33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.12 Sender adaptors [exec.adapt]

33.9.12.1 General [exec.adapt.general]

Subclause [exec.adapt] specifies a set of sender adaptors.

The bitwise inclusive or operator is overloaded for the purpose of creating sender chains.

The adaptors also support function call syntax with equivalent semantics.

Unless otherwise specified:

(3.1)
A sender adaptor is prohibited from causing observable effects, apart from moving and copying its arguments, before the returned sender is connected with a receiver using connect, and start is called on the resulting operation state.
(3.2)
A parent sender ([exec.async.ops]) with a single child sender sndr has an associated attribute object equal to FWD-ENV(get_env(sndr)) ([exec.fwd.env]).
(3.3)
A parent sender with more than one child sender has an associated attributes object equal to env<>{}.
(3.4)
When a parent sender is connected to a receiver rcvr, any receiver used to connect a child sender has an associated environment equal to FWD-ENV(get_env(rcvr)).
(3.5)
An adaptor whose child senders are all non-dependent ([exec.async.ops]) is itself non-dependent.
(3.6)
These requirements apply to any function that is selected by the implementation of the sender adaptor.
(3.7)
Recommended practice: Implementations should use the completion signatures of the adaptors to communicate type errors to users and to propagate any such type errors from child senders.

If a sender returned from a sender adaptor specified in [exec.adapt] is specified to include set_error_t(Err) among its set of completion signatures where decay_t<Err> denotes the type exception_ptr, but the implementation does not potentially evaluate an error completion operation with an exception_ptr argument, the implementation is allowed to omit the exception_ptr error completion signature from the set.

33.9.12.2 Closure objects [exec.adapt.obj]

A pipeable sender adaptor closure object is a function object that accepts one or more sender arguments and returns a sender .

For a pipeable sender adaptor closure object c and an expression sndr such that decltype((sndr)) models sender, the following expressions are equivalent and yield a sender: c(sndr) sndr | c

Given an additional pipeable sender adaptor closure object d, the expression c | d produces another pipeable sender adaptor closure object e:

e is a perfect forwarding call wrapper ([func.require]) with the following properties:

(1.1)
Its target object is an object d2 of type decltype(auto(d)) direct-non-list-initialized with d.
(1.2)
It has one bound argument entity, an object c2 of type decltype(auto(c)) direct-non-list-initialized with c.
(1.3)
Its call pattern is d2(c2(arg)), where arg is the argument used in a function call expression of e.

The expression c | d is well-formed if and only if the initializations of the state entities ([func.def]) of e are all well-formed.

An object t of type T is a pipeable sender adaptor closure object if T models derived_from<sender_adaptor_closure<T>>, T has no other base classes of type sender_adaptor_closure<U> for any other type U, and T does not satisfy sender .

The template parameter D for sender_adaptor_closure can be an incomplete type.

Before any expression of type cv D appears as an operand to the | operator, D shall be complete and model derived_from<sender_adaptor_closure<D>>.

The behavior of an expression involving an object of type cv D as an operand to the | operator is undefined if overload resolution selects a program-defined operator| function.

A pipeable sender adaptor object is a customization point object that accepts a sender as its first argument and returns a sender .

If a pipeable sender adaptor object accepts only one argument, then it is a pipeable sender adaptor closure object.

If a pipeable sender adaptor object adaptor accepts more than one argument, then let sndr be an expression such that decltype((sndr)) models sender, let args... be arguments such that adaptor(sndr, args...) is a well-formed expression as specified below, and let BoundArgs be a pack that denotes decltype(auto(args))....

The expression adaptor(args...) produces a pipeable sender adaptor closure object f that is a perfect forwarding call wrapper with the following properties:

(5.1)
Its target object is a copy of adaptor.
(5.2)
Its bound argument entities bound_args consist of objects of types BoundArgs... direct-non-list-initialized with std::forward<decltype((args))>(args)..., respectively.
(5.3)
Its call pattern is adaptor(rcvr, bound_args...), where rcvr is the argument used in a function call expression of f.

The expression adaptor(args...) is well-formed if and only if the initializations of the bound argument entities of the result, as specified above, are all well-formed.

33.9.12.3 execution::write_env [exec.write.env]

write_env is a sender adaptor that accepts a sender and a queryable object, and that returns a sender that, when connected with a receiver rcvr, connects the adapted sender with a receiver whose execution environment is the result of joining the queryable object to the result of get_env(rcvr).

write_env is a customization point object.

For some subexpressions sndr and env, if decltype((sndr)) does not satisfy sender or if decltype((env)) does not satisfy queryable, the expression write_env(sndr, env) is ill-formed.

Otherwise, it is expression-equivalent to make-sender(write_env, env, sndr).

Let write-env-t denote the type decltype(auto(write_env)).

The exposition-only class template impls-for ([exec.snd.general]) is specialized for write-env-t as follows:

🔗

template<> struct impls-for<write-env-t> : default-impls { static constexpr auto join-env(const auto& state, const auto& env) noexcept { return see below; } static constexpr auto get-env = [](auto, const auto& state, const auto& rcvr) noexcept { return join-env(state, FWD-ENV(get_env(rcvr))); }; template<class Sndr, class... Env> static consteval void check-types(); };

Invocation of impls-for<write-env-t>::join-env returns an object e such that

(3.1)
decltype(e) models queryable and
(3.2)
given a query object q, the expression e.query(q) is expression-equivalent to state.query(q) if that expression is valid, otherwise, e.query(q) is expression-equivalent to env.query(q).
(3.3)
For a type Sndr and a pack of types Env, let State be data-type<Sndr> and let JoinEnv be the pack decltype(join-env(declval<State>(), FWD-ENV(declval<Env>()))).

Then impls-for<write-
env-t>::check-types<Sndr, Env...>() is expression-equivalent to get_completion_signatures<
child-type<Sndr>, JoinEnv...>().

33.9.12.4 execution::unstoppable [exec.unstoppable]

unstoppable is a sender adaptor that connects its inner sender with a receiver that has the execution environment of the outer receiver but with an object of type never_stop_token as the result of the get_stop_token query.

For a subexpression sndr, unstoppable(sndr) is expression-equivalent to write_env(sndr, prop(get_stop_token, never_stop_token{})).

33.9.12.5 execution::starts_on [exec.starts.on]

starts_on adapts an input sender into a sender that will start on an execution agent belonging to a particular scheduler's associated execution resource.

The name starts_on denotes a customization point object.

For subexpressions sch and sndr, if decltype((
sch)) does not satisfy scheduler, or decltype((sndr)) does not satisfy sender, starts_on(sch, sndr) is ill-formed.

Otherwise, the expression starts_on(sch, sndr) is expression-equivalent to: transform_sender( query-with-default(get_domain, sch, default_domain()), make-sender(starts_on, sch, sndr)) except that sch is evaluated only once.

Let out_sndr and env be subexpressions such that OutSndr is decltype((out_sndr)).

If sender-for<OutSndr, starts_on_t> is false, then the expressions starts_on.transform_env(out_sndr, env) and starts_on.transform_sender(out_sndr, env) are ill-formed; otherwise

(4.1)
starts_on.transform_env(out_sndr, env) is equivalent to: auto&& [_, sch, _] = out_sndr; return JOIN-ENV(SCHED-ENV(sch), FWD-ENV(env));
(4.2)
starts_on.transform_sender(out_sndr, env) is equivalent to: auto&& [_, sch, sndr] = out_sndr; return let_value( schedule(sch), [sndr = std::forward_like<OutSndr>(sndr)]() mutable noexcept(is_nothrow_move_constructible_v<decay_t<OutSndr>>) { return std::move(sndr); });

Let out_sndr be a subexpression denoting a sender returned from starts_on(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall start sndr on an execution agent of the associated execution resource of sch.

If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

33.9.12.6 execution::continues_on [exec.continues.on]

continues_on adapts a sender into one that completes on the specified scheduler.

The name continues_on denotes a pipeable sender adaptor object.

For subexpressions sch and sndr, if decltype((sch)) does not satisfy scheduler, or decltype((sndr)) does not satisfy sender, continues_on(sndr, sch) is ill-formed.

