148 lines
6.5 KiB
Markdown
148 lines
6.5 KiB
Markdown
[exec.sync.wait]
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# 33 Execution control library [[exec]](./#exec)
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## 33.9 Senders [[exec.snd]](exec.snd#exec.sync.wait)
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### 33.9.13 Sender consumers [[exec.consumers]](exec.consumers#exec.sync.wait)
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#### 33.9.13.1 this_thread::sync_wait [exec.sync.wait]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5595)
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this_thread::sync_wait and this_thread::sync_wait_with_variant are used
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to block the current thread of execution
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until the specified sender completes and
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to return its async result[.](#1.sentence-1)
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sync_wait mandates
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that the input sender has exactly one value completion signature[.](#1.sentence-2)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5604)
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Let *sync-wait-env* be the following exposition-only class type:namespace std::this_thread {struct *sync-wait-env* { execution::run_loop* *loop*; // *exposition only*auto query(execution::get_scheduler_t) const noexcept {return *loop*->get_scheduler(); }auto query(execution::get_delegation_scheduler_t) const noexcept {return *loop*->get_scheduler(); }};}
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5622)
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Let *sync-wait-result-type* and*sync-wait-with-variant-result-type* be exposition-only alias templates defined as follows:namespace std::this_thread {template<execution::[sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<*sync-wait-env*> Sndr>using *sync-wait-result-type* = optional<execution::value_types_of_t<Sndr, *sync-wait-env*, *decayed-tuple*,
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type_identity_t>>; template<execution::[sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<*sync-wait-env*> Sndr>using *sync-wait-with-variant-result-type* = optional<execution::value_types_of_t<Sndr, *sync-wait-env*>>;}
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5639)
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The name this_thread::sync_wait denotes a customization point object[.](#4.sentence-1)
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For a subexpression sndr, let Sndr be decltype((sndr))[.](#4.sentence-2)
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The expression this_thread::sync_wait(sndr) is expression-equivalent to the following,
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except that sndr is evaluated only once:apply_sender(*get-domain-early*(sndr), sync_wait, sndr)*Mandates*:
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- [(4.1)](#4.1)
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[sender_in](exec.snd.concepts#concept:sender_in "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, *sync-wait-env*> is true[.](#4.1.sentence-1)
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- [(4.2)](#4.2)
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The type *sync-wait-result-type*<Sndr> is well-formed[.](#4.2.sentence-1)
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- [(4.3)](#4.3)
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[same_as](concept.same#concept:same_as "18.4.2 Concept same_as [concept.same]")<decltype(e), *sync-wait-result-type*<Sndr>> is true, where e is the apply_sender expression above[.](#4.3.sentence-1)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5659)
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Let *sync-wait-state* and *sync-wait-receiver* be the following exposition-only class templates:namespace std::this_thread {template<class Sndr>struct *sync-wait-state* { // *exposition only* execution::run_loop *loop*; // *exposition only* exception_ptr *error*; // *exposition only**sync-wait-result-type*<Sndr> *result*; // *exposition only*}; template<class Sndr>struct *sync-wait-receiver* { // *exposition only*using receiver_concept = execution::receiver_t; *sync-wait-state*<Sndr>* *state*; // *exposition only*template<class... Args>void set_value(Args&&... args) && noexcept; template<class Error>void set_error(Error&& err) && noexcept; void set_stopped() && noexcept; *sync-wait-env* get_env() const noexcept { return {&*state*->*loop*}; }};}
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[ð](#itemdecl:1)
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`template<class... Args>
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void set_value(Args&&... args) && noexcept;
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`
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5695)
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*Effects*: Equivalent to:try {*state*->*result*.emplace(std::forward<Args>(args)...);} catch (...) {*state*->*error* = current_exception();}*state*->*loop*.finish();
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[ð](#itemdecl:2)
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`template<class Error>
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void set_error(Error&& err) && noexcept;
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`
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5714)
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*Effects*: Equivalent to:*state*->*error* = *AS-EXCEPT-PTR*(std::forward<Error>(err)); // see [[exec.general]](exec.general "33.1 General")*state*->*loop*.finish();
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[ð](#itemdecl:3)
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`void set_stopped() && noexcept;
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`
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5728)
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*Effects*: Equivalent to *state*->*loop*.finish()[.](#8.sentence-1)
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5733)
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For a subexpression sndr, let Sndr be decltype((sndr))[.](#9.sentence-1)
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If [sender_to](exec.snd.concepts#concept:sender_to "33.9.3 Sender concepts [exec.snd.concepts]")<Sndr, *sync-wait-receiver*<Sndr>> is false,
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the expression sync_wait.apply_sender(sndr) is ill-formed;
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otherwise, it is equivalent to:*sync-wait-state*<Sndr> state;auto op = connect(sndr, *sync-wait-receiver*<Sndr>{&state});
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start(op);
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state.*loop*.run();if (state.*error*) { rethrow_exception(std::move(state.*error*));}return std::move(state.*result*);
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L5751)
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The behavior of this_thread::sync_wait(sndr) is undefined unless:
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- [(10.1)](#10.1)
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It blocks the current thread of execution ([[defns.block]](defns.block "3.6 block"))
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with forward progress guarantee delegation ([[intro.progress]](intro.progress "6.10.2.3 Forward progress"))
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until the specified sender completes[.](#10.1.sentence-1)
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[*Note [1](#note-1)*:
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The default implementation of sync_wait achieves
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forward progress guarantee delegation by providing a run_loop scheduler
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via the get_delegation_scheduler query
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on the *sync-wait-receiver*'s environment[.](#10.1.sentence-2)
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The run_loop is driven by the current thread of execution[.](#10.1.sentence-3)
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â *end note*]
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- [(10.2)](#10.2)
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It returns the specified sender's async results as follows:
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* [(10.2.1)](#10.2.1)
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For a value completion,
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the result datums are returned in
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a tuple in an engaged optional object[.](#10.2.1.sentence-1)
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* [(10.2.2)](#10.2.2)
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For an error completion, an exception is thrown[.](#10.2.2.sentence-1)
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* [(10.2.3)](#10.2.3)
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For a stopped completion, a disengaged optional object is returned[.](#10.2.3.sentence-1)
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