34 Execution control library [exec]

34.9 Senders [exec.snd]

34.9.1 General [exec.snd.general]

For the purposes of this subclause, a sender is an object whose type satisfies the sender concept ([exec.async.ops]).

Subclauses [exec.factories] and [exec.adapt] define customizable algorithms that return senders Each algorithm has a default implementation.

Let sndr be the result of an invocation of such an algorithm or an object equal to the result ([concepts.equality]), and let Sndr be decltype((sndr)).

Let rcvr be a receiver of type Rcvr with associated environment env of type Env such that sender_to<Sndr, Rcvr> is true.

For the default implementation of the algorithm that produced sndr, connecting sndr to rcvr and starting the resulting operation state ([exec.async.ops]) necessarily results in the potential evaluation ([basic.def.odr]) of a set of completion operations whose first argument is a subexpression equal to rcvr.

Let Sigs be a pack of completion signatures corresponding to this set of completion operations.

Then the type of the expression get_completion_signatures(sndr, env) is a specialization of the class template completion_signatures ([exec.util.cmplsig]), the set of whose template arguments is Sigs.

If a user-provided implementation of the algorithm that produced sndr is selected instead of the default, any completion signature that is in the set of types denoted by completion_signatures_of_t<Sndr, Env> and that is not part of Sigs shall correspond to error or stopped completion operations, unless otherwise specified.

34.9.2 Exposition-only entities [exec.snd.expos]

Subclause [exec.snd] makes use of the following exposition-only entities.

For a queryable object env, FWD-ENV(env) is an expression whose type satisfies queryable such that for a query object q and a pack of subexpressions as, the expression FWD-ENV(env).query(q, as...) is ill-formed if forwarding_query(q) is false; otherwise, it is expression-equivalent to env.query(q, as...).

For a query object q and a subexpression v, MAKE-ENV(q, v) is an expression env whose type satisfies queryable such that the result of env.query(q) has a value equal to v ([concepts.equality]).

Unless otherwise stated, the object to which env.query(q) refers remains valid while env remains valid.

For two queryable objects env1 and env2, a query object q, and a pack of subexpressions as, JOIN-ENV(env1, env2) is an expression env3 whose type satisfies queryable such that env3.query(q, as...) is expression-equivalent to:

(4.1)
env1.query(q, as...) if that expression is well-formed,
(4.2)
otherwise, env2.query(q, as...) if that expression is well-formed,
(4.3)
otherwise, env3.query(q, as...) is ill-formed.

The results of FWD-ENV, MAKE-ENV, and JOIN-ENV can be context-dependent; i.e., they can evaluate to expressions with different types and value categories in different contexts for the same arguments.

For a scheduler sch, SCHED-ATTRS(sch) is an expression o1 whose type satisfies queryable such that o1.query(get_completion_scheduler<Tag>) is an expression with the same type and value as sch where Tag is one of set_value_t or set_stopped_t, and such that o1.query(get_domain) is expression-equivalent to sch.query(get_domain).

SCHED-ENV(sch) is an expression o2 whose type satisfies queryable such that o1.query(get_scheduler) is a prvalue with the same type and value as sch, and such that o2.query(get_domain) is expression-equivalent to sch.query(get_domain).

For two subexpressions rcvr and expr, SET-VALUE(rcvr, expr) is expression-equivalent to (expr, set_value(std::move(rcvr))) if the type of expr is void; otherwise, set_value(std::move(rcvr), expr).

TRY-EVAL(rcvr, expr) is equivalent to: try { expr; } catch(...) { set_error(std::move(rcvr), current_exception()); } if expr is potentially-throwing; otherwise, expr.

TRY-SET-VALUE(rcvr, expr) is TRY-EVAL(rcvr, SET-VALUE(rcvr, expr)) except that rcvr is evaluated only once.

🔗

template<class Default = default_domain, class Sndr>
  constexpr auto completion-domain(const Sndr& sndr) noexcept;

COMPL-DOMAIN(T) is the type of the expression get_domain(get_completion_scheduler<T>(get_env(sndr))).

Effects: If all of the types COMPL-DOMAIN(set_value_t), COMPL-DOMAIN(set_error_t), and COMPL-DOMAIN(set_stopped_t) are ill-formed, completion-domain<Default>(sndr) is a default-constructed prvalue of type Default.

Otherwise, if they all share a common type ([meta.trans.other]) (ignoring those types that are ill-formed), then completion-domain<Default>(sndr) is a default-constructed prvalue of that type.

Otherwise, completion-domain<Default>(sndr) is ill-formed.

🔗

template<class Tag, class Env, class Default>
  constexpr decltype(auto) query-with-default(
    Tag, const Env& env, Default&& value) noexcept(see below);

Let e be the expression Tag()(env) if that expression is well-formed; otherwise, it is static_cast<Default>(std::forward<Default>(value)).

Returns: e.

Remarks: The expression in the noexcept clause is noexcept(e).

🔗

template<class Sndr>
  constexpr auto get-domain-early(const Sndr& sndr) noexcept;

Effects: Equivalent to: return Domain(); where Domain is the decayed type of the first of the following expressions that is well-formed:

(13.1)
get_domain(get_env(sndr))
(13.2)
completion-domain(sndr)
(13.3)
default_domain()

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template<class Sndr, class Env>
  constexpr auto get-domain-late(const Sndr& sndr, const Env& env) noexcept;

Effects: Equivalent to:

(14.1)
If sender-for<Sndr, continues_on_t> is true, then return Domain(); where Domain is the type of the following expression: [] { auto [_, sch, _] = sndr; return query-or-default(get_domain, sch, default_domain()); }();
[Note 1:
The continues_on algorithm works in tandem with schedule_from ([exec.schedule.from]) to give scheduler authors a way to customize both how to transition onto (continues_on) and off of (schedule_from) a given execution context.

Thus, continues_on ignores the domain of the predecessor and uses the domain of the destination scheduler to select a customization, a property that is unique to continues_on.

That is why it is given special treatment here.
— end note]
(14.2)
Otherwise, return Domain(); where Domain is the first of the following expressions that is well-formed and whose type is not void:
- (14.2.1)
  get_domain(get_env(sndr))
- (14.2.2)
  completion-domain<void>(sndr)
- (14.2.3)
  get_domain(env)
- (14.2.4)
  get_domain(get_scheduler(env))
- (14.2.5)
  default_domain()

template<callable Fun> requires is_nothrow_move_constructible_v<Fun> struct emplace-from { Fun fun; // exposition only using type = call-result-t<Fun>; constexpr operator type() && noexcept(nothrow-callable<Fun>) { return std::move(fun)(); } constexpr type operator()() && noexcept(nothrow-callable<Fun>) { return std::move(fun)(); } };

[Note 2:

emplace-from is used to emplace non-movable types into tuple, optional, variant, and similar types.

