std::execution::scheduler

 

C++

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Named requirements

Feature test macros(C++20)

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Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Execution control library

Execution concepts scheduler receiver receiver_of operation_state    sender sender_in sender_to Execution components Execution contexts run_loop Execution domains default_domain Forward progress guarantee forward_progress_guarantee Environments empty_env    get_env    env_of_t Queries std::stop_token_of_t std::get_stop_token std::get_allocator std::forwarding_query get_scheduler get_domain get_forward_progress_guarantee get_delegation_scheduler get_completion_scheduler Completion signatures completion_signatures completion_signatures_of_t transform_completion_signatures transform_completion_signatures_of    value_types_of_t error_types_of_t    tag_of_t sends_stopped    Sender adaptor closures sender_adaptor_closure Coroutine utility as_awaitable with_awaitable_senders

Core operations Operation state connect connect_result_t start Completion set_value set_error set_stopped Transformation transform_env transform_sender apply_sender Sender algorithms Sender factories just just_error just_stopped read_env schedule schedule_result_t Sender adaptors starts_on continues_on    on then upon_error upon_stopped let_value let_error let_stopped schedule_from when_all when_all_with_variant into_variant stopped_as_optional stopped_as_error bulk split Sender consumers this_thread::sync_wait this_thread::sync_wait_with_variant

 

Defined in header <execution>
template<class Sch > concept scheduler =     std::derived_from<         typenamestd::remove_cvref_t<Sch>::scheduler_concept,         scheduler_t>&&     /*queryable*/<Sch>&&     requires(Sch&& sch)     {         {             std::execution::schedule(std::forward<Sch>(sch))         }-> std::execution::sender;         {             auto(                 std::execution::get_completion_scheduler<                     std::execution::set_value_t>(                         std::execution::get_env(                             std::execution::schedule(                                 std::forward<Sch>(sch)))))         }->std::same_as<std::remove_cvref_t<Sch>>;     }&&     std::equality_comparable<std::remove_cvref_t<Sch>>&&     std::copy_constructible<std::remove_cvref_t<Sch>>; };	(1)	(since C++26)
Helper tag type
struct scheduler_t {};	(2)	(since C++26)

The concept scheduler is modeled by types that are schedulers, that is, lightweight handlers to execution resources such as thread pools that work with the C++ execution library.

Semantic requirements

Given a scheduler of type Sch and execution environment of type Env such that sender_in<schedule_result_t<Sch>, Env> is satisfied, then /*sender-in-of*/<schedule_result_t<Sch>, Env> is modeled.

The scheduler's copy constructor, destructor, equality comparison, or swap member functions must be non-throwing.

All of those member functions as well as the scheduler type's schedule function must be thread-safe.

Two schedulers are equal only if they represent the same execution resource.

For a given scheduler sch, the expression get_completion_scheduler<set_value_t>(get_env(schedule(sch))) compares equal to sch.

For a given scheduler sch, if the expression get_domain(sch) is well-formed, then the expression get_domain(get_env(schedule(sch))) is also well-formed and has the same type.

The destructor of a scheduler must not block pending completion of any receivers connected to the sender objects returned from schedule (the underlying resource may provide a separate API to wait for completion of submitted function objects)

Examples

simple wrapper for std::execution::run_loop that constantly polls run_loop's queue on a single dedicated thread. Demo using draft reference implementation: https://godbolt.org/z/146fY4Y91

Run this code

#include <execution>#include <iostream>#include <thread>   class single_thread_context { std::execution::run_loop loop_{};std::jthread thread_;   public: single_thread_context(): thread_([this]{ loop_.run();}){} single_thread_context(single_thread_context&&)= delete;   ~single_thread_context(){ loop_.finish();}   std::execution::schedulerauto get_scheduler()noexcept{return loop_.get_scheduler();}};   int main(){ single_thread_context ctx;   std::execution::senderauto snd = std::execution::schedule(ctx.get_scheduler())| std::execution::then([]{std::cout<<"Hello world! Have an int.\n";return015;})| std::execution::then([](int arg){return arg +42;});   auto[i]= std::this_thread::sync_wait(snd).value();   std::cout<<"Back in the main thread, result is "<< i <<'\n';}

Output:

Hello world! Have an int. Back in the main thread, result is 55

See also

execution::schedule (C++26)

prepares a task graph for execution on a given scheduler (customization point object)