Otherwise, the expression continues_on(sndr, sch) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(continues_on, sch, sndr)) except that sndr is evaluated only once.

The exposition-only class template impls-for is specialized for continues_on_t as follows: namespace std::execution { template<> struct impls-for<continues_on_t> : default-impls { static constexpr auto get-attrs = [](const auto& data, const auto& child) noexcept -> decltype(auto) { return JOIN-ENV(SCHED-ATTRS(data), FWD-ENV(get_env(child))); }; }; }

Let sndr and env be subexpressions such that Sndr is decltype((sndr)).

If sender-for<Sndr, continues_on_t> is false, then the expression continues_on.transform_sender(sndr, env) is ill-formed; otherwise, it is equal to: auto [_, data, child] = sndr; return schedule_from(std::move(data), std::move(child));

[Note 1:

This causes the continues_on(sndr, sch) sender to become schedule_from(sch, sndr) when it is connected with a receiver whose execution domain does not customize continues_on.

— end note]

Let out_sndr be a subexpression denoting a sender returned from continues_on(sndr, sch) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall start sndr on the current execution agent and execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch.

If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

33.9.12.7 execution::schedule_from [exec.schedule.from]

schedule_from schedules work dependent on the completion of a sender onto a scheduler's associated execution resource.

[Note 1:

schedule_from is not meant to be used in user code; it is used in the implementation of continues_on.

— end note]

The name schedule_from denotes a customization point object.

For some subexpressions sch and sndr, let Sch be decltype((sch)) and Sndr be decltype((sndr)).

If Sch does not satisfy scheduler, or Sndr does not satisfy sender, schedule_from(sch, sndr) is ill-formed.

Otherwise, the expression schedule_from(sch, sndr) is expression-equivalent to: transform_sender( query-with-default(get_domain, sch, default_domain()), make-sender(schedule_from, sch, sndr)) except that sch is evaluated only once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for schedule_from_t as follows:

🔗

namespace std::execution { template<> struct impls-for<schedule_from_t> : default-impls { static constexpr auto get-attrs = see below; static constexpr auto get-state = see below; static constexpr auto complete = see below; template<class Sndr, class... Env> static consteval void check-types(); }; }

The member impls-for<schedule_from_t>::get-attrs is initialized with a callable object equivalent to the following lambda: [](const auto& data, const auto& child) noexcept -> decltype(auto) { return JOIN-ENV(SCHED-ATTRS(data), FWD-ENV(get_env(child))); }

The member impls-for<schedule_from_t>::get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below) requires sender_in<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)> { auto& [_, sch, child] = sndr; using sched_t = decltype(auto(sch)); using variant_t = see below; using receiver_t = see below; using operation_t = connect_result_t<schedule_result_t<sched_t>, receiver_t>; constexpr bool nothrow = noexcept(connect(schedule(sch), receiver_t{nullptr})); struct state-type { Rcvr& rcvr; // exposition only variant_t async-result; // exposition only operation_t op-state; // exposition only explicit state-type(sched_t sch, Rcvr& rcvr) noexcept(nothrow) : rcvr(rcvr), op-state(connect(schedule(sch), receiver_t{this})) {} }; return state-type{sch, rcvr}; }

🔗

template<class Sndr, class... Env>
  static consteval void check-types();

Effects: Equivalent to: get_completion_signatures<schedule_result_t<data-type<Sndr>>, FWD-ENV-T(Env)...>(); auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>(); decay-copyable-result-datums(cs); // see [exec.snd.expos]

Objects of the local class state-type can be used to initialize a structured binding.

Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>.

Let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Tag, Args...>, and let is-nothrow-decay-copy-sig be a variable template such that auto(is-nothrow-decay-copy-sig<Tag(Args...
)>) is a constant expression of type bool and equal to (is_nothrow_constructible_v<decay_t<Args>, Args> && ...).

Let error-completion be a pack consisting of the type set_error_t(exception_ptr) if (is-nothrow-decay-copy-sig<Sigs> &&...) is false, and an empty pack otherwise.

Then variant_t denotes the type variant<monostate, as-tuple<Sigs>..., error-completion...>, except with duplicate types removed.

receiver_t is an alias for the following exposition-only class: namespace std::execution { struct receiver-type { using receiver_concept = receiver_t; state-type* state; // exposition only void set_value() && noexcept { visit( [this]<class Tuple>(Tuple& result) noexcept -> void { if constexpr (!same_as<monostate, Tuple>) { auto& [tag, ...args] = result; tag(std::move(state->rcvr), std::move(args)...); } }, state->async-result); } template<class Error> void set_error(Error&& err) && noexcept { execution::set_error(std::move(state->rcvr), std::forward<Error>(err)); } void set_stopped() && noexcept { execution::set_stopped(std::move(state->rcvr)); } decltype(auto) get_env() const noexcept { return FWD-ENV(execution::get_env(state->rcvr)); } }; }

The expression in the noexcept clause of the lambda is true if the construction of the returned state-type object is not potentially throwing; otherwise, false.

The member impls-for<schedule_from_t>::complete is initialized with a callable object equivalent to the following lambda: []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void { using result_t = decayed-tuple<Tag, Args...>; constexpr bool nothrow = (is_nothrow_constructible_v<decay_t<Args>, Args> && ...); try { state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...); } catch (...) { if constexpr (!nothrow) state.async-result.template emplace<tuple<set_error_t, exception_ptr>>(set_error, current_exception()); } start(state.op-state); };

Let out_sndr be a subexpression denoting a sender returned from schedule_from(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall start sndr on the current execution agent and execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch.

If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

33.9.12.8 execution::on [exec.on]

The on sender adaptor has two forms:

(1.1)
on(sch, sndr), which starts a sender sndr on an execution agent belonging to a scheduler sch's associated execution resource and that, upon sndr's completion, transfers execution back to the execution resource on which the on sender was started.
(1.2)
on(sndr, sch, closure), which upon completion of a sender sndr, transfers execution to an execution agent belonging to a scheduler sch's associated execution resource, then executes a sender adaptor closure closure with the async results of the sender, and that then transfers execution back to the execution resource on which sndr completed.

The name on denotes a pipeable sender adaptor object.

For subexpressions sch and sndr, on(sch, sndr) is ill-formed if any of the following is true:

(2.1)
decltype((sch)) does not satisfy scheduler, or
(2.2)
decltype((sndr)) does not satisfy sender and sndr is not a pipeable sender adaptor closure object ([exec.adapt.obj]), or
(2.3)
decltype((sndr)) satisfies sender and sndr is also a pipeable sender adaptor closure object.

Otherwise, if decltype((sndr)) satisfies sender, the expression on(sch, sndr) is expression-equivalent to: transform_sender( query-with-default(get_domain, sch, default_domain()), make-sender(on, sch, sndr)) except that sch is evaluated only once.

For subexpressions sndr, sch, and closure, if

(4.1)
decltype((sch)) does not satisfy scheduler, or
(4.2)
decltype((sndr)) does not satisfy sender, or
(4.3)
closure is not a pipeable sender adaptor closure object ([exec.adapt.obj]),

the expression on(sndr, sch, closure) is ill-formed; otherwise, it is expression-equivalent to: transform_sender( get-domain-early(sndr), make-sender(on, product-type{sch, closure}, sndr)) except that sndr is evaluated only once.

Let out_sndr and env be subexpressions, let OutSndr be decltype((out_sndr)), and let Env be decltype((env)).

If sender-for<OutSndr, on_t> is false, then the expressions on.transform_env(out_sndr, env) and on.transform_sender(out_sndr, env) are ill-formed.

Otherwise: Let not-a-scheduler be an unspecified empty class type.