— end note]

struct on-stop-request { inplace_stop_source& stop-src; // exposition only void operator()() noexcept { stop-src.request_stop(); } };

template<class T

_{0}

, class T

_{1}

, ..., class T

_{n}

> struct product-type { // exposition only T

_{0}

_{0}

; // exposition only T

_{1}

_{1}

; // exposition only ... T

_{n}

_{n}

; // exposition only template<size_t I, class Self> constexpr decltype(auto) get(this Self&& self) noexcept; // exposition only template<class Self, class Fn> constexpr decltype(auto) apply(this Self&& self, Fn&& fn) // exposition only noexcept(see below); };

[Note 3:

product-type is presented here in pseudo-code form for the sake of exposition.

It can be approximated in standard C++ with a tuple-like implementation that takes care to keep the type an aggregate that can be used as the initializer of a structured binding declaration.

— end note]

[Note 4:

An expression of type product-type is usable as the initializer of a structured binding declaration ([dcl.struct.bind]).

— end note]

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template<size_t I, class Self>
constexpr decltype(auto) get(this Self&& self) noexcept;

Effects: Equivalent to: auto& [...ts] = self; return std::forward_like<Self>(ts...[I]);

template<class Self, class Fn> constexpr decltype(auto) apply(this Self&& self, Fn&& fn) noexcept(see below);

Constraints: The expression in the return statement below is well-formed.

Effects: Equivalent to: auto& [...ts] = self; return std::forward<Fn>(fn)(std::forward_like<Self>(ts)...);

Remarks: The expression in the noexcept clause is true if the return statement above is not potentially throwing; otherwise, false.

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template<class Tag, class Data = see below, class... Child>
  constexpr auto make-sender(Tag tag, Data&& data, Child&&... child);

Mandates: The following expressions are true:

Returns: A prvalue of type basic-sender<Tag, decay_t<Data>, decay_t<Child>...> that has been direct-list-initialized with the forwarded arguments, where basic-sender is the following exposition-only class template except as noted below.

namespace std::execution { template<class Tag> concept completion-tag = // exposition only same_as<Tag, set_value_t> || same_as<Tag, set_error_t> || same_as<Tag, set_stopped_t>; template<template<class...> class T, class... Args> concept valid-specialization = // exposition only requires { typename T<Args...>; }; struct default-impls { // exposition only static constexpr auto get-attrs = see below; // exposition only static constexpr auto get-env = see below; // exposition only static constexpr auto get-state = see below; // exposition only static constexpr auto start = see below; // exposition only static constexpr auto complete = see below; // exposition only }; template<class Tag> struct impls-for : default-impls {}; // exposition only template<class Sndr, class Rcvr> // exposition only using state-type = decay_t<call-result-t< decltype(impls-for<tag_of_t<Sndr>>::get-state), Sndr, Rcvr&>>; template<class Index, class Sndr, class Rcvr> // exposition only using env-type = call-result-t< decltype(impls-for<tag_of_t<Sndr>>::get-env), Index, state-type<Sndr, Rcvr>&, const Rcvr&>; template<class Sndr, size_t I = 0> using child-type = decltype(declval<Sndr>().template get<I+2>()); // exposition only template<class Sndr> using indices-for = remove_reference_t<Sndr>::indices-for; // exposition only template<class Sndr, class Rcvr> struct basic-state { // exposition only basic-state(Sndr&& sndr, Rcvr&& rcvr) noexcept(see below) : rcvr(std::move(rcvr)) , state(impls-for<tag_of_t<Sndr>>::get-state(std::forward<Sndr>(sndr), rcvr)) { } Rcvr rcvr; // exposition only state-type<Sndr, Rcvr> state; // exposition only }; template<class Sndr, class Rcvr, class Index> requires valid-specialization<env-type, Index, Sndr, Rcvr> struct basic-receiver { // exposition only using receiver_concept = receiver_t; using tag-t = tag_of_t<Sndr>; // exposition only using state-t = state-type<Sndr, Rcvr>; // exposition only static constexpr const auto& complete = impls-for<tag-t>::complete; // exposition only template<class... Args> requires callable<decltype(complete), Index, state-t&, Rcvr&, set_value_t, Args...> void set_value(Args&&... args) && noexcept { complete(Index(), op->state, op->rcvr, set_value_t(), std::forward<Args>(args)...); } template<class Error> requires callable<decltype(complete), Index, state-t&, Rcvr&, set_error_t, Error> void set_error(Error&& err) && noexcept { complete(Index(), op->state, op->rcvr, set_error_t(), std::forward<Error>(err)); } void set_stopped() && noexcept requires callable<decltype(complete), Index, state-t&, Rcvr&, set_stopped_t> { complete(Index(), op->state, op->rcvr, set_stopped_t()); } auto get_env() const noexcept -> env-type<Index, Sndr, Rcvr> { return impls-for<tag-t>::get-env(Index(), op->state, op->rcvr); } basic-state<Sndr, Rcvr>* op; // exposition only }; constexpr auto connect-all = see below; // exposition only template<class Sndr, class Rcvr> using connect-all-result = call-result-t< // exposition only decltype(connect-all), basic-state<Sndr, Rcvr>*, Sndr, indices-for<Sndr>>; template<class Sndr, class Rcvr> requires valid-specialization<state-type, Sndr, Rcvr> && valid-specialization<connect-all-result, Sndr, Rcvr> struct basic-operation : basic-state<Sndr, Rcvr> { // exposition only using operation_state_concept = operation_state_t; using tag-t = tag_of_t<Sndr>; // exposition only connect-all-result<Sndr, Rcvr> inner-ops; // exposition only basic-operation(Sndr&& sndr, Rcvr&& rcvr) noexcept(see below) // exposition only : basic-state<Sndr, Rcvr>(std::forward<Sndr>(sndr), std::move(rcvr)), inner-ops(connect-all(this, std::forward<Sndr>(sndr), indices-for<Sndr>())) {} void start() & noexcept { auto& [...ops] = inner-ops; impls-for<tag-t>::start(this->state, this->rcvr, ops...); } }; template<class Sndr, class Env> using completion-signatures-for = see below; // exposition only template<class Tag, class Data, class... Child> struct basic-sender : product-type<Tag, Data, Child...> { // exposition only using sender_concept = sender_t; using indices-for = index_sequence_for<Child...>; // exposition only decltype(auto) get_env() const noexcept { auto& [_, data, ...child] = *this; return impls-for<Tag>::get-attrs(data, child...); } template<decays-to<basic-sender> Self, receiver Rcvr> auto connect(this Self&& self, Rcvr rcvr) noexcept(see below) -> basic-operation<Self, Rcvr> { return {std::forward<Self>(self), std::move(rcvr)}; } template<decays-to<basic-sender> Self, class Env> auto get_completion_signatures(this Self&& self, Env&& env) noexcept -> completion-signatures-for<Self, Env> { return {}; } }; }

The default template argument for the Data template parameter denotes an unspecified empty trivially copyable class type that models semiregular .

It is unspecified whether a specialization of basic-sender is an aggregate.

An expression of type basic-sender is usable as the initializer of a structured binding declaration ([dcl.struct.bind]).

