cppdraft_translate/cppdraft/exec/then.md

[exec.then]

33 Execution control library [exec]

33.9 Senders [exec.snd]

33.9.12 Sender adaptors [exec.adapt]

33.9.12.9 execution​::​then, execution​::​upon_error, execution​::​upon_stopped [exec.then]

1

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then attaches an invocable as a continuation for an input sender's value completion operation.

upon_error and upon_stopped do the same for the error and stopped completion operations, respectively, sending the result of the invocable as a value completion.

2

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The names then, upon_error, and upon_stopped denote pipeable sender adaptor objects.

Let the expression then-cpo be one ofthen, upon_error, or upon_stopped.

For subexpressions sndr and f, if decltype((sndr)) does not satisfy sender, ordecltype((f)) does not satisfy movable-value,then-cpo(sndr, f) is ill-formed.

3

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Otherwise, the expression then-cpo(sndr, f) is expression-equivalent to:transform_sender(get-domain-early(sndr), make-sender(then-cpo, f, sndr)) except that sndr is evaluated only once.

4

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For then, upon_error, and upon_stopped, let set-cpo beset_value, set_error, and set_stopped, respectively.

The exposition-only class template impls-for ([exec.snd.expos]) is specialized for then-cpo as follows:

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namespace std::execution {template<>struct impls-for<decayed-typeof<then-cpo>> : default-impls {static constexpr auto complete =[]<class Tag, class... Args>(auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void {if constexpr (same_as<Tag, decayed-typeof>) {TRY-SET-VALUE(rcvr, invoke(std::move(fn), std::forward(args)...)); } else { Tag()(std::move(rcvr), std::forward(args)...); }}; template<class Sndr, class... Env>static consteval void check-types(); };}

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template<class Sndr, class... Env> static consteval void check-types();

5

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Effects: Equivalent to:auto cs = get_completion_signatures<child-type, FWD-ENV-T(Env)...>();auto fn = []<class... Ts>(set_value_t(*)(Ts...)) {if constexpr (<remove_cvref_t<data-type>, Ts...>)throw unspecified-exception();}; cs.for-each(overload-set{fn, {}}); where unspecified-exception is a type derived from exception.

6

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The expression then-cpo(sndr, f) has undefined behavior unless it returns a sender out_sndr that

• (6.1)

invokes f or a copy of such with the value, error, or stopped result datums of sndr for then, upon_error, and upon_stopped, respectively, using the result value of f as out_sndr's value completion, and

• (6.2)

forwards all other completion operations unchanged.