cppdraft_translate/cppdraft/exec/envs.md

[exec.envs]

33 Execution control library [exec]

33.11 Queryable utilities [exec.envs]

33.11.1 Class template prop [exec.prop]

namespace std::execution {template<class QueryTag, class ValueType>struct prop { QueryTag query_; // exposition only ValueType value_; // exposition onlyconstexpr const ValueType& query(QueryTag) const noexcept {return value_; }}; template<class QueryTag, class ValueType> prop(QueryTag, ValueType) -> prop<QueryTag, unwrap_reference_t>;}

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Class template prop is for building a queryable object from a query object and a value.

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Mandates: callable<QueryTag, prop-like> is modeled, where prop-like is the following exposition-only class template:templatestruct prop-like { // exposition onlyconst ValueType& query(auto) const noexcept;};

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[Example 1: template<sender 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]

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Specializations of prop are not assignable.

33.11.2 Class template env [exec.env]

namespace std::execution {template<queryable... Envs>struct env { Envs0 envs0; // exposition only Envs1 envs1; // exposition only ⋮ Envsn−1 envsn−1; // exposition onlytemplateconstexpr decltype(auto) query(QueryTag q) const noexcept(see below); }; template<class... Envs> env(Envs...) -> env<unwrap_reference_t...>;}

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The class template env is used to construct a queryable object from several queryable objects.

Query invocations on the resulting object are 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 parenthesized expression-list ([dcl.init]), unless the expression-list consist of a single element of type (possibly const) env.

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[Example 1: template<sender 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]

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template<class QueryTag> constexpr decltype(auto) query(QueryTag q) const noexcept(see below);

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Let has-query be the following exposition-only concept:template<class Env, class QueryTag>concept has-query = // exposition onlyrequires (const Env& env) { env.query(QueryTag()); };

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Let fe be the first element ofenvs0, envs1, …, envsn−1 such that the 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 noexcept(fe.query(q)).