The expression in the noexcept clause of the constructor of basic-state is: is_nothrow_move_constructible_v<Rcvr> && nothrow-callable<decltype(impls-for<tag_of_t<Sndr>>::get-state), Sndr, Rcvr&>

The object connect-all is initialized with a callable object equivalent to the following lambda:

🔗

[]<class Sndr, class Rcvr, size_t... Is>(
  basic-state<Sndr, Rcvr>* op, Sndr&& sndr, index_sequence<Is...>) noexcept(see below)
    -> decltype(auto) {
    auto& [_, data, ...child] = sndr;
    return product-type{connect(
      std::forward_like<Sndr>(child),
      basic-receiver<Sndr, Rcvr, integral_constant<size_t, Is>>{op})...};
  }

Constraints: The expression in the return statement is well-formed.

Remarks: The expression in the noexcept clause is true if the return statement is not potentially throwing; otherwise, false.

The expression in the noexcept clause of the constructor of basic-operation is: is_nothrow_constructible_v<basic-state<Self, Rcvr>, Self, Rcvr> && noexcept(connect-all(this, std::forward<Sndr>(sndr), indices-for<Sndr>()))

The expression in the noexcept clause of the connect member function of basic-sender is: is_nothrow_constructible_v<basic-operation<Self, Rcvr>, Self, Rcvr>

The member default-impls::get-attrs is initialized with a callable object equivalent to the following lambda: [](const auto&, const auto&... child) noexcept -> decltype(auto) { if constexpr (sizeof...(child) == 1) return (FWD-ENV(get_env(child)), ...); else return empty_env(); }

The member default-impls::get-env is initialized with a callable object equivalent to the following lambda: [](auto, auto&, const auto& rcvr) noexcept -> decltype(auto) { return FWD-ENV(get_env(rcvr)); }

The member default-impls::get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept -> decltype(auto) { auto& [_, data, ...child] = sndr; return std::forward_like<Sndr>(data); }

The member default-impls::start is initialized with a callable object equivalent to the following lambda: [](auto&, auto&, auto&... ops) noexcept -> void { (execution::start(ops), ...); }

The member default-impls::complete is initialized with a callable object equivalent to the following lambda: []<class Index, class Rcvr, class Tag, class... Args>( Index, auto& state, Rcvr& rcvr, Tag, Args&&... args) noexcept -> void requires callable<Tag, Rcvr, Args...> { static_assert(Index::value == 0); Tag()(std::move(rcvr), std::forward<Args>(args)...); }

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

Let rcvr be a receiver with an associated environment of type Env such that sender_in<Sndr, Env> is true.

completion-signatures-for<Sndr, Env> denotes a specialization of completion_signatures, the set of whose template arguments correspond to the set of completion operations that are potentially evaluated as a result of starting ([exec.async.ops]) the operation state that results from connecting sndr and rcvr.

When sender_in<Sndr, Env> is false, the type denoted by completion-signatures-for<Sndr, Env>, if any, is not a specialization of completion_signatures.

Recommended practice: When sender_in<Sndr, Env> is false, implementations are encouraged to use the type denoted by completion-signatures-for<Sndr, Env> to communicate to users why.

🔗

template<sender Sndr, queryable Env>
  constexpr auto write-env(Sndr&& sndr, Env&& env);     // exposition only

write-env is an exposition-only sender adaptor that, when connected with a receiver rcvr, connects the adapted sender with a receiver whose execution environment is the result of joining the queryable argument env to the result of get_env(rcvr).

Let write-env-t be an exposition-only empty class type.

Returns: make-sender(write-env-t(), std::forward<Env>(env), std::forward<Sndr>(sndr))

Remarks: 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 get-env = [](auto, const auto& state, const auto& rcvr) noexcept { return JOIN-ENV(state, get_env(rcvr)); }; };

34.9.3 Sender concepts [exec.snd.concepts]

The sender concept defines the requirements for a sender type ([exec.async.ops]).

The sender_in concept defines the requirements for a sender type that can create asynchronous operations given an associated environment type.

The sender_to concept defines the requirements for a sender type that can connect with a specific receiver type.

The get_env customization point object is used to access a sender's associated attributes.

The connect customization point object is used to connect ([exec.async.ops]) a sender and a receiver to produce an operation state.

namespace std::execution { template<class Sigs> concept valid-completion-signatures = see below; // exposition only template<class Sndr> concept is-sender = // exposition only derived_from<typename Sndr::sender_concept, sender_t>; template<class Sndr> concept enable-sender = // exposition only is-sender<Sndr> || is-awaitable<Sndr, env-promise<empty_env>>; // [exec.awaitable] template<class Sndr> concept sender = bool(enable-sender<remove_cvref_t<Sndr>>) && requires (const remove_cvref_t<Sndr>& sndr) { { get_env(sndr) } -> queryable; } && move_constructible<remove_cvref_t<Sndr>> && constructible_from<remove_cvref_t<Sndr>, Sndr>; template<class Sndr, class Env = empty_env> concept sender_in = sender<Sndr> && queryable<Env> && requires (Sndr&& sndr, Env&& env) { { get_completion_signatures(std::forward<Sndr>(sndr), std::forward<Env>(env)) } -> valid-completion-signatures; }; template<class Sndr, class Rcvr> concept sender_to = sender_in<Sndr, env_of_t<Rcvr>> && receiver_of<Rcvr, completion_signatures_of_t<Sndr, env_of_t<Rcvr>>> && requires (Sndr&& sndr, Rcvr&& rcvr) { connect(std::forward<Sndr>(sndr), std::forward<Rcvr>(rcvr)); }; }

Given a subexpression sndr, let Sndr be decltype((sndr)) and let rcvr be a receiver with an associated environment whose type is Env.

A completion operation is a permissible completion for Sndr and Env if its completion signature appears in the argument list of the specialization of completion_signatures denoted by completion_signatures_of_t<Sndr, Env>.

Sndr and Env model sender_in<Sndr, Env> if all the completion operations that are potentially evaluated by connecting sndr to rcvr and starting the resulting operation state are permissible completions for Sndr and Env.

A type models the exposition-only concept valid-completion-signatures if it denotes a specialization of the completion_signatures class template.

The exposition-only concepts sender-of and sender-in-of define the requirements for a sender type that completes with a given unique set of value result types.

namespace std::execution { template<class... As> using value-signature = set_value_t(As...); // exposition only template<class Sndr, class Env, class... Values> concept sender-in-of = sender_in<Sndr, Env> && MATCHING-SIG( // see [exec.general] set_value_t(Values...), value_types_of_t<Sndr, Env, value-signature, type_identity_t>); template<class Sndr, class... Values> concept sender-of = sender-in-of<Sndr, empty_env, Values...>; }

Let sndr be an expression such that decltype((sndr)) is Sndr.

The type tag_of_t<Sndr> is as follows:

(5.1)
If the declaration auto&& [tag, data, ...children] = sndr; would be well-formed, tag_of_t<Sndr> is an alias for decltype(auto(tag)).
(5.2)
Otherwise, tag_of_t<Sndr> is ill-formed.

