34 Execution control library [exec]

34.11 Execution contexts [exec.ctx]

34.11.1 execution::run_loop [exec.run.loop]

34.11.1.1 General [exec.run.loop.general]

A run_loop is an execution resource on which work can be scheduled.

It maintains a thread-safe first-in-first-out queue of work.

Its run member function removes elements from the queue and executes them in a loop on the thread of execution that calls run.

A run_loop instance has an associated count that corresponds to the number of work items that are in its queue.

Additionally, a run_loop instance has an associated state that can be one of starting, running, or finishing.

Concurrent invocations of the member functions of run_loop other than run and its destructor do not introduce data races.

The member functions pop-front, push-back, and finish execute atomically.

Recommended practice: Implementations should use an intrusive queue of operation states to hold the work units to make scheduling allocation-free.

namespace std::execution { class run_loop { // [exec.run.loop.types], associated types class run-loop-scheduler; // exposition only class run-loop-sender; // exposition only struct run-loop-opstate-base { // exposition only virtual void execute() = 0; // exposition only run_loop* loop; // exposition only run-loop-opstate-base* next; // exposition only }; template<class Rcvr> using run-loop-opstate = unspecified; // exposition only // [exec.run.loop.members], member functions run-loop-opstate-base* pop-front(); // exposition only void push-back(run-loop-opstate-base*); // exposition only public: // [exec.run.loop.ctor], constructor and destructor run_loop() noexcept; run_loop(run_loop&&) = delete; ~run_loop(); // [exec.run.loop.members], member functions run-loop-scheduler get_scheduler(); void run(); void finish(); }; }

34.11.1.2 Associated types [exec.run.loop.types]

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class run-loop-scheduler;

run-loop-scheduler is an unspecified type that models scheduler.

Instances of run-loop-scheduler remain valid until the end of the lifetime of the run_loop instance from which they were obtained.

Two instances of run-loop-scheduler compare equal if and only if they were obtained from the same run_loop instance.

Let sch be an expression of type run-loop-scheduler.

The expression schedule(sch) has type run-loop-
sender and is not potentially-throwing if sch is not potentially-throwing.

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class run-loop-sender;

run-loop-sender is an exposition-only type that satisfies sender.

For any type Env, completion_signatures_of_t<run-loop-sender, Env> is completion_signatures<set_value_t(), set_error_t(exception_ptr), set_stopped_t()>

An instance of run-loop-sender remains valid until the end of the lifetime of its associated run_loop instance.

Let sndr be an expression of type run-loop-sender, let rcvr be an expression such that receiver_of<decltype((rcvr)), CS> is true where CS is the completion_signatures specialization above.

Let C be either set_value_t or set_stopped_t.

Then:

(7.1)
The expression connect(sndr, rcvr) has type run-loop-opstate<decay_t<decltype((rcvr))>> and is potentially-throwing if and only if (void(sndr), auto(rcvr)) is potentially-throwing.
(7.2)
The expression get_completion_scheduler<C>(get_env(sndr)) is potentially-throwing if and only if sndr is potentially-throwing, has type run-loop-scheduler, and compares equal to the run-loop-
scheduler instance from which sndr was obtained.

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template<class Rcvr>
  struct run-loop-opstate;

run-loop-opstate<Rcvr> inherits privately and unambiguously from run-loop-opstate-base.

Let o be a non-const lvalue of type run-loop-opstate<Rcvr>, and let REC(o) be a non-const lvalue reference to an instance of type Rcvr that was initialized with the expression rcvr passed to the invocation of connect that returned o.

Then:

(9.1)
The object to which REC(o) refers remains valid for the lifetime of the object to which o refers.
(9.2)
The type run-loop-opstate<Rcvr> overrides run-loop-opstate-base::execute() such that o.execute() is equivalent to: if (get_stop_token(REC(o)).stop_requested()) { set_stopped(std::move(REC(o))); } else { set_value(std::move(REC(o))); }
(9.3)
The expression start(o) is equivalent to: try { o.loop->push-back(addressof(o)); } catch(...) { set_error(std::move(REC(o)), current_exception()); }

34.11.1.3 Constructor and destructor [exec.run.loop.ctor]

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run_loop() noexcept;

Postconditions: count is 0 and state is starting.

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~run_loop();

Effects: If count is not 0 or if state is running, invokes terminate ([except.terminate]).

Otherwise, has no effects.

34.11.1.4 Member functions [exec.run.loop.members]

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run-loop-opstate-base* pop-front();

Effects: Blocks ([defns.block]) until one of the following conditions is true:

(1.1)
count is 0 and state is finishing, in which case pop-front returns nullptr; or
(1.2)
count is greater than 0, in which case an item is removed from the front of the queue, count is decremented by 1, and the removed item is returned.

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void push-back(run-loop-opstate-base* item);

Effects: Adds item to the back of the queue and increments count by 1.

Synchronization: This operation synchronizes with the pop-front operation that obtains item.

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run-loop-scheduler get_scheduler();

Returns: An instance of run-loop-scheduler that can be used to schedule work onto this run_loop instance.

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void run();

Preconditions: state is starting.

Effects: Sets the state to running.

Then, equivalent to: while (auto* op = pop-front()) { op->execute(); }

Remarks: When state changes, it does so without introducing data races.

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void finish();

Effects: Changes state to finishing.

Synchronization: finish synchronizes with the pop-front operation that returns nullptr.