The expression on.transform_env(out_sndr, env) has effects equivalent to: auto&& [_, data, _] = out_sndr; if constexpr (scheduler<decltype(data)>) { return JOIN-ENV(SCHED-ENV(std::forward_like<OutSndr>(data)), FWD-ENV(std::forward<Env>(env))); } else { return std::forward<Env>(env); }

The expression on.transform_sender(out_sndr, env) has effects equivalent to: auto&& [_, data, child] = out_sndr; if constexpr (scheduler<decltype(data)>) { auto orig_sch = query-with-default(get_scheduler, env, not-a-scheduler()); if constexpr (same_as<decltype(orig_sch), not-a-scheduler>) { return not-a-sender{}; } else { return continues_on( starts_on(std::forward_like<OutSndr>(data), std::forward_like<OutSndr>(child)), std::move(orig_sch)); } } else { auto& [sch, closure] = data; auto orig_sch = query-with-default( get_completion_scheduler<set_value_t>, get_env(child), query-with-default(get_scheduler, env, not-a-scheduler())); if constexpr (same_as<decltype(orig_sch), not-a-scheduler>) { return not-a-sender{}; } else { return write_env( continues_on( std::forward_like<OutSndr>(closure)( continues_on( write_env(std::forward_like<OutSndr>(child), SCHED-ENV(orig_sch)), sch)), orig_sch), SCHED-ENV(sch)); } }

Let out_sndr be a subexpression denoting a sender returned from on(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall

(9.1)
remember the current scheduler, get_scheduler(get_env(rcvr));
(9.2)
start sndr on an execution agent belonging to sch's associated execution resource;
(9.3)
upon sndr's completion, transfer execution back to the execution resource associated with the scheduler remembered in step 1; and
(9.4)
forward sndr's async result to out_rcvr.

If any scheduling operation fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

Let out_sndr be a subexpression denoting a sender returned from on(sndr, sch, closure) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall

(10.1)
remember the current scheduler, which is the first of the following expressions that is well-formed:
- (10.1.1)
  get_completion_scheduler<set_value_t>(get_env(sndr))
- (10.1.2)
  get_scheduler(get_env(rcvr));
(10.2)
start sndr on the current execution agent;
(10.3)
upon sndr's completion, transfer execution to an agent owned by sch's associated execution resource;
(10.4)
forward sndr's async result as if by connecting and starting a sender closure(S), where S is a sender that completes synchronously with sndr's async result; and
(10.5)
upon completion of the operation started in the previous step, transfer execution back to the execution resource associated with the scheduler remembered in step 1 and forward the operation's async result to out_rcvr.

If any scheduling operation fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.

33.9.12.9 execution::then, execution::upon_error, execution::upon_stopped [exec.then]

then attaches an invocable as a continuation for an input sender's value completion operation.

upon_error and upon_stopped do the same for the error and stopped completion operations, respectively, sending the result of the invocable as a value completion.

The names then, upon_error, and upon_stopped denote pipeable sender adaptor objects.

Let the expression then-cpo be one of then, upon_error, or upon_stopped.

For subexpressions sndr and f, if decltype((sndr)) does not satisfy sender, or decltype((f)) does not satisfy movable-value, then-cpo(sndr, f) is ill-formed.

Otherwise, the expression then-cpo(sndr, f) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(then-cpo, f, sndr)) except that sndr is evaluated only once.

For then, upon_error, and upon_stopped, let set-cpo be set_value, set_error, and set_stopped, respectively.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for then-cpo as follows:

🔗

namespace std::execution { template<> struct impls-for<decayed-typeof<then-cpo>> : default-impls { static constexpr auto complete = []<class Tag, class... Args> (auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void { if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) { TRY-SET-VALUE(rcvr, invoke(std::move(fn), std::forward<Args>(args)...)); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }; template<class Sndr, class... Env> static consteval void check-types(); }; }

🔗

template<class Sndr, class... Env>
  static consteval void check-types();

Effects: Equivalent to: auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>(); auto fn = []<class... Ts>(set_value_t(*)(Ts...)) { if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts...>) throw unspecified-exception(); }; cs.for-each(overload-set{fn, [](auto){}}); where unspecified-exception is a type derived from exception.

The expression then-cpo(sndr, f) has undefined behavior unless it returns a sender out_sndr that

(6.1)
invokes f or a copy of such with the value, error, or stopped result datums of sndr for then, upon_error, and upon_stopped, respectively, using the result value of f as out_sndr's value completion, and
(6.2)
forwards all other completion operations unchanged.

33.9.12.10 execution::let_value, execution::let_error, execution::let_stopped [exec.let]

let_value, let_error, and let_stopped transform a sender's value, error, and stopped completions, respectively, into a new child asynchronous operation by passing the sender's result datums to a user-specified callable, which returns a new sender that is connected and started.

For let_value, let_error, and let_stopped, let set-cpo be set_value, set_error, and set_stopped, respectively.

Let the expression let-cpo be one of let_value, let_error, or let_stopped.

For a subexpression sndr, let let-env(sndr) be expression-equivalent to the first well-formed expression below:

(2.1)
SCHED-ENV(get_completion_scheduler<decayed-typeof<set-cpo>>(get_env(sndr)))
(2.2)
MAKE-ENV(get_domain, get_domain(get_env(sndr)))
(2.3)
(void(sndr), env<>{})

The names let_value, let_error, and let_stopped denote pipeable sender adaptor objects.

For subexpressions sndr and f, let F be the decayed type of f.

If decltype((sndr)) does not satisfy sender or if decltype((f)) does not satisfy movable-value, the expression let-cpo(sndr, f) is ill-formed.

If F does not satisfy invocable, the expression let_stopped(sndr, f) is ill-formed.

Otherwise, the expression let-cpo(sndr, f) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(let-cpo, f, sndr)) except that sndr is evaluated only once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for let-cpo as follows:

🔗

namespace std::execution { template<class State, class Rcvr, class... Args> void let-bind(State& state, Rcvr& rcvr, Args&&... args); // exposition only template<> struct impls-for<decayed-typeof<let-cpo>> : default-impls { static constexpr auto get-state = see below; static constexpr auto complete = see below; template<class Sndr, class... Env> static consteval void check-types(); }; }

Let receiver2 denote the following exposition-only class template: namespace std::execution { template<class Rcvr, class Env> struct receiver2 { using receiver_concept = receiver_t; template<class... Args> void set_value(Args&&... args) && noexcept { execution::set_value(std::move(rcvr), std::forward<Args>(args)...); } template<class Error> void set_error(Error&& err) && noexcept { execution::set_error(std::move(rcvr), std::forward<Error>(err)); } void set_stopped() && noexcept { execution::set_stopped(std::move(rcvr)); } decltype(auto) get_env() const noexcept { return see below; } Rcvr& rcvr; // exposition only Env env; // exposition only }; }

Invocation of the function receiver2::get_env returns an object e such that

(6.1)
decltype(e) models queryable and
(6.2)
given a query object q, the expression e.query(q) is expression-equivalent to env.query(q) if that expression is valid; otherwise, if the type of q satisfies forwarding-query, e.query(q) is expression-equivalent to get_env(rcvr).query(q); otherwise, e.query(q) is ill-formed.

🔗

template<class Sndr, class... Env>
  static consteval void check-types();

Effects: Equivalent to: using LetFn = remove_cvref_t<data-type<Sndr>>; auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>(); auto fn = []<class... Ts>(decayed-typeof<set-cpo>(*)(Ts...)) { if constexpr (!is-valid-let-sender) // see below throw unspecified-exception(); }; cs.for-each(overload-set(fn, [](auto){})); where unspecified-exception is a type derived from exception, and where is-valid-let-sender is true if and only if all of the following are true:

(7.1)
(constructible_from<decay_t<Ts>, Ts> &&...)
(7.2)
invocable<LetFn, decay_t<Ts>&...>
(7.3)
sender<invoke_result_t<LetFn, decay_t<Ts>&...>>
(7.4)
sizeof...(Env) == 0 || sender_in<invoke_result_t<LetFn, decay_t<Ts>&...>, env-t
...>

where env-t is the pack decltype(let-cpo.transform_env(declval<Sndr>(), declval<Env>())).

impls-for<decayed-typeof<let-cpo>>::get-state is initialized with a callable object equivalent to the following: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) requires see below { auto& [_, fn, child] = sndr; using fn_t = decay_t<decltype(fn)>; using env_t = decltype(let-env(child)); using args_variant_t = see below; using ops2_variant_t = see below; struct state-type { fn_t fn; // exposition only env_t env; // exposition only args_variant_t args; // exposition only ops2_variant_t ops2; // exposition only }; return state-type{allocator-aware-forward(std::forward_like<Sndr>(fn), rcvr), let-env(child), {}, {}}; }

Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>.