Let sender-for be an exposition-only concept defined as follows: namespace std::execution { template<class Sndr, class Tag> concept sender-for = sender<Sndr> && same_as<tag_of_t<Sndr>, Tag>; }

For a type T, SET-VALUE-SIG(T) denotes the type set_value_t() if T is cv void; otherwise, it denotes the type set_value_t(T).

Library-provided sender types

(8.1)
always expose an overload of a member connect that accepts an rvalue sender and
(8.2)
only expose an overload of a member connect that accepts an lvalue sender if they model copy_constructible.

34.9.4 Awaitable helpers [exec.awaitable]

The sender concepts recognize awaitables as senders.

For [exec], an awaitable is an expression that would be well-formed as the operand of a co_await expression within a given context.

For a subexpression c, let GET-AWAITER(c, p) be expression-equivalent to the series of transformations and conversions applied to c as the operand of an await-expression in a coroutine, resulting in lvalue e as described by [expr.await], where p is an lvalue referring to the coroutine's promise, which has type Promise.

[Note 1:

This includes the invocation of the promise type's await_transform member if any, the invocation of the operator co_await picked by overload resolution if any, and any necessary implicit conversions and materializations.

— end note]

Let is-awaitable be the following exposition-only concept: namespace std { template<class T> concept await-suspend-result = see below; // exposition only template<class A, class Promise> concept is-awaiter = // exposition only requires (A& a, coroutine_handle<Promise> h) { a.await_ready() ? 1 : 0; { a.await_suspend(h) } -> await-suspend-result; a.await_resume(); }; template<class C, class Promise> concept is-awaitable = // exposition only requires (C (*fc)() noexcept, Promise& p) { { GET-AWAITER(fc(), p) } -> is-awaiter<Promise>; }; }

await-suspend-result<T> is true if and only if one of the following is true:

(3.1)
T is void, or
(3.2)
T is bool, or
(3.3)
T is a specialization of coroutine_handle.

For a subexpression c such that decltype((c)) is type C, and an lvalue p of type Promise, await-result-
type<C, Promise> denotes the type decltype(GET-AWAITER(c, p).await_resume()).

Let with-await-transform be the exposition-only class template: namespace std::execution { template<class T, class Promise> concept has-as-awaitable = // exposition only requires (T&& t, Promise& p) { { std::forward<T>(t).as_awaitable(p) } -> is-awaitable<Promise&>; }; template<class Derived> struct with-await-transform { // exposition only template<class T> T&& await_transform(T&& value) noexcept { return std::forward<T>(value); } template<has-as-awaitable<Derived> T> decltype(auto) await_transform(T&& value) noexcept(noexcept(std::forward<T>(value).as_awaitable(declval<Derived&>()))) { return std::forward<T>(value).as_awaitable(static_cast<Derived&>(*this)); } }; }

Let env-promise be the exposition-only class template: namespace std::execution { template<class Env> struct env-promise : with-await-transform<env-promise<Env>> { // exposition only unspecified get_return_object() noexcept; unspecified initial_suspend() noexcept; unspecified final_suspend() noexcept; void unhandled_exception() noexcept; void return_void() noexcept; coroutine_handle<> unhandled_stopped() noexcept; const Env& get_env() const noexcept; }; }

[Note 2:

of env-promise are used only for the purpose of type computation; its members need not be defined.

— end note]

34.9.5 execution::default_domain [exec.domain.default]

namespace std::execution { struct default_domain { template<sender Sndr, queryable... Env> requires (sizeof...(Env) <= 1) static constexpr sender decltype(auto) transform_sender(Sndr&& sndr, const Env&... env) noexcept(see below); template<sender Sndr, queryable Env> static constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept; template<class Tag, sender Sndr, class... Args> static constexpr decltype(auto) apply_sender(Tag, Sndr&& sndr, Args&&... args) noexcept(see below); }; }

🔗

template<sender Sndr, queryable... Env>
  requires (sizeof...(Env) <= 1)
constexpr sender decltype(auto) transform_sender(Sndr&& sndr, const Env&... env)
  noexcept(see below);

Let e be the expression tag_of_t<Sndr>().transform_sender(std::forward<Sndr>(sndr), env...) if that expression is well-formed; otherwise, std::forward<Sndr>(sndr).

Returns: e.

Remarks: The exception specification is equivalent to noexcept(e).

🔗

template<sender Sndr, queryable Env>
  constexpr queryable decltype(auto) transform_env(Sndr&& sndr, Env&& env) noexcept;

Let e be the expression tag_of_t<Sndr>().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env)) if that expression is well-formed; otherwise, static_cast<Env>(std::forward<Env>(env)).

Mandates: noexcept(e) is true.

Returns: e.

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template<class Tag, sender Sndr, class... Args>
constexpr decltype(auto) apply_sender(Tag, Sndr&& sndr, Args&&... args)
  noexcept(see below);

Let e be the expression Tag().apply_sender(std::forward<Sndr>(sndr), std::forward<Args>(args)...)

Constraints: e is a well-formed expression.

Returns: e.

Remarks: The exception specification is equivalent to noexcept(e).

34.9.6 execution::transform_sender [exec.snd.transform]

🔗

namespace std::execution {
  template<class Domain, sender Sndr, queryable... Env>
      requires (sizeof...(Env) <= 1)
    constexpr sender decltype(auto) transform_sender(Domain dom, Sndr&& sndr, const Env&... env)
      noexcept(see below);
}

Let transformed-sndr be the expression dom.transform_sender(std::forward<Sndr>(sndr), env...) if that expression is well-formed; otherwise, default_domain().transform_sender(std::forward<Sndr>(sndr), env...)

Let final-sndr be the expression transformed-sndr if transformed-sndr and sndr have the same type ignoring cv-qualifiers; otherwise, it is the expression transform_sender(dom, transformed-sndr, env...).

Returns: final-sndr.

Remarks: The exception specification is equivalent to noexcept(final-sndr).

34.9.7 execution::transform_env [exec.snd.transform.env]

🔗

namespace std::execution {
  template<class Domain, sender Sndr, queryable Env>
    constexpr queryable decltype(auto) transform_env(Domain dom, Sndr&& sndr, Env&& env) noexcept;
}

Let e be the expression dom.transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env)) if that expression is well-formed; otherwise, default_domain().transform_env(std::forward<Sndr>(sndr), std::forward<Env>(env))

Mandates: noexcept(e) is true.

Returns: e.

34.9.8 execution::apply_sender [exec.snd.apply]

🔗

namespace std::execution {
  template<class Domain, class Tag, sender Sndr, class... Args>
    constexpr decltype(auto) apply_sender(Domain dom, Tag, Sndr&& sndr, Args&&... args)
      noexcept(see below);
}

Let e be the expression dom.apply_sender(Tag(), std::forward<Sndr>(sndr), std::forward<Args>(args)...) if that expression is well-formed; otherwise, default_domain().apply_sender(Tag(), std::forward<Sndr>(sndr), std::forward<Args>(args)...)

