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;
4
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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;
6
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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>>; }

34.10.2 execution​::​transform_completion_signatures [exec.util.cmplsig.trans]

1
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transform_completion_signatures is an alias template used to transform one set of completion signatures into another.
It takes a set of completion signatures and several other template arguments that apply modifications to each completion signature in the set to generate a new specialization of completion_signatures.
2
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[Example 1: 
Given a sender Sndr and an environment Env, adapt the completion signatures of Sndr by lvalue-ref qualifying the values, adding an additional exception_ptr error completion if its not already there, and leaving the other completion signatures alone.
template<class... Args> using my_set_value_t = completion_signatures< set_value_t(add_lvalue_reference_t<Args>...)>; using my_completion_signatures = transform_completion_signatures< completion_signatures_of_t<Sndr, Env>, completion_signatures<set_error_t(exception_ptr)>, my_set_value_t>; — end example]
3
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This subclause makes use of the following exposition-only entities: template<class... As> using default-set-value = completion_signatures<set_value_t(As...)>; template<class Err> using default-set-error = completion_signatures<set_error_t(Err)>;
4
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namespace std::execution { template<valid-completion-signatures InputSignatures, valid-completion-signatures AdditionalSignatures = completion_signatures<>, template<class...> class SetValue = default-set-value, template<class> class SetError = default-set-error, valid-completion-signatures SetStopped = completion_signatures<set_stopped_t()>> using transform_completion_signatures = completion_signatures<see below>; }
5
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SetValue shall name an alias template such that for any pack of types As, the type SetValue<As...> is either ill-formed or else valid-completion-signatures<SetValue<As...>> is satisfied.
SetError shall name an alias template such that for any type Err, SetError<Err> is either ill-formed or else valid-completion-signatures<SetError<Err>> is satisfied.
6
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Let Vs be a pack of the types in the type-list named by gather-signatures<set_value_t, InputSignatures, SetValue, type-list>.
7
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Let Es be a pack of the types in the type-list named by gather-signatures<set_error_t, InputSignatures, type_identity_t, error-list>, where error-list is an alias template such that error-list<
Ts...> is type-list<SetError<Ts>...>.
8
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Let Ss name the type completion_signatures<> if gather-signatures<set_stopped_t, InputSignatures, type-list, type-list> is an alias for the type type-list<>; otherwise, SetStopped.
9
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If any of the above types are ill-formed, then transform_completion_signatures<InputSignatures, AdditionalSignatures, SetValue, SetError, SetStopped> is ill-formed.
Otherwise, transform_completion_signatures<InputSignatures, AdditionalSignatures, SetValue, SetError, SetStopped> is the type completion_signatures<Sigs...> where Sigs... is the unique set of types in all the template arguments of all the completion_signatures specializations in the set AdditionalSignatures, Vs..., Es..., Ss.