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cppdraft/exec/sysctxrepl/query.md

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+[exec.sysctxrepl.query]
+
+# 33 Execution control library [[exec]](./#exec)
+
+## 33.16 Namespace system_context_replaceability [[exec.sysctxrepl]](exec.sysctxrepl#query)
+
+### 33.16.2 query_parallel_scheduler_backend [exec.sysctxrepl.query]
+
+[🔗](#lib:query_parallel_scheduler_backend)
+
+`shared_ptr<parallel_scheduler_backend> query_parallel_scheduler_backend();
+`
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8443)
+
+query_parallel_scheduler_backend() returns
+the implementation object for a parallel scheduler[.](#1.sentence-1)
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8447)
+
+*Returns*: A non-null shared pointer to an object
+that implements the parallel_scheduler_backend interface[.](#2.sentence-1)
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8452)
+
+*Remarks*: This function is replaceable ([[dcl.fct.def.replace]](dcl.fct.def.replace#term.replaceable.function "9.6.5 Replaceable function definitions"))[.](#3.sentence-1)
+
+namespace std::execution::system_context_replaceability {struct [receiver_proxy](#lib:receiver_proxy "33.16.2 query_­parallel_­scheduler_­backend [exec.sysctxrepl.query]") {virtual ~receiver_proxy() = default; protected:virtual bool *query-env*(*unspecified*) noexcept = 0; // *exposition only*public:virtual void set_value() noexcept = 0; virtual void set_error(exception_ptr) noexcept = 0; virtual void set_stopped() noexcept = 0; template<class P, [*class-type*](execution.syn#concept:class-type "33.4 Header <execution> synopsis [execution.syn]") Query> optional<P> try_query(Query q) noexcept; }; struct [bulk_item_receiver_proxy](#lib:bulk_item_receiver_proxy "33.16.2 query_­parallel_­scheduler_­backend [exec.sysctxrepl.query]") : receiver_proxy {virtual void execute(size_t, size_t) noexcept = 0; };}
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8480)
+
+receiver_proxy represents a receiver
+that will be notified
+by the implementations of parallel_scheduler_backend to trigger the completion operations[.](#4.sentence-1)
+
+bulk_item_receiver_proxy is derived from receiver_proxy and
+is used for bulk_chunked and bulk_unchunked customizations
+that will also receive notifications
+from implementations of parallel_scheduler_backend corresponding to different iterations[.](#4.sentence-2)
+
+[🔗](#itemdecl:2)
+
+`template<class P, [class-type](execution.syn#concept:class-type "33.4 Header <execution> synopsis [execution.syn]") Query>
+  optional<P> [try_query](#lib:try_query "33.16.2 query_­parallel_­scheduler_­backend [exec.sysctxrepl.query]")(Query q) noexcept;
+`
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8497)
+
+*Mandates*: P is a cv-unqualified non-array object type[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8501)
+
+*Returns*: Let env be the environment of the receiver represented by *this[.](#6.sentence-1)
+
+If
+
+- [(6.1)](#6.1)
+
+Query is not a member of an implementation-defined set
+of supported queries; or
+
+- [(6.2)](#6.2)
+
+P is not a member of an implementation-defined set
+of supported result types for Query; or
+
+- [(6.3)](#6.3)
+
+the expression q(env) is not well-formed or
+does not have type cv P,
+
+then returns nullopt[.](#6.sentence-2)
+
+Otherwise, returns q(env)[.](#6.sentence-3)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8519)
+
+*Remarks*: get_stop_token_t is
+in the implementation-defined set of supported queries, andinplace_stop_token is a member
+of the implementation-defined set of supported result types
+for get_stop_token_t[.](#7.sentence-1)