[exec.prop]

namespace std::execution { template<class QueryTag, class ValueType> struct 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>>; }

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

Mandates: callable<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; };

[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]

Specializations of prop are not assignable.

33 Execution control library [exec]

33.11 Queryable utilities [exec.envs]

33.11.1 Class template prop [exec.prop]