34 Execution control library [exec]

34.10 Sender/receiver utilities [exec.util]

34.10.1 execution​::​completion_signatures [exec.util.cmplsig]

1
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completion_signatures is a type that encodes a set of completion signatures ([exec.async.ops]).
2
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[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]
3
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This subclause makes use of the following exposition-only entities: template<class Fn> concept completion-signature = see below;
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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()
5
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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;
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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 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...), META-APPLY(Tuple, Ts...), …, META-APPLY(Tuple, Ts...)) 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...>
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[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]
8
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namespace std::execution { template<completion-signature... Fns> struct completion_signatures {}; template<class Sndr, class Env = empty_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 = empty_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 = empty_env> requires sender_in<Sndr, Env> inline constexpr bool sends_stopped = !same_as<type-list<>, gather-signatures<set_stopped_t, completion_signatures_of_t<Sndr, Env>, type-list, type-list>>; }