[stopcallback]

# 32 Concurrency support library [[thread]](./#thread)

## 32.3 Stop tokens [[thread.stoptoken]](thread.stoptoken#stopcallback)

### 32.3.6 Class template stop_callback [stopcallback]

#### [32.3.6.1](#general) General [[stopcallback.general]](stopcallback.general)

[1](#general-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1039)

[🔗](#lib:stop_callback_)

namespace std {template<class CallbackFn>class stop_callback {public:using callback_type = CallbackFn; // [[stopcallback.cons]](#cons "32.3.6.2 Constructors and destructor"), constructors and destructortemplate<class Initializer>explicit stop_callback(const stop_token& st, Initializer&& init)noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>); template<class Initializer>explicit stop_callback(stop_token&& st, Initializer&& init)noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>); ~stop_callback();

 stop_callback(const stop_callback&) = delete;
 stop_callback(stop_callback&&) = delete;
 stop_callback& operator=(const stop_callback&) = delete;
 stop_callback& operator=(stop_callback&&) = delete; private: CallbackFn *callback-fn*; // *exposition only*}; template<class CallbackFn> stop_callback(stop_token, CallbackFn) -> stop_callback<CallbackFn>;}

[2](#general-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1071)

*Mandates*: stop_callback is instantiated with an argument for the
template parameter CallbackFn that satisfies both [invocable](concept.invocable#concept:invocable "18.7.2 Concept invocable [concept.invocable]") and [destructible](concept.destructible#concept:destructible "18.4.10 Concept destructible [concept.destructible]")[.](#general-2.sentence-1)

[3](#general-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1078)

*Remarks*: For a type Initializer,
if[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]")<CallbackFn, stop_token, Initializer> is satisfied, then[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]")<CallbackFn, stop_token, Initializer> is modeled[.](#general-3.sentence-1)

The exposition-only *callback-fn* member is
the associated callback function ([[stoptoken.concepts]](stoptoken.concepts "32.3.3 Stop token concepts")) ofstop_callback<  
CallbackFn> objects[.](#general-3.sentence-2)

#### [32.3.6.2](#cons) Constructors and destructor [[stopcallback.cons]](stopcallback.cons)

[🔗](#lib:stop_callback,constructor)

`template<class Initializer>
  explicit stop_callback(const stop_token& st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);

template<class Initializer>
  explicit stop_callback(stop_token&& st, Initializer&& init)
    noexcept(is_nothrow_constructible_v<CallbackFn, Initializer>);
`

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1103)

*Constraints*: CallbackFn and Initializer satisfy[constructible_from](concept.constructible#concept:constructible_from "18.4.11 Concept constructible_­from [concept.constructible]")<CallbackFn, Initializer>[.](#cons-1.sentence-1)

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1108)

*Effects*: Initializes *callback-fn* with std​::​forward<Initializer>(init) and executes a stoppable callback registration ([[stoptoken.concepts]](stoptoken.concepts "32.3.3 Stop token concepts"))[.](#cons-2.sentence-1)

If a callback is registered with st's shared stop state,
then *this acquires shared ownership of that stop state[.](#cons-2.sentence-2)

[🔗](#lib:stop_callback,destructor)

`~stop_callback();
`

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L1122)

*Effects*: Executes a stoppable callback deregistration ([[stoptoken.concepts]](stoptoken.concepts "32.3.3 Stop token concepts")) and
releases ownership of the stop state, if any[.](#cons-3.sentence-1)