cppdraft_translate/cppdraft/exec/envs.md

[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)