Constraints: The expression e is well-formed.

Returns: e.

Remarks: The exception specification is equivalent to noexcept(e).

34.9.9 execution::get_completion_signatures [exec.getcomplsigs]

get_completion_signatures is a customization point object.

Let sndr be an expression such that decltype((sndr)) is Sndr, and let env be an expression such that decltype((env)) is Env.

Let new_sndr be the expression transform_sender(decltype(get-domain-late(sndr, env)), sndr, env), and let NewSndr be decltype((new_sndr)).

Then get_completion_signatures(sndr, env) is expression-equivalent to (void(sndr), void(env), CS()) except that void(sndr) and void(env) are indeterminately sequenced, where CS is:

(1.1)
decltype(new_sndr.get_completion_signatures(env)) if that type is well-formed,
(1.2)
Otherwise, remove_cvref_t<NewSndr>::completion_signatures if that type is well-formed,
(1.3)
Otherwise, if is-awaitable<NewSndr, env-promise<Env>> is true, then: completion_signatures< SET-VALUE-SIG(await-result-type<NewSndr, env-promise<Env>>), // ([exec.snd.concepts]) set_error_t(exception_ptr), set_stopped_t()>
(1.4)
Otherwise, CS is ill-formed.

Let rcvr be an rvalue whose type Rcvr models receiver, and let Sndr be the type of a sender such that sender_in<Sndr, env_of_t<Rcvr>> is true.

Let Sigs... be the template arguments of the completion_signatures specialization named by completion_signatures_of_t<Sndr, env_of_t<Rcvr>>.

Let CSO be a completion function.

If sender Sndr or its operation state cause the expression CSO(rcvr, args...) to be potentially evaluated ([basic.def.odr]) then there shall be a signature Sig in Sigs... such that MATCHING-SIG(decayed-typeof<CSO>(decltype(args)...), Sig) is true ([exec.general]).

34.9.10 execution::connect [exec.connect]

connect connects ([exec.async.ops]) a sender with a receiver.

The name connect denotes a customization point object.

For subexpressions sndr and rcvr, let Sndr be decltype((sndr)) and Rcvr be decltype((rcvr)), let new_sndr be the expression transform_sender(decltype(get-domain-late(sndr, get_env(rcvr))){}, sndr, get_env(rcvr)) and let DS and DR be decay_t<decltype((new_sndr))> and decay_t<Rcvr>, respectively.

Let connect-awaitable-promise be the following exposition-only class:

namespace std::execution { struct connect-awaitable-promise : with-await-transform<connect-awaitable-promise> { connect-awaitable-promise(DS&, DR& rcvr) noexcept : rcvr(rcvr) {} suspend_always initial_suspend() noexcept { return {}; } [[noreturn]] suspend_always final_suspend() noexcept { terminate(); } [[noreturn]] void unhandled_exception() noexcept { terminate(); } [[noreturn]] void return_void() noexcept { terminate(); } coroutine_handle<> unhandled_stopped() noexcept { set_stopped(std::move(rcvr)); return noop_coroutine(); } operation-state-task get_return_object() noexcept { return operation-state-task{ coroutine_handle<connect-awaitable-promise>::from_promise(*this)}; } env_of_t<DR> get_env() const noexcept { return execution::get_env(rcvr); } private: DR& rcvr; // exposition only }; }

Let operation-state-task be the following exposition-only class: namespace std::execution { struct operation-state-task { using operation_state_concept = operation_state_t; using promise_type = connect-awaitable-promise; explicit operation-state-task(coroutine_handle<> h) noexcept : coro(h) {} operation-state-task(operation-state-task&& o) noexcept : coro(exchange(o.coro, {})) {} ~operation-state-task() { if (coro) coro.destroy(); } void start() & noexcept { coro.resume(); } private: coroutine_handle<> coro; // exposition only }; }

Let V name the type await-result-type<DS, connect-awaitable-promise>, let Sigs name the type completion_signatures< SET-VALUE-SIG(V), // see ([exec.snd.concepts]) set_error_t(exception_ptr), set_stopped_t()> and let connect-awaitable be an exposition-only coroutine defined as follows: namespace std::execution { template<class Fun, class... Ts> auto suspend-complete(Fun fun, Ts&&... as) noexcept { // exposition only auto fn = [&, fun]() noexcept { fun(std::forward<Ts>(as)...); }; struct awaiter { decltype(fn) fn; // exposition only static constexpr bool await_ready() noexcept { return false; } void await_suspend(coroutine_handle<>) noexcept { fn(); } [[noreturn]] void await_resume() noexcept { unreachable(); } }; return awaiter{fn}; } operation-state-task connect-awaitable(DS sndr, DR rcvr) requires receiver_of<DR, Sigs> { exception_ptr ep; try { if constexpr (same_as<V, void>) { co_await std::move(sndr); co_await suspend-complete(set_value, std::move(rcvr)); } else { co_await suspend-complete(set_value, std::move(rcvr), co_await std::move(sndr)); } } catch(...) { ep = current_exception(); } co_await suspend-complete(set_error, std::move(rcvr), std::move(ep)); } }

The expression connect(sndr, rcvr) is expression-equivalent to:

(6.1)
new_sndr.connect(rcvr) if that expression is well-formed.
Mandates: The type of the expression above satisfies operation_state .
(6.2)
Otherwise, connect-awaitable(new_sndr, rcvr).

Mandates: sender<Sndr> && receiver<Rcvr> is true.

34.9.11 Sender factories [exec.factories]

34.9.11.1 execution::schedule [exec.schedule]

schedule obtains a schedule sender ([exec.async.ops]) from a scheduler.

The name schedule denotes a customization point object.

For a subexpression sch, the expression schedule(sch) is expression-equivalent to sch.schedule().

Mandates: The type of sch.schedule() satisfies sender .

If the expression get_completion_scheduler<set_value_t>(get_env(sch.schedule())) == sch is ill-formed or evaluates to false, the behavior of calling schedule(sch) is undefined.

34.9.11.2 execution::just, execution::just_error, execution::just_stopped [exec.just]

just, just_error, and just_stopped are sender factories whose asynchronous operations complete synchronously in their start operation with a value completion operation, an error completion operation, or a stopped completion operation, respectively.

The names just, just_error, and just_stopped denote customization point objects.

Let just-cpo be one of just, just_error, or just_stopped.

For a pack of subexpressions ts, let Ts be the pack of types decltype((ts)).

The expression just-cpo(ts...) is ill-formed if

(2.1)
(movable-value<Ts> &&...) is false, or
(2.2)
just-cpo is just_error and sizeof...(ts) == 1 is false, or
(2.3)
just-cpo is just_stopped and sizeof...(ts) == 0 is false.

Otherwise, it is expression-equivalent to make-sender(just-cpo, product-typets...).