Let LetSigs be a pack of those types in Sigs with a return type of decayed-typeof<set-cpo>.

Let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Args...>.

Then args_variant_t denotes the type variant<monostate, as-tuple<LetSigs>...> except with duplicate types removed.

Given a type Tag and a pack Args, let as-sndr2 be an alias template such that as-sndr2<Tag(Args...)> denotes the type call-result-t<F, decay_t<Args>&...>.

Then ops2_variant_t denotes the type variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, env_t>>...> except with duplicate types removed.

The requires-clause constraining the above lambda is satisfied if and only if the types args_variant_t and ops2_variant_t are well-formed.

The exposition-only function template let-bind has effects equivalent to: using args_t = decayed-tuple<Args...>; auto mkop2 = [&] { return connect( apply(std::move(state.fn), state.args.template emplace<args_t>(std::forward<Args>(args)...)), receiver2{rcvr, std::move(state.env)}); }; start(state.ops2.template emplace<decltype(mkop2())>(emplace-from{mkop2}));

impls-for<decayed-typeof<let-cpo>>::complete is initialized with a callable object equivalent to the following: []<class Tag, class... Args> (auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void { if constexpr (same_as<Tag, decayed-typeof<set-cpo>>) { TRY-EVAL(rcvr, let-bind(state, rcvr, std::forward<Args>(args)...)); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }

Let sndr and env be subexpressions, and let Sndr be decltype((sndr)).

If sender-for<Sndr, decayed-typeof<let-cpo>> is false, then the expression let-cpo.transform_env(sndr, env) is ill-formed.

Otherwise, it is equal to: auto& [_, _, child] = sndr; return JOIN-ENV(let-env(child), FWD-ENV(env));

Let the subexpression out_sndr denote the result of the invocation let-cpo(sndr, f) or an object equal to such, and let the subexpression rcvr denote a receiver such that the expression connect(out_sndr, rcvr) is well-formed.

The expression connect(out_sndr, rcvr) has undefined behavior unless it creates an asynchronous operation ([exec.async.ops]) that, when started:

(15.1)
invokes f when set-cpo is called with sndr's result datums,
(15.2)
makes its completion dependent on the completion of a sender returned by f, and
(15.3)
propagates the other completion operations sent by sndr.

33.9.12.11 execution::bulk, execution::bulk_chunked, and execution::bulk_unchunked [exec.bulk]

bulk, bulk_chunked, and bulk_unchunked run a task repeatedly for every index in an index space.

The names bulk, bulk_chunked, and bulk_unchunked denote pipeable sender adaptor objects.

Let bulk-algo be either bulk, bulk_chunked, or bulk_unchunked.

For subexpressions sndr, policy, shape, and f, let Policy be remove_cvref_t<decltype(policy)>, Shape be decltype(auto(shape)), and Func be decay_t<decltype((f))>.

(2.1)
decltype((sndr)) does not satisfy sender, or
(2.2)
is_execution_policy_v<Policy> is false, or
(2.3)
Shape does not satisfy integral, or
(2.4)
Func does not model copy_constructible,

bulk-algo(sndr, policy, shape, f) is ill-formed.

Otherwise, the expression bulk-algo(sndr, policy, shape, f) is expression-equivalent to:

transform_sender(get-domain-early(sndr), make-sender( bulk-algo, product-type<see below, Shape, Func>{policy, shape, f}, sndr)) except that sndr is evaluated only once.

The first template argument of product-type is Policy if Policy models copy_constructible, and const Policy& otherwise.

Let sndr and env be subexpressions such that Sndr is decltype((sndr)).

If sender-for<Sndr, bulk_t> is false, then the expression bulk.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: auto [_, data, child] = sndr; auto& [policy, shape, f] = data; auto new_f = [func = std::move(f)](Shape begin, Shape end, auto&&... vs) noexcept(noexcept(f(begin, vs...))) { while (begin != end) func(begin++, vs...); } return bulk_chunked(std::move(child), policy, shape, std::move(new_f));

[Note 1:

This causes the bulk(sndr, policy, shape, f) sender to be expressed in terms of bulk_chunked(sndr, policy, shape, f) when it is connected to a receiver whose execution domain does not customize bulk.

— end note]

The exposition-only class template impls-for ([exec.snd.general]) is specialized for bulk_chunked_t as follows:

🔗

namespace std::execution { template<> struct impls-for<bulk_chunked_t> : default-impls { static constexpr auto complete = see below; template<class Sndr, class... Env> static consteval void check-types(); }; }

The member impls-for<bulk_chunked_t>::complete is initialized with a callable object equivalent to the following lambda: []<class Index, class State, class Rcvr, class Tag, class... Args> (Index, State& state, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void requires see below { if constexpr (same_as<Tag, set_value_t>) { auto& [policy, shape, f] = state; constexpr bool nothrow = noexcept(f(auto(shape), auto(shape), args...)); TRY-EVAL(rcvr, [&]() noexcept(nothrow) { f(static_cast<decltype(auto(shape))>(0), auto(shape), args...); Tag()(std::move(rcvr), std::forward<Args>(args)...); }()); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }

The expression in the requires-clause of the lambda above is true if and only if Tag denotes a type other than set_value_t or if the expression f(auto(shape), auto(shape), args...) is well-formed.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for bulk_unchunked_t as follows: namespace std::execution { template<> struct impls-for<bulk_unchunked_t> : default-impls { static constexpr auto complete = see below; }; }

The member impls-for<bulk_unchunked_t>::complete is initialized with a callable object equivalent to the following lambda: []<class Index, class State, class Rcvr, class Tag, class... Args> (Index, State& state, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void requires see below { if constexpr (same_as<Tag, set_value_t>) { auto& [shape, f] = state; constexpr bool nothrow = noexcept(f(auto(shape), args...)); TRY-EVAL(rcvr, [&]() noexcept(nothrow) { for (decltype(auto(shape)) i = 0; i < shape; ++i) { f(auto(i), args...); } Tag()(std::move(rcvr), std::forward<Args>(args)...); }()); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }

The expression in the requires-clause of the lambda above is true if and only if Tag denotes a type other than set_value_t or if the expression f(auto(shape), args...) is well-formed.

🔗

template<class Sndr, class... Env>
  static consteval void check-types();

Effects: Equivalent to: auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>(); auto fn = []<class... Ts>(set_value_t(*)(Ts...)) { if constexpr (!invocable<remove_cvref_t<data-type<Sndr>>, Ts&...>) throw unspecified-exception(); }; cs.for-each(overload-set(fn, [](auto){})); where unspecified-exception is a type derived from exception.

Let the subexpression out_sndr denote the result of the invocation bulk-algo(sndr, policy, shape, f) or an object equal to such, and let the subexpression rcvr denote a receiver such that the expression connect(out_sndr, rcvr) is well-formed.

