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 only constexpr const ValueType& query(QueryTag) const noexcept { return value_; } }; template<class QueryTag, class ValueType> prop(QueryTag, ValueType) -> prop<QueryTag, unwrap_reference_t<ValueType>>; }
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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<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; };
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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.