33 Execution control library [exec]

33.10 Completion signatures [exec.cmplsig]

completion_signatures is a type that encodes a set of completion signatures ([exec.async.ops]).

[Example 1: struct my_sender { using sender_concept = sender_t; using completion_signatures = execution::completion_signatures< set_value_t(), set_value_t(int, float), set_error_t(exception_ptr), set_error_t(error_code), set_stopped_t()>; };

Declares my_sender to be a sender that can complete by calling one of the following for a receiver expression rcvr:

(2.1)
set_value(rcvr)
(2.2)
set_value(rcvr, int{...}, float{...})
(2.3)
set_error(rcvr, exception_ptr{...})
(2.4)
set_error(rcvr, error_code{...})
(2.5)
set_stopped(rcvr)

— end example]

This subclause makes use of the following exposition-only entities: template<class Fn> concept completion-signature = see below;

A type Fn satisfies completion-signature if and only if it is a function type with one of the following forms:

(4.1)
set_value_t(Vs...), where Vs is a pack of object or reference types.
(4.2)
set_error_t(Err), where Err is an object or reference type.
(4.3)
set_stopped_t()

template<bool> struct indirect-meta-apply { template<template<class...> class T, class... As> using meta-apply = T<As...>; // exposition only }; template<class...> concept always-true = true; // exposition only template<class Tag, valid-completion-signatures Completions, template<class...> class Tuple, template<class...> class Variant> using gather-signatures = see below;

Let Fns be a pack of the arguments of the completion_signatures specialization named by Completions, let TagFns be a pack of the function types in Fns whose return types are Tag, and let

Ts_{n}

be a pack of the function argument types in the n-th type in TagFns.

Then, given two variadic templates Tuple and Variant, the type gather-signatures<Tag, Completions, Tuple, Variant> names the type META-APPLY(Variant, META-APPLY(Tuple, Ts

_{0}

...), META-APPLY(Tuple, Ts

_{1}

...), …, META-APPLY(Tuple, Ts

_{m - 1}

...)) where m is the size of the pack TagFns and META-APPLY(T, As...) is equivalent to: typename indirect-meta-apply<always-true<As...>>::template meta-apply<T, As...>

[Note 1:

The purpose of META-APPLY is to make it valid to use non-variadic templates as Variant and Tuple arguments to gather-signatures.

— end note]

🔗

namespace std::execution { template<completion-signature... Fns> struct completion_signatures { template<class Tag> static constexpr size_t count-of(Tag) { return see below; } template<class Fn> static constexpr void for-each(Fn&& fn) { // exposition only (std::forward<Fn>(fn)(static_cast<Fns*>(nullptr)), ...); } }; template<class Sndr, class Env = env<>, template<class...> class Tuple = decayed-tuple, template<class...> class Variant = variant-or-empty> requires sender_in<Sndr, Env> using value_types_of_t = gather-signatures<set_value_t, completion_signatures_of_t<Sndr, Env>, Tuple, Variant>; template<class Sndr, class Env = env<>, template<class...> class Variant = variant-or-empty> requires sender_in<Sndr, Env> using error_types_of_t = gather-signatures<set_error_t, completion_signatures_of_t<Sndr, Env>, type_identity_t, Variant>; template<class Sndr, class Env = env<>> requires sender_in<Sndr, Env> constexpr bool sends_stopped = !same_as<type-list<>, gather-signatures<set_stopped_t, completion_signatures_of_t<Sndr, Env>, type-list, type-list>>; }

For a subexpression tag, let Tag be the decayed type of tag.

completion_signatures<Fns...>::count-of(
tag) returns the count of function types in Fns... that are of the form Tag(Ts...) where Ts is a pack of types.