The expression connect(out_sndr, rcvr) has undefined behavior unless it creates an asynchronous operation ([exec.async.ops]) that, when started:

(8.1)
If sndr has a successful completion, where args is a pack of lvalue subexpressions referring to the value completion result datums of sndr, or decayed copies of those values if they model copy_constructible, then:
- (8.1.1)
  If out_sndr also completes successfully, then:
  - (8.1.1.1)
    for bulk, invokes f(i, args...) for every i of type Shape from 0 to shape;
  - (8.1.1.2)
    for bulk_unchunked, invokes f(i, args...) for every i of type Shape from 0 to shape;
    Recommended practice: The underlying scheduler should execute each iteration on a distinct execution agent.
  - (8.1.1.3)
    for bulk_chunked, invokes f(b, e, args...) zero or more times with pairs of b and e of type Shape in range [0, shape], such that $b < e$ and for every i of type Shape from 0 to shape, there is exactly one invocation with a pair b and e, such that i is in the range [b, e).
- (8.1.2)
  If out_sndr completes with set_error(rcvr, eptr), then the asynchronous operation may invoke a subset of the invocations of f before the error completion handler is called, and eptr is an exception_ptr containing either:
  (8.1.2.1)
  an exception thrown by an invocation of f, or
  (8.1.2.2)
  a bad_alloc exception if the implementation fails to allocate required resources, or
  (8.1.2.3)
  an exception derived from runtime_error.
- (8.1.3)
  If out_sndr completes with set_stopped(rcvr), then the asynchronous operation may invoke a subset of the invocations of f before the stopped completion handler.
(8.2)
If sndr does not complete with set_value, then the completion is forwarded to recv.
(8.3)
For bulk-algo, the parameter policy describes the manner in which the execution of the asynchronous operations corresponding to these algorithms may be parallelized and the manner in which they apply f.

Permissions and requirements on parallel algorithm element access functions ([algorithms.parallel.exec]) apply to f.

[Note 2:

The asynchronous operation corresponding to bulk-algo(sndr, policy, shape, f) can complete with set_stopped if cancellation is requested or ignore cancellation requests.

— end note]

33.9.12.12 execution::when_all [exec.when.all]

when_all and when_all_with_variant both adapt multiple input senders into a sender that completes when all input senders have completed.

when_all only accepts senders with a single value completion signature and on success concatenates all the input senders' value result datums into its own value completion operation.

when_all_with_variant(sndrs...) is semantically equivalent to when_all(into_variant(sndrs)...), where sndrs is a pack of subexpressions whose types model sender .

The names when_all and when_all_with_variant denote customization point objects.

Let sndrs be a pack of subexpressions, let Sndrs be a pack of the types decltype((sndrs))..., and let CD be the type common_type_t<decltype(get-domain-early(sndrs))...>.

Let CD2 be CD if CD is well-formed, and default_domain otherwise.

The expressions when_all(sndrs...) and when_all_with_variant(sndrs...) are ill-formed if any of the following is true:

(2.1)
sizeof...(sndrs) is 0, or
(2.2)
(sender<Sndrs> && ...) is false.

The expression when_all(sndrs...) is expression-equivalent to: transform_sender(CD2(), make-sender(when_all, {}, sndrs...))

The exposition-only class template impls-for ([exec.snd.general]) is specialized for when_all_t as follows:

🔗

namespace std::execution { template<> struct impls-for<when_all_t> : default-impls { static constexpr auto get-attrs = see below; static constexpr auto get-env = see below; static constexpr auto get-state = see below; static constexpr auto start = see below; static constexpr auto complete = see below; template<class Sndr, class... Env> static consteval void check-types(); }; }

Let make-when-all-env be the following exposition-only function template: template<class Env> constexpr auto make-when-all-env(inplace_stop_source& stop_src, // exposition only Env&& env) noexcept { return see below; }

Returns an object e such that

(5.1)
decltype(e) models queryable, and
(5.2)
e.query(get_stop_token) is expression-equivalent to state.stop-src.get_token(), and
(5.3)
given a query object q with type other than cv stop_token_t and whose type satisfies forwarding-query, e.query(q) is expression-equivalent to get_env(rcvr).query(q).

Let when-all-env be an alias template such that when-all-env<Env> denotes the type decltype(make-
when-all-env(declval<inplace_stop_source&>(), declval<Env>())).

🔗

template<class Sndr, class... Env>
  static consteval void check-types();

Let Is be the pack of integral template arguments of the integer_sequence specialization denoted by indices-for<Sndr>.

Effects: Equivalent to: auto fn = []<class Child>() { auto cs = get_completion_signatures<Child, when-all-env<Env>...>(); if constexpr (cs.count-of(set_value) >= 2) throw unspecified-exception(); decay-copyable-result-datums(cs); // see [exec.snd.expos] }; (fn.template operator()<child-type<Sndr, Is>>(), ...); where unspecified-exception is a type derived from exception.

Throws: Any exception thrown as a result of evaluating the Effects, or an exception of an unspecified type derived from exception when CD is ill-formed.

The member impls-for<when_all_t>::get-attrs is initialized with a callable object equivalent to the following lambda expression: [](auto&&, auto&&... child) noexcept { if constexpr (same_as<CD, default_domain>) { return env<>(); } else { return MAKE-ENV(get_domain, CD()); } }

The member impls-for<when_all_t>::get-env is initialized with a callable object equivalent to the following lambda expression: []<class State, class Rcvr>(auto&&, State& state, const Receiver& rcvr) noexcept { return make-when-all-env(state.stop-src, get_env(rcvr)); }

The member impls-for<when_all_t>::get-state is initialized with a callable object equivalent to the following lambda expression: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(noexcept(e)) -> decltype(e) { return e; } where e is the expression std::forward<Sndr>(sndr).apply(make-state<Rcvr>()) and where make-state is the following exposition-only class template: enum class disposition { started, error, stopped }; // exposition only template<class Rcvr> struct make-state { template<class... Sndrs> auto operator()(auto, auto, Sndrs&&... sndrs) const { using values_tuple = see below; using errors_variant = see below; using stop_callback = stop_callback_for_t<stop_token_of_t<env_of_t<Rcvr>>, on-stop-request>; struct state-type { void arrive(Rcvr& rcvr) noexcept { // exposition only if (0 == --count) { complete(rcvr); } } void complete(Rcvr& rcvr) noexcept; // exposition only atomic<size_t> count{sizeof...(sndrs)}; // exposition only inplace_stop_source stop_src{}; // exposition only atomic<disposition> disp{disposition::started}; // exposition only errors_variant errors{}; // exposition only values_tuple values{}; // exposition only optional<stop_callback> on_stop{nullopt}; // exposition only }; return state-type{}; } };

Let copy-fail be exception_ptr if decay-copying any of the child senders' result datums can potentially throw; otherwise, none-such, where none-such is an unspecified empty class type.

The alias values_tuple denotes the type tuple<value_types_of_t<Sndrs, FWD-ENV-T(env_of_t<Rcvr>), decayed-tuple, optional>...> if that type is well-formed; otherwise, tuple<>.

The alias errors_variant denotes the type variant<none-such, copy-fail, Es...> with duplicate types removed, where Es is the pack of the decayed types of all the child senders' possible error result datums.

The member void state-type::complete(Rcvr& rcvr) noexcept behaves as follows:

(16.1)
If disp is equal to disposition::started, evaluates: auto tie = []<class... T>(tuple<T...>& t) noexcept { return tuple<T&...>(t); }; auto set = [&](auto&... t) noexcept { set_value(std::move(rcvr), std::move(t)...); }; on_stop.reset(); apply( [&](auto&... opts) noexcept { apply(set, tuple_cat(tie(*opts)...)); }, values);
(16.2)
Otherwise, if disp is equal to disposition::error, evaluates: on_stop.reset(); visit( [&]<class Error>(Error& error) noexcept { if constexpr (!same_as<Error, none-such>) { set_error(std::move(rcvr), std::move(error)); } }, errors);
(16.3)
Otherwise, evaluates: on_stop.reset(); set_stopped(std::move(rcvr));

The member impls-for<when_all_t>::start is initialized with a callable object equivalent to the following lambda expression: []<class State, class Rcvr, class... Ops>( State& state, Rcvr& rcvr, Ops&... ops) noexcept -> void { state.on_stop.emplace( get_stop_token(get_env(rcvr)), on-stop-request{state.stop_src}); if (state.stop_src.stop_requested()) { state.on_stop.reset(); set_stopped(std::move(rcvr)); } else { (start(ops), ...); } }

