33 Execution control library [exec]

33.1 General [exec.general]

This Clause describes components supporting execution of function objects ([function.objects]).

The following subclauses describe the requirements, concepts, and components for execution control primitives as summarized in Table 157 .

Table 157 — Execution control library summary [tab:exec.summary]

🔗	Subclause	Header
🔗 [exec.sched]	Schedulers	<execution>
🔗 [exec.recv]	Receivers
🔗 [exec.opstate]	Operation states
🔗 [exec.snd]	Senders

Table 158 shows the types of customization point objects ([customization.point.object]) used in the execution control library.

Table 158 — Types of customization point objects in the execution control library [tab:exec.pos]

🔗 Customization point	Purpose	Examples
🔗 object type
🔗 core	provide core execution functionality, and connection between core components	e.g., connect, start
🔗 completion functions	called by senders to announce the completion of the work (success, error, or cancellation)	set_value, set_error, set_stopped
🔗 senders	allow the specialization of the provided sender algorithms	sender factories (e.g., schedule, just, read_env) sender adaptors (e.g., continues_on, then, let_value) sender consumers (e.g., sync_wait)
🔗 queries	allow querying different properties of objects	general queries (e.g., get_allocator, get_stop_token) environment queries (e.g., get_scheduler, get_delegation_scheduler) scheduler queries (e.g., get_forward_progress_guarantee) sender attribute queries (e.g., get_completion_scheduler)

This clause makes use of the following exposition-only entities.

For a subexpression expr, let MANDATE-NOTHROW(expr) be expression-equivalent to expr.

Mandates: noexcept(expr) is true.

namespace std { template<class T> concept movable-value = // exposition only move_constructible<decay_t<T>> && constructible_from<decay_t<T>, T> && (!is_array_v<remove_reference_t<T>>); }

For function types F1 and F2 denoting R1(Args1...) and R2(Args2...), respectively, MATCHING-SIG(F1, F2) is true if and only if same_as<R1(Args1&&...), R2(Args2&&...)> is true.

For a subexpression err, let Err be decltype((err)) and let AS-EXCEPT-PTR(err) be:

(8.1)
err if decay_t<Err> denotes the type exception_ptr.
Preconditions: !err is false.
(8.2)
Otherwise, make_exception_ptr(system_error(err)) if decay_t<Err> denotes the type error_code.
(8.3)
Otherwise, make_exception_ptr(err).

For a subexpression expr, let AS-CONST(expr) be expression-equivalent to [](const auto& x) noexcept -> const auto& { return x; }(expr)