34 Execution control library [exec]

34.10 Sender/receiver utilities [exec.util]

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

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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.
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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]
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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)>;
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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>; }
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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.
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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>.
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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>...>.
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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.
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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.