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4485. Move specification for task::stop_token_type

Section: 33.13.6.4 [task.state] Status: New Submitter: Tomasz Kamiński Opened: 2025-12-08 Last modified: 2025-12-12

Priority: Not Prioritized

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Discussion:

It is not clear what bullet [ask.stat] p4.6 is – it reads like a requirement on stop_token_type, but if so, this paragraph is a poor place for it.

[2025-12-05 Tomasz comments]

The paragraph expresses requirements on user supplied Environment::stop_source_type, that needs to model stoppable-source, which includes stoppable-callback-for on associated token. We should also require that Environment::scheduler_type shall satisfy scheduler.

We also need to clarify that stop_possible and stop_requested on prom.token returns same value as st during lifetime of asynchronous operation.

Proposed resolution:

This wording is relative to N5014.

1. Modify 33.13.6 [exec.task] as indicated:

   -5- allocator_type shall meet the Cpp17Allocator requirements, scheduler_type shall model scheduler, and stop_source_type shall model stoppable-source.

2. Modify 33.13.6.4 [task.state] as indicated:

   void start() & noexcept;
   

   -4- Effects: Effects: Let prom be the object handle.promise(). Associates STATE(prom), RCVR(prom), and SCHED(prom) with *this as follows:

   • -4.1- […]
   • -4.2- […]
   • -4.3- […]

   Let st be get_stop_token(get_env(rcvr)). Initializes prom.token and prom.source such that during lifetime of the asynchronous operation (33.3 [exec.async.ops]) associated with *this

   • -4.4- prom.token.stop_requested() returns st.stop_requested();
   • -4.5- prom.token.stop_possible() returns st.stop_possible();.
   • -4.6- for types Fn and Init such that both invocable<Fn> and constructible_from<Fn, Init> are modeled, stop_token_type::callback_type<Fn> models stoppable-callback-for<Fn, stop_token_type, Init>.