For just, just_error, and just_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 just-cpo as follows: namespace std::execution { template<> struct impls-for<decayed-typeof<just-cpo>> : default-impls { static constexpr auto start = [](auto& state, auto& rcvr) noexcept -> void { auto& [...ts] = state; set-cpo(std::move(rcvr), std::move(ts)...); }; }; }

34.9.11.3 execution::read_env [exec.read.env]

read_env is a sender factory for a sender whose asynchronous operation completes synchronously in its start operation with a value completion result equal to a value read from the receiver's associated environment.

read_env is a customization point object.

For some query object q, the expression read_env(q) is expression-equivalent to make-sender(read_env, q).

The exposition-only class template impls-for ([exec.snd.general]) is specialized for read_env as follows: namespace std::execution { template<> struct impls-for<decayed-typeof<read_env>> : default-impls { static constexpr auto start = [](auto query, auto& rcvr) noexcept -> void { TRY-SET-VALUE(rcvr, query(get_env(rcvr))); }; }; }

34.9.12 Sender adaptors [exec.adapt]

34.9.12.1 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 empty_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)
These requirements apply to any function that is selected by the implementation of the sender adaptor.

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.

34.9.12.2 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.

34.9.12.3 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-or-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.

34.9.12.4 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.

34.9.12.5 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-or-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; }; }

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>, 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}; }

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>, env_of_t<Rcvr>>.

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

Then variant_t denotes the type variant<monostate, as-tuple<Sigs>...>, 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<result_t, Tag, Args...>; TRY-EVAL(rcvr, [&]() noexcept(nothrow) { state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...); }()); if (state.async-result.valueless_by_exception()) return; if (state.async-result.index() == 0) return; 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.

34.9.12.6 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-or-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, and let not-a-sender be the exposition-only type: struct not-a-sender { using sender_concept = sender_t; auto get_completion_signatures(auto&&) const { return see below; } }; where the member function get_completion_signatures returns an object of a type that is not a specialization of the completion_signatures class template.

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)); } }

Recommended practice: Implementations should use the return type of not-a-sender::get_completion_signatures to inform users that their usage of on is incorrect because there is no available scheduler onto which to restore execution.

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

(10.1)
remember the current scheduler, get_scheduler(get_env(rcvr));
(10.2)
start sndr on an execution agent belonging to sch's associated execution resource;
(10.3)
upon sndr's completion, transfer execution back to the execution resource associated with the scheduler remembered in step 1; and
(10.4)
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

(11.1)
remember the current scheduler, which is the first of the following expressions that is well-formed:
- (11.1.1)
  get_completion_scheduler<set_value_t>(get_env(sndr))
- (11.1.2)
  get_scheduler(get_env(rcvr));
(11.2)
start sndr on the current execution agent;
(11.3)
upon sndr's completion, transfer execution to an agent owned by sch's associated execution resource;
(11.4)
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
(11.5)
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.

34.9.12.7 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)...); } }; }; }

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

(5.1)
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
(5.2)
forwards all other completion operations unchanged.

34.9.12.8 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), empty_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; }; }

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 JOIN-ENV(env, FWD-ENV(execution::get_env(rcvr))); } Rcvr& rcvr; // exposition only Env env; // exposition only }; }

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{std::forward_like<Sndr>(fn), 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>, 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<Fn, decay_t<Args>&...>.

Then ops2_variant_t denotes the type variant<monostate, connect_result_t<as-sndr2<LetSigs>, receiver2<Rcvr, Env>>...> 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 JOIN-ENV(let-env(sndr), 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:

(14.1)
invokes f when set-cpo is called with sndr's result datums,
(14.2)
makes its completion dependent on the completion of a sender returned by f, and
(14.3)
propagates the other completion operations sent by sndr.

34.9.12.9 execution::bulk [exec.bulk]

bulk runs a task repeatedly for every index in an index space.

The name bulk denotes a pipeable sender adaptor object.

For subexpressions sndr, shape, and f, let Shape be decltype(auto(shape)).

(1.1)
decltype((sndr)) does not satisfy sender, or
(1.2)
Shape does not satisfy integral, or
(1.3)
decltype((f)) does not satisfy movable-value,

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

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

transform_sender(get-domain-early(sndr), make-sender(bulk, product-type{shape, f}, sndr)) except that sndr is evaluated only once.

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

The member impls-for<bulk_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.

Let the subexpression out_sndr denote the result of the invocation bulk(sndr, 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,

(6.1)
on a value completion operation, invokes f(i, args...) for every i of type Shape from 0 to shape, where args is a pack of lvalue subexpressions referring to the value completion result datums of the input sender, and
(6.2)
propagates all completion operations sent by sndr.

34.9.12.10 execution::split [exec.split]

split adapts an arbitrary sender into a sender that can be connected multiple times.

Let split-env be the type of an environment such that, given an instance env, the expression get_stop_token(env) is well-formed and has type inplace_stop_token.

The name split denotes a pipeable sender adaptor object.

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

If sender_in<Sndr, split-env> is false, split(sndr) is ill-formed.

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

[Note 1:

The default implementation of transform_sender will have the effect of connecting the sender to a receiver.

It will return a sender with a different tag type.

— end note]

Let local-state denote the following exposition-only class template:

namespace std::execution { struct local-state-base { // exposition only virtual ~local-state-base() = default; virtual void notify() noexcept = 0; // exposition only }; template<class Sndr, class Rcvr> struct local-state : local-state-base { // exposition only using on-stop-callback = // exposition only stop_callback_of_t<stop_token_of_t<env_of_t<Rcvr>>, on-stop-request>; local-state(Sndr&& sndr, Rcvr& rcvr) noexcept; ~local-state(); void notify() noexcept override; private: optional<on-stop-callback> on_stop; // exposition only shared-state<Sndr>* sh_state; // exposition only Rcvr* rcvr; // exposition only }; }

🔗

local-state(Sndr&& sndr, Rcvr& rcvr) noexcept;

Effects: Equivalent to: auto& [_, data, _] = sndr; this->sh_state = data.sh_state.get(); this->sh_state->inc-ref(); this->rcvr = addressof(rcvr);

🔗

~local-state();

Effects: Equivalent to: sh_state->dec-ref();

🔗

void notify() noexcept override;

Effects: Equivalent to: on_stop.reset(); visit( [this](const auto& tupl) noexcept -> void { apply( [this](auto tag, const auto&... args) noexcept -> void { tag(std::move(*rcvr), args...); }, tupl); }, sh_state->result);

Let split-receiver denote the following exposition-only class template: namespace std::execution { template<class Sndr> struct split-receiver { // exposition only using receiver_concept = receiver_t; template<class Tag, class... Args> void complete(Tag, Args&&... args) noexcept { // exposition only using tuple_t = decayed-tuple<Tag, Args...>; try { sh_state->result.template emplace<tuple_t>(Tag(), std::forward<Args>(args)...); } catch (...) { using tuple_t = tuple<set_error_t, exception_ptr>; sh_state->result.template emplace<tuple_t>(set_error, current_exception()); } sh_state->notify(); } template<class... Args> void set_value(Args&&... args) && noexcept { complete(execution::set_value, std::forward<Args>(args)...); } template<class Error> void set_error(Error&& err) && noexcept { complete(execution::set_error, std::forward<Error>(err)); } void set_stopped() && noexcept { complete(execution::set_stopped); } struct env { // exposition only shared-state<Sndr>* sh-state; // exposition only inplace_stop_token query(get_stop_token_t) const noexcept { return sh-state->stop_src.get_token(); } }; env get_env() const noexcept { return env{sh_state}; } shared-state<Sndr>* sh_state; // exposition only }; }

