34 Execution control library [exec]

schedule_from schedules work dependent on the completion of a sender onto a scheduler's associated execution resource.

The name schedule_from denotes a customization point object.

For some subexpressions sch and sndr, let Sch be decltype((sch)) and Sndr be decltype((sndr)).

If Sch does not satisfy scheduler, or Sndr does not satisfy sender, schedule_from(sch, sndr) is ill-formed.

Otherwise, the expression schedule_from(sch, sndr) is expression-equivalent to: transform_sender( query-or-default(get_domain, sch, default_domain()), make-sender(schedule_from, sch, sndr)) except that sch is evaluated only once.

The exposition-only class template impls-for ([exec.snd.general]) is specialized for schedule_from_t as follows: namespace std::execution { template<> struct impls-for<schedule_from_t> : default-impls { static constexpr auto get-attrs = see below; static constexpr auto get-state = see below; static constexpr auto complete = see below; }; }

The member impls-for<schedule_from_t>​::​get-attrs is initialized with a callable object equivalent to the following lambda: [](const auto& data, const auto& child) noexcept -> decltype(auto) { return JOIN-ENV(SCHED-ATTRS(data), FWD-ENV(get_env(child))); }

The member impls-for<schedule_from_t>​::​get-state is initialized with a callable object equivalent to the following lambda: []<class Sndr, class Rcvr>(Sndr&& sndr, Rcvr& rcvr) noexcept(see below) requires sender_in<child-type<Sndr>, env_of_t<Rcvr>> { auto& [_, sch, child] = sndr; using sched_t = decltype(auto(sch)); using variant_t = see below; using receiver_t = see below; using operation_t = connect_result_t<schedule_result_t<sched_t>, receiver_t>; constexpr bool nothrow = noexcept(connect(schedule(sch), receiver_t{nullptr})); struct state-type { Rcvr& rcvr; // exposition only variant_t async-result; // exposition only operation_t op-state; // exposition only explicit state-type(sched_t sch, Rcvr& rcvr) noexcept(nothrow) : rcvr(rcvr), op-state(connect(schedule(sch), receiver_t{this})) {} }; return state-type{sch, rcvr}; }

Objects of the local class state-type can be used to initialize a structured binding.

Let Sigs be a pack of the arguments to the completion_signatures specialization named by completion_signatures_of_t<child-type<Sndr>, env_of_t<Rcvr>>.

Let as-tuple be an alias template that transforms a completion signature Tag(Args...) into the tuple specialization decayed-tuple<Tag, Args...>.

Then variant_t denotes the type variant<monostate, as-tuple<Sigs>...>, except with duplicate types removed.

receiver_t is an alias for the following exposition-only class: namespace std::execution { struct receiver-type { using receiver_concept = receiver_t; state-type* state; // exposition only void set_value() && noexcept { visit( [this]<class Tuple>(Tuple& result) noexcept -> void { if constexpr (!same_as<monostate, Tuple>) { auto& [tag, ...args] = result; tag(std::move(state->rcvr), std::move(args)...); } }, state->async-result); } template<class Error> void set_error(Error&& err) && noexcept { execution::set_error(std::move(state->rcvr), std::forward<Error>(err)); } void set_stopped() && noexcept { execution::set_stopped(std::move(state->rcvr)); } decltype(auto) get_env() const noexcept { return FWD-ENV(execution::get_env(state->rcvr)); } }; }

The expression in the noexcept clause of the lambda is true if the construction of the returned state-type object is not potentially throwing; otherwise, false.

The member impls-for<schedule_from_t>​::​complete is initialized with a callable object equivalent to the following lambda: []<class Tag, class... Args>(auto, auto& state, auto& rcvr, Tag, Args&&... args) noexcept -> void { using result_t = decayed-tuple<Tag, Args...>; constexpr bool nothrow = is_nothrow_constructible_v<result_t, Tag, Args...>; TRY-EVAL(rcvr, [&]() noexcept(nothrow) { state.async-result.template emplace<result_t>(Tag(), std::forward<Args>(args)...); }()); if (state.async-result.valueless_by_exception()) return; if (state.async-result.index() == 0) return; start(state.op-state); };

Let out_sndr be a subexpression denoting a sender returned from schedule_from(sch, sndr) or one equal to such, and let OutSndr be the type decltype((out_sndr)).

Let out_rcvr be a subexpression denoting a receiver that has an environment of type Env such that sender_in<OutSndr, Env> is true.

Let op be an lvalue referring to the operation state that results from connecting out_sndr with out_rcvr.

Calling start(op) shall start sndr on the current execution agent and execute completion operations on out_rcvr on an execution agent of the execution resource associated with sch.

If scheduling onto sch fails, an error completion on out_rcvr shall be executed on an unspecified execution agent.