The member impls-for<when_all_t>::complete is initialized with a callable object equivalent to the following lambda expression: []<class Index, class State, class Rcvr, class Set, class... Args>( this auto& complete, Index, State& state, Rcvr& rcvr, Set, Args&&... args) noexcept -> void { if constexpr (same_as<Set, set_error_t>) { if (disposition::error != state.disp.exchange(disposition::error)) { state.stop_src.request_stop(); TRY-EMPLACE-ERROR(state.errors, std::forward<Args>(args)...); } } else if constexpr (same_as<Set, set_stopped_t>) { auto expected = disposition::started; if (state.disp.compare_exchange_strong(expected, disposition::stopped)) { state.stop_src.request_stop(); } } else if constexpr (!same_as<decltype(State::values), tuple<>>) { if (state.disp == disposition::started) { auto& opt = get<Index::value>(state.values); TRY-EMPLACE-VALUE(complete, opt, std::forward<Args>(args)...); } } state.arrive(rcvr); } where TRY-EMPLACE-ERROR(v, e), for subexpressions v and e, is equivalent to: try { v.template emplace<decltype(auto(e))>(e); } catch (...) { v.template emplace<exception_ptr>(current_exception()); } if the expression decltype(auto(e))(e) is potentially throwing; otherwise, v.template emplace<decltype(auto(e))>(e); and where TRY-EMPLACE-VALUE(c, o, as...), for subexpressions c, o, and pack of subexpressions as, is equivalent to: try { o.emplace(as...); } catch (...) { c(Index(), state, rcvr, set_error, current_exception()); return; } if the expression decayed-tuple<decltype(as)...>{as...} is potentially throwing; otherwise, o.emplace(as...).

The expression when_all_with_variant(sndrs...) is expression-equivalent to: transform_sender(CD2(), make-sender(when_all_with_variant, {}, sndrs...));

Given subexpressions sndr and env, if sender-for<decltype((sndr)), when_all_with_variant_t> is false, then the expression when_all_with_variant.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: auto&& [_, _, ...child] = sndr; return when_all(into_variant(std::forward_like<decltype((sndr))>(child))...);

[Note 1:

This causes the when_all_with_variant(sndrs...) sender to become when_all(into_variant(sndrs)...) when it is connected with a receiver whose execution domain does not customize when_all_with_variant.

— end note]

33.9.12.13 execution::into_variant [exec.into.variant]

into_variant adapts a sender with multiple value completion signatures into a sender with just one value completion signature consisting of a variant of tuples.

The name into_variant denotes a pipeable sender adaptor object.

For a subexpression sndr, let Sndr be decltype((sndr)).

If Sndr does not satisfy sender, into_variant(sndr) is ill-formed.

Otherwise, the expression into_variant(sndr) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(into_variant, {}, sndr)) except that sndr is only evaluated once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for into_variant as follows:

🔗

namespace std::execution { template<> struct impls-for<into_variant_t> : default-impls { static constexpr auto get-state = see below; static constexpr auto complete = see below; template<class Sndr, class... Env> static consteval void check-types() { auto cs = get_completion_signatures<child-type<Sndr>, FWD-ENV-T(Env)...>(); decay-copyable-result-datums(cs); // see [exec.snd.expos] } }; }

The member impls-for<into_variant_t>::get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> type_identity<value_types_of_t<child-type<Sndr>, FWD-ENV-T(env_of_t<Rcvr>)>> { return {}; }

The member impls-for<into_variant_t>::complete is initialized with a callable object equivalent to the following lambda: []<class State, class Rcvr, class Tag, class... Args>( auto, State, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void { if constexpr (same_as<Tag, set_value_t>) { using variant_type = typename State::type; TRY-SET-VALUE(rcvr, variant_type(decayed-tuple<Args...>{std::forward<Args>(args)...})); } else { Tag()(std::move(rcvr), std::forward<Args>(args)...); } }

33.9.12.14 execution::stopped_as_optional [exec.stopped.opt]

stopped_as_optional maps a sender's stopped completion operation into a value completion operation as a disengaged optional.

The sender's value completion operation is also converted into an optional.

The result is a sender that never completes with stopped, reporting cancellation by completing with a disengaged optional.

The name stopped_as_optional denotes a pipeable sender adaptor object.

For a subexpression sndr, let Sndr be decltype((sndr)).

The expression stopped_as_optional(sndr) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(stopped_as_optional, {}, sndr)) except that sndr is only evaluated once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for stopped_as_optional_t as follows:

🔗

namespace std::execution { template<> struct impls-for<stopped_as_optional_t> : default-impls { template<class Sndr, class... Env> static consteval void check-types() { default-impls::check-types<Sndr, Env...>(); if constexpr (!requires { requires (!same_as<void, single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)...>>); }) throw unspecified-exception(); } }; } where unspecified-exception is a type derived from exception.

Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)).

If sender-for<Sndr, stopped_as_optional_t> is false then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, if sender_in<child-type<Sndr>, FWD-ENV-T(Env)> is false, the expression stopped_as_optional.transform_sender(sndr, env) is equivalent to not-a-sender(); otherwise, it is equivalent to: auto&& [_, _, child] = sndr; using V = single-sender-value-type<child-type<Sndr>, FWD-ENV-T(Env)>; return let_stopped( then(std::forward_like<Sndr>(child), []<class... Ts>(Ts&&... ts) noexcept(is_nothrow_constructible_v<V, Ts...>) { return optional<V>(in_place, std::forward<Ts>(ts)...); }), []() noexcept { return just(optional<V>()); });

33.9.12.15 execution::stopped_as_error [exec.stopped.err]

stopped_as_error maps an input sender's stopped completion operation into an error completion operation as a custom error type.

The result is a sender that never completes with stopped, reporting cancellation by completing with an error.

The name stopped_as_error denotes a pipeable sender adaptor object.

For some subexpressions sndr and err, let Sndr be decltype((sndr)) and let Err be decltype((err)).

If the type Sndr does not satisfy sender or if the type Err does not satisfy movable-value, stopped_as_error(sndr, err) is ill-formed.

Otherwise, the expression stopped_as_error(sndr, err) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(stopped_as_error, err, sndr)) except that sndr is only evaluated once.

Let sndr and env be subexpressions such that Sndr is decltype((sndr)) and Env is decltype((env)).

If sender-for<Sndr, stopped_as_error_t> is false, then the expression stopped_as_error.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: auto&& [_, err, child] = sndr; using E = decltype(auto(err)); return let_stopped( std::forward_like<Sndr>(child), [err = std::forward_like<Sndr>(err)]() mutable noexcept(is_nothrow_move_constructible_v<E>) { return just_error(std::move(err)); });

33.9.12.16 std::execution::associate [exec.associate]

associate tries to associate a sender with an async scope such that the scope can track the lifetime of any asynchronous operations created with the sender.

Let associate-data be the following exposition-only class template:

🔗

namespace std::execution { template<scope_token Token, sender Sender> struct associate-data { // exposition only using wrap-sender = // exposition only remove_cvref_t<decltype(declval<Token&>().wrap(declval<Sender>()))>; explicit associate-data(Token t, Sender&& s) : sndr(t.wrap(std::forward<Sender>(s))), token(t) { if (!token.try_associate()) sndr.reset(); } associate-data(const associate-data& other) noexcept(is_nothrow_copy_constructible_v<wrap-sender> && noexcept(other.token.try_associate())); associate-data(associate-data&& other) noexcept(is_nothrow_move_constructible_v<wrap-sender>); ~associate-data(); optional<pair<Token, wrap-sender>> release() && noexcept(is_nothrow_move_constructible_v<wrap-sender>); private: optional<wrap-sender> sndr; // exposition only Token token; // exposition only }; template<scope_token Token, sender Sender> associate-data(Token, Sender&&) -> associate-data<Token, Sender>; }

For an associate-data object a, a.sndr.has_value() is true if and only if an association was successfully made and is owned by a.

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associate-data(const associate-data& other)
  noexcept(is_nothrow_copy_constructible_v<wrap-sender> &&
           noexcept(other.token.try_associate()));

Constraints: copy_constructible<wrap-sender> is true.

Effects: Value-initializes sndr and initializes token with other.token.

If other.sndr.has_value() is false, no further effects; otherwise, calls token.try_associate() and, if that returns true, calls sndr.emplace(*other.sndr) and, if that exits with an exception, calls token.disassociate() before propagating the exception.