Let shared-state denote the following exposition-only class template: namespace std::execution { template<class Sndr> struct shared-state { using variant-type = see below; // exposition only using state-list-type = see below; // exposition only explicit shared-state(Sndr&& sndr); void start-op() noexcept; // exposition only void notify() noexcept; // exposition only void inc-ref() noexcept; // exposition only void dec-ref() noexcept; // exposition only inplace_stop_source stop_src{}; // exposition only variant-type result{}; // exposition only state-list-type waiting_states; // exposition only atomic<bool> completed{false}; // exposition only atomic<size_t> ref_count{1}; // exposition only connect_result_t<Sndr, split-receiver<Sndr>> op_state; // exposition only }; }

Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<Sndr>.

For type Tag and pack Args, let as-tuple be an alias template such that as-tuple<Tag(Args...)> denotes the type decayed-tuple<Tag, Args...>.

Then variant-type denotes the type variant<tuple<set_stopped_t>, tuple<set_error_t, exception_ptr>, as-tuple<Sigs>...> but with duplicate types removed.

Let state-list-type be a type that stores a list of pointers to local-state-base objects and that permits atomic insertion.

🔗

explicit shared-state(Sndr&& sndr);

Effects: Initializes op_state with the result of connect(std::forward<Sndr>(sndr), split-receiver{this}).

Postconditions: waiting_states is empty, and completed is false.

🔗

void start-op() noexcept;

Effects: Evaluates inc-ref().

If stop_src.stop_requested() is true, evaluates notify(); otherwise, evaluates start(op_state).

🔗

void notify() noexcept;

Effects: Atomically does the following:

(16.1)
Sets completed to true, and
(16.2)
Exchanges waiting_states with an empty list, storing the old value in a local prior_states.

Then, for each pointer p in prior_states, evaluates p->notify().

Finally, evaluates dec-ref().

🔗

void inc-ref() noexcept;

Effects: Increments ref_count.

🔗

void dec-ref() noexcept;

Effects: Decrements ref_count.

If the new value of ref_count is 0, calls delete this.

Synchronization: If an evaluation of dec-ref() does not decrement the ref_count to 0 then synchronizes with the evaluation of dec-ref() that decrements ref_count to 0.

Let split-impl-tag be an empty exposition-only class type.

Given an expression sndr, the expression split.transform_sender(sndr) is equivalent to: auto&& [tag, _, child] = sndr; auto* sh_state = new shared-state{std::forward_like<decltype((sndr))>(child)}; return make-sender(split-impl-tag(), shared-wrapper{sh_state, tag}); where shared-wrapper is an exposition-only class that manages the reference count of the shared-state object pointed to by sh_state.

shared-wrapper models copyable with move operations nulling out the moved-from object, copy operations incrementing the reference count by calling sh_state->inc-ref(), and assignment operations performing a copy-and-swap operation.

The destructor has no effect if sh_state is null; otherwise, it decrements the reference count by evaluating sh_state->dec-ref().

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

The member impls-for<split-impl-tag>::get-state is initialized with a callable object equivalent to the following lambda expression: []<class Sndr>(Sndr&& sndr, auto& rcvr) noexcept { return local-state{std::forward<Sndr>(sndr), rcvr}; }

The member impls-for<split-impl-tag>::start is initialized with a callable object that has a function call operator equivalent to the following: template<class Sndr, class Rcvr> void operator()(local-state<Sndr, Rcvr>& state, Rcvr& rcvr) const noexcept;

Effects: If state.sh_state->completed is true, evaluates state.notify() and returns.

Otherwise, does the following in order:

(23.1)
Evaluates state.on_stop.emplace( get_stop_token(get_env(rcvr)), on-stop-request{state.sh_state->stop_src});
(23.2)
Then atomically does the following:
- (23.2.1)
  Reads the value c of state.sh_state->completed, and
- (23.2.2)
  Inserts addressof(state) into state.sh_state->waiting_states if c is false.
(23.3)
If c is true, calls state.notify() and returns.
(23.4)
Otherwise, if addressof(state) is the first item added to state.sh_state->waiting_states, evaluates state.sh_state->start-op().

34.9.12.11 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))...>.

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, or
(2.3)
CD is ill-formed.

The expression when_all(sndrs...) is expression-equivalent to: transform_sender(CD(), 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; }; }

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 empty_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 JOIN-ENV( MAKE-ENV(get_stop_token, state.stop_src.get_token()), 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(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: template<class Sndr, class Env> concept max-1-sender-in = sender_in<Sndr, Env> && // exposition only (tuple_size_v<value_types_of_t<Sndr, Env, tuple, tuple>> <= 1); enum class disposition { started, error, stopped }; // exposition only template<class Rcvr> struct make-state { template<max-1-sender-in<env_of_t<Rcvr>>... Sndrs> auto operator()(auto, auto, Sndrs&&... sndrs) const { using values_tuple = see below; using errors_variant = see below; using stop_callback = stop_callback_of_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, 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:

(11.1)
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);
(11.2)
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);
(11.3)
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(CD(), 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]

34.9.12.12 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; }; }

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>, 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)...); } }

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

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

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, or if the type single-sender-value-type<Sndr, Env> is ill-formed or void, then the expression stopped_as_optional.transform_sender(sndr, env) is ill-formed; otherwise, it is equivalent to: auto&& [_, _, child] = sndr; using V = single-sender-value-type<Sndr, 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>()); });

34.9.12.14 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)); });

34.9.13 Sender consumers [exec.consumers]

34.9.13.1 this_thread::sync_wait [exec.sync.wait]

this_thread::sync_wait and this_thread::sync_wait_with_variant are used to block the current thread of execution until the specified sender completes and to return its async result.

sync_wait mandates that the input sender has exactly one value completion signature.

Let sync-wait-env be the following exposition-only class type: namespace std::this_thread { struct sync-wait-env { execution::run_loop* loop; // exposition only auto query(execution::get_scheduler_t) const noexcept { return loop->get_scheduler(); } auto query(execution::get_delegation_scheduler_t) const noexcept { return loop->get_scheduler(); } }; }

Let sync-wait-result-type and sync-wait-with-variant-result-type be exposition-only alias templates defined as follows: namespace std::this_thread { template<execution::sender_in<sync-wait-env> Sndr> using sync-wait-result-type = optional<execution::value_types_of_t<Sndr, sync-wait-env, decayed-tuple, type_identity_t>>; template<execution::sender_in<sync-wait-env> Sndr> using sync-wait-with-variant-result-type = optional<execution::value_types_of_t<Sndr, sync-wait-env>>; }

The name this_thread::sync_wait denotes a customization point object.

