[exec.counting.scopes]

# 33 Execution control library [[exec]](./#exec)

## 33.14 Execution scope utilities [[exec.scope]](exec.scope#exec.counting.scopes)

### 33.14.2 Counting Scopes [exec.counting.scopes]

#### [33.14.2.1](#general) General [[exec.counting.scopes.general]](exec.counting.scopes.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7769)

Scopes of type simple_counting_scope and counting_scope maintain counts of associations[.](#general-1.sentence-1)

Let:

- [(1.1)](#general-1.1)

Scope be either simple_counting_scope or counting_scope,

- [(1.2)](#general-1.2)

scope be an object of type Scope,

- [(1.3)](#general-1.3)

tkn be an object of type Scope​::​token obtained from scope.get_token(),

- [(1.4)](#general-1.4)

jsndr be a sender obtained from scope.join(), and

- [(1.5)](#general-1.5)

op be an operation state obtained from
connecting jsndr to a receiver[.](#general-1.sentence-2)

During its lifetime scope goes through different states
which govern what operations are allowed and the result of these operations:

- [(1.6)](#general-1.6)

  *unused*:
a newly constructed object starts in the *unused* state[.](#general-1.6.sentence-1)

- [(1.7)](#general-1.7)

  *open*:
when tkn.try_associate() is called
while scope is in the *unused* state,scope moves to the *open* state[.](#general-1.7.sentence-1)

- [(1.8)](#general-1.8)

  *open-and-joining*:
when the operation state op is started
while scope is in the *unused* or *open* state,scope moves to the *open-and-joining* state[.](#general-1.8.sentence-1)

- [(1.9)](#general-1.9)

  *closed*:
when scope.close() is called
while scope is in the *open* state,scope moves to the *closed* state[.](#general-1.9.sentence-1)

- [(1.10)](#general-1.10)

  *unused-and-closed*:
when scope.close() is called
while scope is in the *unused* state,scope moves to the *unused-and-closed* state[.](#general-1.10.sentence-1)

- [(1.11)](#general-1.11)

  *closed-and-joining*:
when scope.close() is called
while scope is in the *open-and-joining* state or
the operation state op is started
while scope is in
the *closed* or *unused-and-closed* state,scope moves to the *closed-and-joining* state[.](#general-1.11.sentence-1)

- [(1.12)](#general-1.12)

  *joined*:
when the count of associations drops to zero
while scope is in
the *open-and-joining* or *closed-and-joining* state,scope moves to the *joined* state[.](#general-1.12.sentence-1)

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7836)

*Recommended practice*: For simple_counting_scope and counting_scope,
implementations should store the state and the count of associations
in a single member of type size_t[.](#general-2.sentence-1)

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7842)

Subclause [exec.counting.scopes] makes use of
the following exposition-only entities:

struct *scope-join-t* {}; // *exposition only*enum *scope-state-type* { // *exposition only**unused*, // *exposition only**open*, // *exposition only**closed*, // *exposition only**open-and-joining*, // *exposition only**closed-and-joining*, // *exposition only**unused-and-closed*, // *exposition only**joined*, // *exposition only*};

[4](#general-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7860)

The exposition-only class template *impls-for* ([[exec.snd.expos]](exec.snd.expos "33.9.2 Exposition-only entities"))
is specialized for *scope-join-t* as follows:

namespace std::execution {template<>struct *impls-for*<*scope-join-t*> : *default-impls* {template<class Scope, class Rcvr>struct *state* { // *exposition only*struct *rcvr-t* { // *exposition only*using receiver_concept = receiver_t;

 Rcvr& *rcvr*; // *exposition only*void set_value() && noexcept { execution::set_value(std::move(*rcvr*)); }template<class E>void set_error(E&& e) && noexcept { execution::set_error(std::move(*rcvr*), std::forward<E>(e)); }void set_stopped() && noexcept { execution::set_stopped(std::move(*rcvr*)); }decltype(auto) get_env() const noexcept {return execution::get_env(*rcvr*); }}; using *sched-sender* = // *exposition only*decltype(schedule(get_scheduler(get_env(declval<Rcvr&>())))); using *op-t* = // *exposition only* connect_result_t<*sched-sender*, *rcvr-t*>;

 Scope* *scope*; // *exposition only* Rcvr& *receiver*; // *exposition only**op-t* *op*; // *exposition only**state*(Scope* scope, Rcvr& rcvr) // *exposition only*noexcept([*nothrow-callable*](functional.syn#concept:nothrow-callable "22.10.2 Header <functional> synopsis [functional.syn]")<connect_t, *sched-sender*, *rcvr-t*>): *scope*(scope), *receiver*(rcvr), *op*(connect(schedule(get_scheduler(get_env(rcvr))), *rcvr-t*(rcvr))) {}void *complete*() noexcept { // *exposition only* start(*op*); }void *complete-inline*() noexcept { // *exposition only* set_value(std::move(*receiver*)); }}; static constexpr auto *get-state* = // *exposition only*[]<class Rcvr>(auto&& sender, Rcvr& receiver)noexcept(is_nothrow_constructible_v<*state*<Rcvr>, *data-type*<decltype(sender)>, Rcvr&>) {auto[_, self] = sender; return *state*(self, receiver); }; static constexpr auto *start* = // *exposition only*[](auto& s, auto&) noexcept {if (s.*scope*->*start-join-sender*(s)) s.*complete-inline*(); }; };}

