[thread.stoptoken.intro]

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

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

### 32.3.1 Introduction [thread.stoptoken.intro]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L449)

Subclause [[thread.stoptoken]](thread.stoptoken "32.3 Stop tokens") describes components that can be used
to asynchronously request that an operation stops execution in a timely manner,
typically because the result is no longer required[.](#1.sentence-1)

Such a request is called a [*stop request*](#def:stop_request "32.3.1 Introduction [thread.stoptoken.intro]")[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L455)

The concepts[*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]"),[stoppable_token](stoptoken.concepts#concept:stoppable_token "32.3.3 Stop token concepts [stoptoken.concepts]"), and[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]") specify the required syntax and semantics of
shared access to a [*stop state*](#def:stop_state "32.3.1 Introduction [thread.stoptoken.intro]")[.](#2.sentence-1)

Any object modeling [*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]"),[stoppable_token](stoptoken.concepts#concept:stoppable_token "32.3.3 Stop token concepts [stoptoken.concepts]"), or[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]") that refers to the same stop state is
an [*associated*](#def:associated "32.3.1 Introduction [thread.stoptoken.intro]")[*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]"),[stoppable_token](stoptoken.concepts#concept:stoppable_token "32.3.3 Stop token concepts [stoptoken.concepts]"), or[*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]"),
respectively[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L472)

An object of a type that models [stoppable_token](stoptoken.concepts#concept:stoppable_token "32.3.3 Stop token concepts [stoptoken.concepts]") can be passed to an operation that can either

- [(3.1)](#3.1)

actively poll the token to check if there has been a stop request, or

- [(3.2)](#3.2)

register a callback that
 will be called in the event that a stop request is made[.](#3.sentence-1)

A stop request made via an object
whose type models [*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]") will be visible to
all associated [stoppable_token](stoptoken.concepts#concept:stoppable_token "32.3.3 Stop token concepts [stoptoken.concepts]") and[*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]") objects[.](#3.sentence-2)

Once a stop request has been made it cannot be withdrawn
(a subsequent stop request has no effect)[.](#3.sentence-3)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L488)

Callbacks registered via an object
whose type models [*stoppable-callback-for*](stoptoken.concepts#concept:stoppable-callback-for "32.3.3 Stop token concepts [stoptoken.concepts]") are called when a stop request is first made
by any associated [*stoppable-source*](stoptoken.concepts#concept:stoppable-source "32.3.3 Stop token concepts [stoptoken.concepts]") object[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L494)

The types stop_source and stop_token and
the class template stop_callback implement
the semantics of shared ownership of a stop state[.](#5.sentence-1)

The last remaining owner of the stop state automatically releases
the resources associated with the stop state[.](#5.sentence-2)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/threads.tex#L501)

An object of type inplace_stop_source is the sole owner of its stop state[.](#6.sentence-1)

An object of type inplace_stop_token or
of a specialization of the class template inplace_stop_callback does not participate in ownership of its associated stop state[.](#6.sentence-2)

[*Note [1](#note-1)*:

They are for use when all uses of the associated token and callback objects
are known to nest within the lifetime of the inplace_stop_source object[.](#6.sentence-3)

— *end note*]