tmp/tmpuw5gdocu/{from.md → to.md}
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| 1 |
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### Class template `env` <a id="exec.env">[[exec.env]]</a>
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``` cpp
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namespace std::execution {
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template<queryable... Envs>
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struct env {
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Envs_0 envs_0; // exposition only
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Envs_1 envs_1; // exposition only
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⋮
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Envs_{n-1} envs_{n-1}; // exposition only
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template<class QueryTag>
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constexpr decltype(auto) query(QueryTag q) const noexcept(see below);
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};
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template<class... Envs>
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env(Envs...) -> env<unwrap_reference_t<Envs>...>;
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}
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```
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The class template `env` is used to construct a queryable object from
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several queryable objects. Query invocations on the resulting object are
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resolved by attempting to query each subobject in lexical order.
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Specializations of `env` are not assignable.
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It is unspecified whether `env` supports initialization using a
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parenthesized *expression-list* [[dcl.init]], unless the
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*expression-list* consist of a single element of type (possibly const)
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`env`.
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[*Example 1*:
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``` cpp
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template<sender Sndr>
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sender auto parameterize_work(Sndr sndr) {
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// Make an environment such that:
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// get_allocator(env) returns a reference to a copy of my_alloc{}
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// get_scheduler(env) returns a reference to a copy of my_sched{}
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auto e = env{prop(get_allocator, my_alloc{}),
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prop(get_scheduler, my_sched{})};
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// Parameterize the input sender so that it will use our custom execution environment.
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return write_env(sndr, e);
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}
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```
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— *end example*]
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``` cpp
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template<class QueryTag>
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constexpr decltype(auto) query(QueryTag q) const noexcept(see below);
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```
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Let `has-query` be the following exposition-only concept:
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``` cpp
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template<class Env, class QueryTag>
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concept has-query = // exposition only
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requires (const Env& env) {
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env.query(QueryTag());
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};
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```
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Let *fe* be the first element of envs₀, envs₁, …, envsₙ₋₁ such that the
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expression *`fe`*`.query(q)` is well-formed.
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*Constraints:* `(has-query<Envs, QueryTag> || ...)` is `true`.
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*Effects:* Equivalent to: `return `*`fe`*`.query(q);`
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*Remarks:* The expression in the `noexcept` clause is equivalent to
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`noexcept(`*`fe`*`.query(q))`.
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