[9](#task.promise-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7566)

*Returns*: An awaitable object of unspecified type ([[expr.await]](expr.await "7.6.2.4 Await")) whose
member functions arrange for the calling coroutine to be suspended
and then completes the asynchronous operation associated with*STATE*(*this) by invoking set_error(std​::​move(*RCVR*(*this)),*Cerr*(std​::​move(err.error)))[.](#task.promise-9.sentence-1)

[ð](#lib:await_transform,task::promise_type)

`template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sender>
  auto await_transform(Sender&& sndr) noexcept;
`

[10](#task.promise-10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7581)

*Returns*: If [same_as](concept.same#concept:same_as "18.4.2 Concept same_­as [concept.same]")<inline_scheduler, scheduler_type> is true returns as_awaitable(​std​::​​forward<Sender>(sndr), *this);
otherwise returnsas_awaitable(affine_on(​std​::​​forward<Sender>(sndr), *SCHED*(*this)), *this)[.](#task.promise-10.sentence-1)

[ð](#lib:await_transform,task::promise_type_)

`template<class Sch>
  auto await_transform(change_coroutine_scheduler<Sch> sch) noexcept;
`

[11](#task.promise-11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7595)

*Effects*: Equivalent to:return await_transform(just(exchange(*SCHED*(*this), scheduler_type(sch.scheduler))), *this);

[ð](#lib:uncaught_exception,task::promise_type)

`void uncaught_exception();
`

[12](#task.promise-12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7608)

*Effects*: If the signature set_error_t(exception_ptr) is not an element
of error_types, calls terminate() ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#task.promise-12.sentence-1)

Otherwise, stores current_exception() into *errors*[.](#task.promise-12.sentence-2)

[ð](#lib:unhandled_stopped,task::promise_type)

`coroutine_handle<> unhandled_stopped();
`

[13](#task.promise-13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7620)

*Effects*: Completes the asynchronous operation associated with *STATE*(*this) by invoking set_stopped(std​::​move(*RCVR*(*this)))[.](#task.promise-13.sentence-1)

[14](#task.promise-14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7626)

*Returns*: noop_coroutine()[.](#task.promise-14.sentence-1)

[ð](#lib:get_env,task::promise_type)

`unspecified get_env() const noexcept;
`

[15](#task.promise-15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7636)

*Returns*: An object env such that queries are forwarded as follows:

- [(15.1)](#task.promise-15.1)

  env.query(get_scheduler) returns scheduler_type(*SCHED*(*this))[.](#task.promise-15.1.sentence-1)

- [(15.2)](#task.promise-15.2)

  env.query(get_allocator) returns *alloc*[.](#task.promise-15.2.sentence-1)

- [(15.3)](#task.promise-15.3)

  env.query(get_stop_token) returns *token*[.](#task.promise-15.3.sentence-1)

- [(15.4)](#task.promise-15.4)

  For any other query q and arguments a... a
call to env.query(q, a...) returns*STATE*(*this)[.](#task.promise-15.4.sentence-1)
  environment.query(q, a...) if this expression
is well-formed and forwarding_query(q) is well-formed and is true[.](#task.promise-15.4.sentence-2)
  Otherwise env.query(q, a...) is ill-formed[.](#task.promise-15.4.sentence-3)

[ð](#lib:operator_new,task::promise_type)

`template<class... Args>
  void* operator new(size_t size, const Args&... args);
`

[16](#task.promise-16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7657)

If there is no parameter with type allocator_arg_t then letalloc be allocator_type()[.](#task.promise-16.sentence-1)

Otherwise, let arg_next be the parameter
following the first allocator_arg_t parameter,
and let alloc be allocator_type(arg_next)[.](#task.promise-16.sentence-2)

Let PAlloc be allocator_traits<allocator_type>​::​template rebind_alloc<U>, where U is an unspecified type
whose size and alignment are both __STDCPP_DEFAULT_NEW_ALIGNMENT__[.](#task.promise-16.sentence-3)

[17](#task.promise-17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7667)

*Mandates*: 

- [(17.1)](#task.promise-17.1)

  The first parameter of type allocator_arg_t (if any) is not the last parameter[.](#task.promise-17.1.sentence-1)

- [(17.2)](#task.promise-17.2)

  allocator_type(arg_next) is a valid expression if there is a parameter
of type allocator_arg_t[.](#task.promise-17.2.sentence-1)

- [(17.3)](#task.promise-17.3)

  allocator_traits<PAlloc>​::​pointer is a pointer type[.](#task.promise-17.3.sentence-1)

[18](#task.promise-18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7676)

*Effects*: Initializes an allocator palloc of type PAlloc withalloc[.](#task.promise-18.sentence-1)

Uses palloc to allocate storage for the
smallest array of U sufficient to provide storage for a
coroutine state of size size, and unspecified additional
state necessary to ensure that operator delete can later
deallocate this memory block with an allocator equal to palloc[.](#task.promise-18.sentence-2)

[19](#task.promise-19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7686)

*Returns*: A pointer to the allocated storage[.](#task.promise-19.sentence-1)

[ð](#lib:operator_delete,task::promise_type)

`void operator delete(void* pointer, size_t size) noexcept;
`

[20](#task.promise-20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7696)

*Preconditions*: pointer was returned from an invocation of the above overload
of operator new with a size argument equal to size[.](#task.promise-20.sentence-1)

[21](#task.promise-21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7701)

*Effects*: Deallocates the storage pointed to by pointer using an
allocator equal to that used to allocate it[.](#task.promise-21.sentence-1)
