[exec.env]

# 33 Execution control library [[exec]](./#exec)

## 33.11 Queryable utilities [[exec.envs]](exec.envs#exec.env)

### 33.11.2 Class template 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](#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[.](#1.sentence-1)

Query invocations on the resulting object are resolved
by attempting to query each subobject in lexical order[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6252)

Specializations of env are not assignable[.](#2.sentence-1)

[3](#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[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6262)

[*Example [1](#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](#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](#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[.](#6.sentence-1)

[7](#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[.](#7.sentence-1)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L6305)

*Effects*: Equivalent to: return *fe*.query(q);

[9](#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))[.](#9.sentence-1)