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

If sender_in<Sndr, sync-wait-env> is false, the expression this_thread::sync_wait(sndr) is ill-formed.

Otherwise, it is expression-equivalent to the following, except that sndr is evaluated only once: apply_sender(get-domain-early(sndr), sync_wait, sndr) Mandates:

(4.1)
The type sync-wait-result-type<Sndr> is well-formed.
(4.2)
same_as<decltype(e), sync-wait-result-type<Sndr>> is true, where e is the apply_sender expression above.

Let sync-wait-state and sync-wait-receiver be the following exposition-only class templates: namespace std::this_thread { template<class Sndr> struct sync-wait-state { // exposition only execution::run_loop loop; // exposition only exception_ptr error; // exposition only sync-wait-result-type<Sndr> result; // exposition only }; template<class Sndr> struct sync-wait-receiver { // exposition only using receiver_concept = execution::receiver_t; sync-wait-state<Sndr>* state; // exposition only template<class... Args> void set_value(Args&&... args) && noexcept; template<class Error> void set_error(Error&& err) && noexcept; void set_stopped() && noexcept; sync-wait-env get_env() const noexcept { return {&state->loop}; } }; }

🔗

template<class... Args>
void set_value(Args&&... args) && noexcept;

Effects: Equivalent to: try { state->result.emplace(std::forward<Args>(args)...); } catch (...) { state->error = current_exception(); } state->loop.finish();

🔗

template<class Error>
void set_error(Error&& err) && noexcept;

Effects: Equivalent to: state->error = AS-EXCEPT-PTR(std::forward<Error>(err)); // see [exec.general] state->loop.finish();

🔗

void set_stopped() && noexcept;

Effects: Equivalent to state->loop.finish().

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

If sender_to<Sndr, sync-wait-receiver<
Sndr>> is false, the expression sync_wait.apply_sender(sndr) is ill-formed; otherwise, it is equivalent to: sync-wait-state<Sndr> state; auto op = connect(sndr, sync-wait-receiver<Sndr>{&state}); start(op); state.loop.run(); if (state.error) { rethrow_exception(std::move(state.error)); } return std::move(state.result);

The behavior of this_thread::sync_wait(sndr) is undefined unless:

(10.1)
It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes.

[Note 1:
The default implementation of sync_wait achieves forward progress guarantee delegation by providing a run_loop scheduler via the get_delegation_scheduler query on the sync-wait-receiver's environment.

The run_loop is driven by the current thread of execution.
— end note]
(10.2)
It returns the specified sender's async results as follows:
- (10.2.1)
  For a value completion, the result datums are returned in a tuple in an engaged optional object.
- (10.2.2)
  For an error completion, an exception is thrown.
- (10.2.3)
  For a stopped completion, a disengaged optional object is returned.

34.9.13.2 this_thread::sync_wait_with_variant [exec.sync.wait.var]

The name this_thread::sync_wait_with_variant denotes a customization point object.

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

If sender_in<Sndr, sync-wait-env> is false, this_thread::sync_wait_with_variant(sndr) is ill-formed.

Otherwise, it is expression-equivalent to the following, except sndr is evaluated only once: apply_sender(get-domain-early(sndr), sync_wait_with_variant, sndr) Mandates:

(1.1)
The type sync-wait-with-variant-result-type<Sndr> is well-formed.
(1.2)
same_as<decltype(e), sync-wait-with-variant-result-type<Sndr>> is true, where e is the apply_sender expression above.

If callable<sync_wait_t, Sndr> is false, the expression sync_wait_with_variant.apply_sender(
sndr) is ill-formed.

Otherwise, it is equivalent to: using result_type = sync-wait-with-variant-result-type<Sndr>; if (auto opt_value = sync_wait(into_variant(sndr))) { return result_type(std::move(get<0>(*opt_value))); } return result_type(nullopt);

The behavior of this_thread::sync_wait_with_variant(sndr) is undefined unless:

(3.1)
It blocks the current thread of execution ([defns.block]) with forward progress guarantee delegation ([intro.progress]) until the specified sender completes.

[Note 1:
The default implementation of sync_wait_with_variant achieves forward progress guarantee delegation by relying on the forward progress guarantee delegation provided by sync_wait.
— end note]
(3.2)
It returns the specified sender's async results as follows:
- (3.2.1)
  For a value completion, the result datums are returned in an engaged optional object that contains a variant of tuples.
- (3.2.2)
  For an error completion, an exception is thrown.
- (3.2.3)
  For a stopped completion, a disengaged optional object is returned.

34 Execution control library [exec]

34.9 Senders [exec.snd]

34.9.1 General [exec.snd.general]

34.9.2 Exposition-only entities [exec.snd.expos]

34.9.3 Sender concepts [exec.snd.concepts]

34.9.4 Awaitable helpers [exec.awaitable]

34.9.5 execution​::​default_domain [exec.domain.default]

34.9.6 execution​::​transform_sender [exec.snd.transform]

34.9.7 execution​::​transform_env [exec.snd.transform.env]

34.9.8 execution​::​apply_sender [exec.snd.apply]

34.9.9 execution​::​get_completion_signatures [exec.getcomplsigs]

34.9.10 execution​::​connect [exec.connect]

34.9.11 Sender factories [exec.factories]

34.9.11.1 execution​::​schedule [exec.schedule]

34.9.11.2 execution​::​just, execution​::​just_error, execution​::​just_stopped [exec.just]

34.9.11.3 execution​::​read_env [exec.read.env]

34.9.12 Sender adaptors [exec.adapt]

34.9.12.1 General [exec.adapt.general]

34.9.12.2 Closure objects [exec.adapt.obj]

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

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

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

34.9.12.6 execution​::​on [exec.on]

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

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

34.9.12.9 execution​::​bulk [exec.bulk]

34.9.12.10 execution​::​split [exec.split]

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

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

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

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

34.9.13 Sender consumers [exec.consumers]

34.9.13.1 this_thread​::​sync_wait [exec.sync.wait]

34.9.13.2 this_thread​::​sync_wait_with_variant [exec.sync.wait.var]

34.9.5 execution::default_domain [exec.domain.default]

34.9.6 execution::transform_sender [exec.snd.transform]

34.9.7 execution::transform_env [exec.snd.transform.env]

34.9.8 execution::apply_sender [exec.snd.apply]

34.9.9 execution::get_completion_signatures [exec.getcomplsigs]

34.9.10 execution::connect [exec.connect]

34.9.11.1 execution::schedule [exec.schedule]

34.9.11.2 execution::just, execution::just_error, execution::just_stopped [exec.just]

34.9.11.3 execution::read_env [exec.read.env]

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

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

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

34.9.12.6 execution::on [exec.on]

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

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

34.9.12.9 execution::bulk [exec.bulk]

34.9.12.10 execution::split [exec.split]

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

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

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

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

34.9.13.1 this_thread::sync_wait [exec.sync.wait]

34.9.13.2 this_thread::sync_wait_with_variant [exec.sync.wait.var]