Init

cppdraft/exec/envs.md

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+[exec.envs]
+
+# 33 Execution control library [[exec]](./#exec)
+
+## 33.11 Queryable utilities [exec.envs]
+
+### [33.11.1](#exec.prop) Class template prop [[exec.prop]](exec.prop)
+
+namespace std::execution {template<class QueryTag, class ValueType>struct [prop](#lib:prop "33.11.1 Class template prop [exec.prop]") { QueryTag *query_*; // *exposition only* ValueType *value_*; // *exposition only*constexpr const ValueType& query(QueryTag) const noexcept {return *value_*; }}; template<class QueryTag, class ValueType> prop(QueryTag, ValueType) -> prop<QueryTag, unwrap_reference_t<ValueType>>;}
+
+[1](#exec.prop-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6193)
+
+Class template prop is for building a queryable object
+from a query object and a value[.](#exec.prop-1.sentence-1)
+
+[2](#exec.prop-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6197)
+
+*Mandates*: [*callable*](functional.syn#concept:callable "22.10.2 Header <functional> synopsis [functional.syn]")<QueryTag, *prop-like*<ValueType>> is modeled,
+where *prop-like* is the following exposition-only class template:template<class ValueType>struct *prop-like* { // *exposition only*const ValueType& query(auto) const noexcept;};
+
+[3](#exec.prop-3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6209)
+
+[*Example [1](#exec.prop-example-1)*: template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr> sender auto parameterize_work(Sndr sndr) {// Make an environment such that get_allocator(env) returns a reference to a copy of my_alloc{}.auto e = prop(get_allocator, my_alloc{}); // Parameterize the input sender so that it will use our custom execution environment.return write_env(sndr, e);} — *end example*]
+
+[4](#exec.prop-4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6223)
+
+Specializations of prop are not assignable[.](#exec.prop-4.sentence-1)
+
+### [33.11.2](#exec.env) Class template env [[exec.env]](exec.env)
+
+namespace std::execution {template<[*queryable*](exec.queryable.concept#concept:queryable "33.2.2 queryable concept [exec.queryable.concept]")... Envs>struct [env](#lib:env "33.11.2 Class template env [exec.env]") { Envs0 envs0; // *exposition only* Envs1 envs1; // *exposition only* ⋮
+ Envsn−1 envsn−1; // *exposition only*template<class QueryTag>constexpr decltype(auto) query(QueryTag q) const noexcept(*see below*); }; template<class... Envs> env(Envs...) -> env<unwrap_reference_t<Envs>...>;}
+
+[1](#exec.env-1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6246)
+
+The class template env is used to construct a queryable object
+from several queryable objects[.](#exec.env-1.sentence-1)
+
+Query invocations on the resulting object are resolved
+by attempting to query each subobject in lexical order[.](#exec.env-1.sentence-2)
+
+[2](#exec.env-2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6252)
+
+Specializations of env are not assignable[.](#exec.env-2.sentence-1)
+
+[3](#exec.env-3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6255)
+
+It is unspecified
+whether env supports initialization
+using a parenthesized [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") ([[dcl.init]](dcl.init "9.5 Initializers")),
+unless the [*expression-list*](expr.post.general#nt:expression-list "7.6.1.1 General [expr.post.general]") consist of
+a single element of type (possibly const) env[.](#exec.env-3.sentence-1)
+
+[4](#exec.env-4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6262)
+
+[*Example [1](#exec.env-example-1)*: template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sndr> sender auto parameterize_work(Sndr sndr) {// Make an environment such that:// get_allocator(env) returns a reference to a copy of my_alloc{}// get_scheduler(env) returns a reference to a copy of my_sched{}auto e = env{prop(get_allocator, my_alloc{}),
+ prop(get_scheduler, my_sched{})}; // Parameterize the input sender so that it will use our custom execution environment.return write_env(sndr, e);} — *end example*]
+
+[🔗](#lib:query,env)
+
+`template<class QueryTag>
+constexpr decltype(auto) query(QueryTag q) const noexcept(see below);
+`
+
+[5](#exec.env-5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6286)
+
+Let [*has-query*](#concept:has-query "33.11.2 Class template env [exec.env]") be the following exposition-only concept:template<class Env, class QueryTag>concept [*has-query*](#concept:has-query "33.11.2 Class template env [exec.env]") = // *exposition only*requires (const Env& env) { env.query(QueryTag()); };
+
+[6](#exec.env-6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6296)
+
+Let *fe* be the first element ofenvs0, envs1, …, envsn−1 such that the expression *fe*.query(q) is well-formed[.](#exec.env-6.sentence-1)
+
+[7](#exec.env-7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6301)
+
+*Constraints*: ([*has-query*](#concept:has-query "33.11.2 Class template env [exec.env]")<Envs, QueryTag> || ...) is true[.](#exec.env-7.sentence-1)
+
+[8](#exec.env-8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6305)
+
+*Effects*: Equivalent to: return *fe*.query(q);
+
+[9](#exec.env-9)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6309)
+
+*Remarks*: The expression in the noexcept clause is equivalent
+to noexcept(*fe*.query(q))[.](#exec.env-9.sentence-1)