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associate-data(associate-data&& other)
  noexcept(is_nothrow_move_constructible_v<wrap-sender>);

Effects: Initializes sndr with std::move(other.sndr) and initializes token with std::move(other.token) and then calls other.sndr.reset().

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~associate-data();

Effects: If sndr.has_value() returns false then no effect; otherwise, invokes sndr.reset() before invoking token.disassociate().

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optional<pair<Token, wrap-sender>>
  release() && noexcept(is_nothrow_move_constructible_v<wrap-sender>);

Effects: If sndr.has_value() returns false then returns an optional that does not contain a value; otherwise returns an optional containing a value of type pair<Token, wrap-sender> as if by: return optional(pair(token, std::move(*sndr)));

Postconditions: sndr does not contain a value.

The name associate denotes a pipeable sender adaptor object.

For subexpressions sndr and token:

(10.1)
If decltype((sndr)) does not satisfy sender, or remove_cvref_t<decltype((token))> does not satisfy scope_token, then associate(sndr, token) is ill-formed.
(10.2)
Otherwise, the expression associate(sndr, token) is expression-equivalent to: transform_sender(get-domain-early(sndr), make-sender(associate, associate-data(token, sndr))) except that sndr is evaluated only once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for associate_t as follows:

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namespace std::execution { template<> struct impls-for<associate_t> : default-impls { static constexpr auto get-state = see below; // exposition only static constexpr auto start = see below; // exposition only template<class Sndr, class... Env> static consteval void check-types() { // exposition only using associate_data_t = remove_cvref_t<data-type<Sndr>>; using child_type_t = typename associate_data_t::wrap-sender; (void)get_completion_signatures<child_type_t, FWD-ENV-T(Env)...>(); } }; }

The member impls-for<associate_t>::get-state is initialized with a callable object equivalent to the following lambda:

[]<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below) { auto [_, data] = std::forward<Sndr>(sndr); auto dataParts = std::move(data).release(); using scope_tkn = decltype(dataParts->first); using wrap_sender = decltype(dataParts->second); using op_t = connect_result_t<wrap_sender, Rcvr>; struct op_state { bool associated = false; // exposition only union { Rcvr* rcvr; // exposition only struct { scope_tkn token; // exposition only op_t op; // exposition only } assoc; // exposition only }; explicit op_state(Rcvr& r) noexcept : rcvr(addressof(r)) {} explicit op_state(scope_tkn tkn, wrap_sender&& sndr, Rcvr& r) try : associated(true), assoc(tkn, connect(std::move(sndr), std::move(r))) { } catch (...) { tkn.disassociate(); throw; } op_state(op_state&&) = delete; ~op_state() { if (associated) { assoc.op.~op_t(); assoc.token.disassociate(); assoc.token.~scope_tkn(); } } void run() noexcept { // exposition only if (associated) start(assoc.op); else set_stopped(std::move(*rcvr)); } }; if (dataParts) return op_state{std::move(dataParts->first), std::move(dataParts->second), rcvr}; else return op_state{rcvr}; }

The expression in the noexcept clause of impls-for<associate_t>::get-state is is_nothrow_constructible_v<remove_cvref_t<Sndr>, Sndr> && is_nothrow_move_constructible_v<wrap-sender> && nothrow-callable<connect_t, wrap-sender, Rcvr> where wrap-sender is the type remove_cvref_t<data-type<Sndr>>::wrap-sender.

The member impls-for<associate_t>::start is initialized with a callable object equivalent to the following lambda: [](auto& state, auto&) noexcept -> void { state.run(); }

The evaluation of associate(sndr, token) may cause side effects observable via token's associated async scope object.

33.9.12.17 Exposition-only std::execution::stop-when [exec.stop.when]

stop-when fuses an additional stop token t into a sender so that, upon connecting to a receiver r, the resulting operation state receives stop requests from both t and the token returned from get_stop_token(get_env(r)).

The name stop-when denotes an exposition-only sender adaptor.

For subexpressions sndr and token:

(2.1)
If decltype((sndr)) does not satisfy sender, or remove_cvref_t<decltype((token))> does not satisfy stoppable_token, then stop-when(sndr, token) is ill-formed.
(2.2)
Otherwise, if remove_cvref_t<decltype((token))> models unstoppable_token then stop-when(sndr, token) is expression-equivalent to sndr.
(2.3)
Otherwise, stop-when(sndr, token) returns a sender osndr.

If osndr is connected to a receiver r, let rtoken be the result of get_stop_token(get_env(r)).
- (2.3.1)
  If the type of rtoken models unstoppable_token then the effects of connecting osndr to r are equivalent to connect(write_env(sndr, prop(get_stop_token, token)), r).
- (2.3.2)
  Otherwise, the effects of connecting osndr to r are equivalent to connect(write_env(sndr, prop(get_stop_token, stoken)), r) where stoken is an object of an exposition-only type stoken-t such that:
  (2.3.2.1)
  stoken-t models stoppable_token;
  (2.3.2.2)
  stoken.stop_requested() returns token.stop_requested() || rtoken.stop_reques-
  ted();
  (2.3.2.3)
  stoken.stop_possible() returns token.stop_possible() || rtoken.stop_possible(); and
  (2.3.2.4)
  for types Fn and Init such that both invocable<Fn> and constructible_from<Fn, Init> are modeled, stoken-t::callback_type<Fn> models stoppable-callback-for<Fn, stoken-t, Init>.
  [Note 1:
  For an object fn of type Fn constructed from a value, init, of type Init, registering fn using stoken-t::callback_type<Fn>(stoken, init) results in an invocation of fn when a callback registered with token or rtoken would be invoked.
  
  fn is invoked at most once.
  — end note]

33.9.12.18 std::execution::spawn_future [exec.spawn.future]

spawn_future attempts to associate the given input sender with the given token's async scope and, on success, eagerly starts the input sender; the return value is a sender that, when connected and started, completes with either the result of the eagerly-started input sender or with set_stopped if the input sender was not started.

The name spawn_future denotes a customization point object.

For subexpressions sndr, token, and env,

(2.1)
let Sndr be decltype((sndr)),
(2.2)
let Token be remove_cvref_t<decltype((token))>, and
(2.3)
let Env be remove_cvref_t<decltype((env))>.

If any of sender<Sndr>, scope_token<Token>, or queryable<Env> are not satisfied, the expression spawn_future(sndr, token, env) is ill-formed.

Let spawn-future-state-base be the exposition-only class template:

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namespace std::execution { template<class Completions> struct spawn-future-state-base; // exposition only template<class... Sigs> struct spawn-future-state-base<completion_signatures<Sigs...>> { // exposition only using variant-t = see below; // exposition only variant-t result; // exposition only virtual void complete() noexcept = 0; // exposition only }; }

Let Sigs be the pack of arguments to the completion_signatures specialization provided as a parameter to the spawn-future-state-base class template.

Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>.

(4.1)
If is_nothrow_constructible_v<decay_t<Arg>, Arg> is true for every type Arg in every parameter pack Args in every completion signature Tag(Args...) in Sigs then variant-t denotes the type variant<monostate, tuple<set_stopped_t>, as-tuple<Sigs>...>, except with duplicate types removed.
(4.2)
Otherwise variant-t denotes the type variant<monostate, tuple<set_stopped_t>, tuple<set_error_t, exception_ptr>, as-tuple<Sigs>...>, except with duplicate types removed.

Let spawn-future-receiver be the exposition-only class template:

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namespace std::execution { template<class Completions> struct spawn-future-receiver { // exposition only using receiver_concept = receiver_t; spawn-future-state-base<Completions>* state; // exposition only template<class... T> void set_value(T&&... t) && noexcept { set-complete<set_value_t>(std::forward<T>(t)...); } template<class E> void set_error(E&& e) && noexcept { set-complete<set_error_t>(std::forward<E>(e)); } void set_stopped() && noexcept { set-complete<set_stopped_t>(); } private: template<class CPO, class... T> void set-complete(T&&... t) noexcept { // exposition only constexpr bool nothrow = (is_nothrow_constructible_v<decay_t<T>, T> && ...); try { state->result.template emplace<decayed-tuple<CPO, T...>>(CPO{}, std::forward<T>(t)...); } catch (...) { if constexpr (!nothrow) { using tuple_t = decayed-tuple<set_error_t, exception_ptr>; state->result.template emplace<tuple_t>(set_error_t{}, current_exception()); } } state->complete(); } }; }

Let ssource-t be an unspecified type that models stoppable-source and let ssource be an lvalue of type ssource-t.