#### [33.14.2.2](#exec.scope.simple.counting) Simple Counting Scope [[exec.scope.simple.counting]](exec.scope.simple.counting)

#### [33.14.2.2.1](#exec.scope.simple.counting.general) General [[exec.scope.simple.counting.general]](exec.scope.simple.counting.general)

[ð](#lib:execution::simple_counting_scope)

namespace std::execution {class simple_counting_scope {public:// [[exec.simple.counting.token]](#exec.simple.counting.token "33.14.2.2.4 Token"), tokenstruct token; static constexpr size_t max_associations = *implementation-defined*; // [[exec.simple.counting.ctor]](#exec.simple.counting.ctor "33.14.2.2.2 Constructor and Destructor"), constructor and destructor simple_counting_scope() noexcept;
 simple_counting_scope(simple_counting_scope&&) = delete; ~simple_counting_scope(); // [[exec.simple.counting.mem]](#exec.simple.counting.mem "33.14.2.2.3 Members"), members token get_token() noexcept; void close() noexcept; [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") 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*};}

[1](#exec.scope.simple.counting.general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7971)

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]](intro.multithread "6.10.2 Multi-threaded executions and data races"))[.](#exec.scope.simple.counting.general-1.sentence-1)

These operations on a single object of
type simple_counting_scope appear to occur in a single total order[.](#exec.scope.simple.counting.general-1.sentence-2)

#### [33.14.2.2.2](#exec.simple.counting.ctor) Constructor and Destructor [[exec.simple.counting.ctor]](exec.simple.counting.ctor)

[ð](#lib:execution::simple_counting_scope,constructor)

`simple_counting_scope() noexcept;
`

[1](#exec.simple.counting.ctor-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L7991)

*Postconditions*: *count* is 0 and *state* is *unused*[.](#exec.simple.counting.ctor-1.sentence-1)

[ð](#lib:execution::simple_counting_scope,destructor)

`~simple_counting_scope();
`

[2](#exec.simple.counting.ctor-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8002)

*Effects*: If *state* is not one of*joined*, *unused*, or *unused-and-closed*,
invokes terminate ([[except.terminate]](except.terminate "14.6.2 The std​::​terminate function"))[.](#exec.simple.counting.ctor-2.sentence-1)

Otherwise, has no effects[.](#exec.simple.counting.ctor-2.sentence-2)

#### [33.14.2.2.3](#exec.simple.counting.mem) Members [[exec.simple.counting.mem]](exec.simple.counting.mem)

[ð](#lib:get_token,execution::simple_counting_scope)

`token get_token() noexcept;
`

[1](#exec.simple.counting.mem-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8018)

*Returns*: An object t of type simple_counting_scope​::​token such thatt.*scope* == this is true[.](#exec.simple.counting.mem-1.sentence-1)

[ð](#lib:close,execution::simple_counting_scope)

`void close() noexcept;
`

[2](#exec.simple.counting.mem-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8030)

*Effects*: If *state* is

- [(2.1)](#exec.simple.counting.mem-2.1)

*unused*, then changes *state* to *unused-and-closed*;

- [(2.2)](#exec.simple.counting.mem-2.2)

*open*, then changes *state* to *closed*;

- [(2.3)](#exec.simple.counting.mem-2.3)

*open-and-joining*,
then changes *state* to *closed-and-joining*;

- [(2.4)](#exec.simple.counting.mem-2.4)

otherwise, no effects[.](#exec.simple.counting.mem-2.sentence-1)

[3](#exec.simple.counting.mem-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8045)

*Postconditions*: Any subsequent call to *try-associate*() on *this returns false[.](#exec.simple.counting.mem-3.sentence-1)

[ð](#lib:join,execution::simple_counting_scope)

`[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") auto join() noexcept;
`

[4](#exec.simple.counting.mem-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8057)

*Returns*: *make-sender*(*scope-join-t*(), this)[.](#exec.simple.counting.mem-4.sentence-1)

[ð](#lib:try-associate,execution::simple_counting_scope)

`bool try-associate() noexcept;
`

[5](#exec.simple.counting.mem-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8068)

*Effects*: If *count* is equal to max_associations, then no effects[.](#exec.simple.counting.mem-5.sentence-1)

Otherwise, if *state* is

- [(5.1)](#exec.simple.counting.mem-5.1)

*unused*,
then increments *count* and changes *state* to *open*;

- [(5.2)](#exec.simple.counting.mem-5.2)

*open* or *open-and-joining*,
then increments *count*;

- [(5.3)](#exec.simple.counting.mem-5.3)

otherwise, no effects[.](#exec.simple.counting.mem-5.sentence-2)

[6](#exec.simple.counting.mem-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8083)

