33 Execution control library [exec]

33.14 Execution scope utilities [exec.scope]

33.14.2 Counting Scopes [exec.counting.scopes]

33.14.2.2 Simple Counting Scope [exec.scope.simple.counting]

33.14.2.2.1 General [exec.scope.simple.counting.general]

namespace std::execution { class simple_counting_scope { public: // [exec.simple.counting.token], token struct token; static constexpr size_t max_associations = implementation-defined; // [exec.simple.counting.ctor], constructor and destructor simple_counting_scope() noexcept; simple_counting_scope(simple_counting_scope&&) = delete; ~simple_counting_scope(); // [exec.simple.counting.mem], members token get_token() noexcept; void close() noexcept; sender auto join() noexcept; private: size_t count; // exposition only scope-state-type state; // exposition only bool try-associate() noexcept; // exposition only void disassociate() noexcept; // exposition only template<class State> bool start-join-sender(State& state) noexcept; // exposition only }; }

For purposes of determining the existence of a data race, get_token, close, join, try-associate, disassociate, and start-join-sender behave as atomic operations ([intro.multithread]).

These operations on a single object of type simple_counting_scope appear to occur in a single total order.

33.14.2.2.2 Constructor and Destructor [exec.simple.counting.ctor]

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simple_counting_scope() noexcept;

Postconditions: count is 0 and state is unused.

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~simple_counting_scope();

Effects: If state is not one of joined, unused, or unused-and-closed, invokes terminate ([except.terminate]).

Otherwise, has no effects.

33.14.2.2.3 Members [exec.simple.counting.mem]

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token get_token() noexcept;

Returns: An object t of type simple_counting_scope::token such that t.scope == this is true.

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void close() noexcept;

Effects: If state is

(2.1)
unused, then changes state to unused-and-closed;
(2.2)
open, then changes state to closed;
(2.3)
open-and-joining, then changes state to closed-and-joining;
(2.4)
otherwise, no effects.

Postconditions: Any subsequent call to try-associate() on *this returns false.

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sender auto join() noexcept;

Returns: make-sender(scope-join-t(), this).

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bool try-associate() noexcept;

Effects: If count is equal to max_associations, then no effects.

Otherwise, if state is

(5.1)
unused, then increments count and changes state to open;
(5.2)
open or open-and-joining, then increments count;
(5.3)
otherwise, no effects.

Returns: true if count was incremented, false otherwise.

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void disassociate() noexcept;

Preconditions: count is greater than zero.

Effects: Decrements count.

If count is zero after decrementing and state is open-and-joining or closed-and-joining, changes state to joined and calls complete() on all objects registered with *this.

[Note 1:

Calling complete() on any registered object can cause *this to be destroyed.

— end note]

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template<class State>
  bool start-join-sender(State& st) noexcept;

Effects: If state is

(9.1)
unused, unused-and-closed, or joined, then changes state to joined and returns true;
(9.2)
open or open-and-joining, then changes state to open-and-joining, registers st with *this and returns false;
(9.3)
closed or closed-and-joining, then changes state to closed-and-joining, registers st with *this and returns false.

33.14.2.2.4 Token [exec.simple.counting.token]

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namespace std::execution { struct simple_counting_scope::token { template<sender Sender> Sender&& wrap(Sender&& snd) const noexcept; bool try_associate() const noexcept; void disassociate() const noexcept; private: simple_counting_scope* scope; // exposition only }; }

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template<sender Sender>
  Sender&& wrap(Sender&& snd) const noexcept;

Returns: std::forward<Sender>(snd).

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bool try_associate() const noexcept;

Effects: Equivalent to: return scope->try-associate();

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void disassociate() const noexcept;

Effects: Equivalent to scope->disassociate().