Let stoken-t be decltype(ssource.get_token()).

Let future-spawned-sender be the alias template:

template<sender Sender, class Env> using future-spawned-sender = // exposition only decltype(write_env(stop-when(declval<Sender>(), declval<stoken-t>()), declval<Env>()));

Let spawn-future-state be the exposition-only class template:

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namespace std::execution { template<class Alloc, scope_token Token, sender Sender, class Env> struct spawn-future-state // exposition only : spawn-future-state-base<completion_signatures_of_t<future-spawned-sender<Sender, Env>>> { using sigs-t = // exposition only completion_signatures_of_t<future-spawned-sender<Sender, Env>>; using receiver-t = // exposition only spawn-future-receiver<sigs-t>; using op-t = // exposition only connect_result_t<future-spawned-sender<Sender, Env>, receiver-t>; spawn-future-state(Alloc alloc, Sender&& sndr, Token token, Env env) // exposition only : alloc(std::move(alloc)), op(connect( write_env(stop-when(std::forward<Sender>(sndr), ssource.get_token()), std::move(env)), receiver-t(this))), token(std::move(token)), associated(token.try_associate()) { if (associated) start(op); else set_stopped(receiver-t(this)); } void complete() noexcept override; // exposition only void consume(receiver auto& rcvr) noexcept; // exposition only void abandon() noexcept; // exposition only private: using alloc-t = // exposition only typename allocator_traits<Alloc>::template rebind_alloc<spawn-future-state>; alloc-t alloc; // exposition only ssource-t ssource; // exposition only op-t op; // exposition only Token token; // exposition only bool associated; // exposition only void destroy() noexcept; // exposition only }; }

For purposes of determining the existence of a data race, complete, consume, and abandon behave as atomic operations ([intro.multithread]).

These operations on a single object of a type that is a specialization of spawn-future-state appear to occur in a single total order.

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void complete() noexcept;

Effects:

(9.1)
No effects if this invocation of complete happens before an invocation of consume or abandon on *this;
(9.2)
otherwise, if an invocation of consume on *this happens before this invocation of complete then there is a receiver, rcvr, registered and that receiver is completed as if by consume(rcvr);
(9.3)
otherwise, destroy is invoked.

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void consume(receiver auto& rcvr) noexcept;

Effects:

(10.1)
If this invocation of consume happens before an invocation of complete on *this then rcvr is registered to be completed when complete is subsequently invoked on *this;
(10.2)
otherwise, rcvr is completed as if by: std::move(this->result).visit( [&rcvr](auto&& tuple) noexcept { if constexpr (!same_as<remove_reference_t<decltype(tuple)>, monostate>) { apply([&rcvr](auto cpo, auto&&... vals) { cpo(std::move(rcvr), std::move(vals)...); }, std::move(tuple)); } });

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void abandon() noexcept;

Effects:

(11.1)
If this invocation of abandon happens before an invocation of complete on *this then equivalent to: ssource.request_stop();
(11.2)
otherwise, destroy is invoked.

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void destroy() noexcept;

Effects: Equivalent to: auto token = std::move(this->token); bool associated = this->associated; { auto alloc = std::move(this->alloc); allocator_traits<alloc-t>::destroy(alloc, this); allocator_traits<alloc-t>::deallocate(alloc, this, 1); } if (associated) token.disassociate();

The exposition-only class template impls-for ([exec.snd.general]) is specialized for spawn_future_t as follows:

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namespace std::execution { template<> struct impls-for<spawn_future_t> : default-impls { static constexpr auto start = see below; // exposition only }; }

The member impls-for<spawn_future_t>::start is initialized with a callable object equivalent to the following lambda: [](auto& state, auto& rcvr) noexcept -> void { state->consume(rcvr); }

For the expression spawn_future(sndr, token, env) let new_sender be the expression token.wrap(sndr) and let alloc and senv be defined as follows:

(15.1)
if the expression get_allocator(env) is well-formed, then alloc is the result of get_allocator(env) and senv is the expression env;
(15.2)
otherwise, if the expression get_allocator(get_env(new_sender)) is well-formed, then alloc is the result of get_allocator(get_env(new_sender)) and senv is the expression JOIN-ENV(prop(get_allocator, alloc), env);
(15.3)
otherwise, alloc is allocator<void>() and senv is the expression env.

The expression spawn_future(sndr, token, env) has the following effects:

(16.1)
Uses alloc to allocate and construct an object s of a type that is a specialization of spawn-future-state from alloc, token.wrap(sndr), token, and senv.

If an exception is thrown then any constructed objects are destroyed and any allocated memory is deallocated.
(16.2)
Constructs an object u of a type that is a specialization of unique_ptr such that:
- (16.2.1)
  u.get() is equal to the address of s, and
- (16.2.2)
  u.get_deleter()(u.release()) is equivalent to u.release()->abandon().
(16.3)
Returns make-sender(spawn_future, std::move(u)).

The expression spawn_future(sndr, token) is expression-equivalent to spawn_future(sndr, token, execution::env<>()).

33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.12 Sender adaptors [exec.adapt]

33.9.12.1 General [exec.adapt.general]

33.9.12.2 Closure objects [exec.adapt.obj]

33.9.12.3 execution​::​write_env [exec.write.env]

33.9.12.4 execution​::​unstoppable [exec.unstoppable]

33.9.12.5 execution​::​starts_on [exec.starts.on]

33.9.12.6 execution​::​continues_on [exec.continues.on]

33.9.12.7 execution​::​schedule_from [exec.schedule.from]

33.9.12.8 execution​::​on [exec.on]

33.9.12.9 execution​::​then, execution​::​upon_error, execution​::​upon_stopped [exec.then]

33.9.12.10 execution​::​let_value, execution​::​let_error, execution​::​let_stopped [exec.let]

33.9.12.11 execution​::​bulk, execution​::​bulk_chunked, and execution​::​bulk_unchunked [exec.bulk]

33.9.12.12 execution​::​when_all [exec.when.all]

33.9.12.13 execution​::​into_variant [exec.into.variant]

33.9.12.14 execution​::​stopped_as_optional [exec.stopped.opt]

33.9.12.15 execution​::​stopped_as_error [exec.stopped.err]

33.9.12.16 std​::​execution​::​associate [exec.associate]

33.9.12.17 Exposition-only std​::​execution​::​stop-when [exec.stop.when]

33.9.12.18 std​::​execution​::​spawn_future [exec.spawn.future]

33.9.12.3 execution::write_env [exec.write.env]

33.9.12.4 execution::unstoppable [exec.unstoppable]

33.9.12.5 execution::starts_on [exec.starts.on]

33.9.12.6 execution::continues_on [exec.continues.on]

33.9.12.7 execution::schedule_from [exec.schedule.from]

33.9.12.8 execution::on [exec.on]

33.9.12.9 execution::then, execution::upon_error, execution::upon_stopped [exec.then]

33.9.12.10 execution::let_value, execution::let_error, execution::let_stopped [exec.let]

33.9.12.11 execution::bulk, execution::bulk_chunked, and execution::bulk_unchunked [exec.bulk]

33.9.12.12 execution::when_all [exec.when.all]

33.9.12.13 execution::into_variant [exec.into.variant]

33.9.12.14 execution::stopped_as_optional [exec.stopped.opt]

33.9.12.15 execution::stopped_as_error [exec.stopped.err]

33.9.12.16 std::execution::associate [exec.associate]

33.9.12.17 Exposition-only std::execution::stop-when [exec.stop.when]

33.9.12.18 std::execution::spawn_future [exec.spawn.future]