*Returns*: true if *count* was incremented, false otherwise[.](#exec.simple.counting.mem-6.sentence-1)

[ð](#lib:disassociate,execution::simple_counting_scope)

`void disassociate() noexcept;
`

[7](#exec.simple.counting.mem-7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8094)

*Preconditions*: *count* is greater than zero[.](#exec.simple.counting.mem-7.sentence-1)

[8](#exec.simple.counting.mem-8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8098)

*Effects*: Decrements *count*[.](#exec.simple.counting.mem-8.sentence-1)

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[.](#exec.simple.counting.mem-8.sentence-2)

[*Note [1](#exec.simple.counting.mem-note-1)*:

Calling *complete*() on any registered object
can cause *this to be destroyed[.](#exec.simple.counting.mem-8.sentence-3)

â *end note*]

[ð](#lib:start-join-sender,execution::simple_counting_scope)

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

[9](#exec.simple.counting.mem-9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8118)

*Effects*: If *state* is

- [(9.1)](#exec.simple.counting.mem-9.1)

*unused*, *unused-and-closed*, or *joined*, then
changes *state* to *joined* and returns true;

- [(9.2)](#exec.simple.counting.mem-9.2)

*open* or *open-and-joining*, then
changes *state* to *open-and-joining*,
registers st with *this and returns false;

- [(9.3)](#exec.simple.counting.mem-9.3)

*closed* or *closed-and-joining*, then
changes *state* to *closed-and-joining*,
registers st with *this and returns false[.](#exec.simple.counting.mem-9.sentence-1)

#### [33.14.2.2.4](#exec.simple.counting.token) Token [[exec.simple.counting.token]](exec.simple.counting.token)

[ð](#lib:execution::simple_counting_scope::token)

namespace std::execution {struct simple_counting_scope::token {template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sender> Sender&& wrap(Sender&& snd) const noexcept; bool try_associate() const noexcept; void disassociate() const noexcept; private: simple_counting_scope* *scope*; // *exposition only*};}

[ð](#lib:wrap,execution::simple_counting_scope::token)

`template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sender>
  Sender&& wrap(Sender&& snd) const noexcept;
`

[1](#exec.simple.counting.token-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8160)

*Returns*: std​::​forward<Sender>(snd)[.](#exec.simple.counting.token-1.sentence-1)

[ð](#lib:try_associate,execution::simple_counting_scope::token)

`bool try_associate() const noexcept;
`

[2](#exec.simple.counting.token-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8171)

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

[ð](#lib:disassociate,execution::simple_counting_scope::token)

`void disassociate() const noexcept;
`

[3](#exec.simple.counting.token-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8182)

*Effects*: Equivalent to *scope*->*disassociate*()[.](#exec.simple.counting.token-3.sentence-1)

#### [33.14.2.3](#exec.scope.counting) Counting Scope [[exec.scope.counting]](exec.scope.counting)

[ð](#lib:execution::counting_scope)

namespace std::execution {class counting_scope {public:struct token {template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sender>[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") auto wrap(Sender&& snd) const noexcept(*see below*); bool try_associate() const noexcept; void disassociate() const noexcept; private: counting_scope* *scope*; // *exposition only*}; static constexpr size_t max_associations = *implementation-defined*;

 counting_scope() noexcept;
 counting_scope(counting_scope&&) = delete; ~counting_scope();

 token get_token() noexcept; void close() noexcept; [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") auto join() noexcept; void request_stop() noexcept; private: size_t *count*; // *exposition only**scope-state-type* *state*; // *exposition only* inplace_stop_source *s_source*; // *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*};}

[1](#exec.scope.counting-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8230)

counting_scope differs from simple_counting_scope by
adding support for cancellation[.](#exec.scope.counting-1.sentence-1)

Unless specified below, the semantics of members of counting_scope are the same as the corresponding members of simple_counting_scope[.](#exec.scope.counting-1.sentence-2)

[ð](#lib:get_token,execution::counting_scope)

`token get_token() noexcept;
`

[2](#exec.scope.counting-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8242)

*Returns*: An object t of type counting_scope​::​token such thatt.*scope* == this is true[.](#exec.scope.counting-2.sentence-1)

[ð](#lib:request_stop,execution::counting_scope)

`void request_stop() noexcept;
`

[3](#exec.scope.counting-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8254)

*Effects*: Equivalent to *s_source*.request_stop()[.](#exec.scope.counting-3.sentence-1)

[4](#exec.scope.counting-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8258)

*Remarks*: Calls to request_stop do not introduce data races[.](#exec.scope.counting-4.sentence-1)

[ð](#lib:wrap,execution::counting_scope::token)

`template<[sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") Sender>
  [sender](exec.snd.concepts#concept:sender "33.9.3 Sender concepts [exec.snd.concepts]") auto counting_scope::token::wrap(Sender&& snd) const
    noexcept(is_nothrow_constructible_v<remove_cvref_t<Sender>, Sender>);
`

[5](#exec.scope.counting-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/exec.tex#L8271)

*Effects*: Equivalent to:return *stop-when*(std::forward<Sender>(snd), *scope*->*s_source*.get_token());
