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### 3458. Is `shared_future` intended to work with arrays or function types?

Section: 32.9.7 [futures.unique.future], 32.9.8 [futures.shared.future] Status: Resolved Submitter: Tomasz Kamiński Opened: 2020-06-28 Last modified: 2020-11-09

Priority: 0

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Discussion:

Currently there is no prohibition of creating `shared_future<T>` where `T` is either an array type or a function type. However such objects cannot be constructed, as the corresponding `future<T>`, is ill-formed due the signature `get()` method being ill-formed — it will be declared as function returning an array or function type, respectively. [Note: For `shared_future` `get` always returns an reference, so it is well-formed for array or function types.]

[2020-07-17; Reflector prioritization]

Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4861.

> > Ideally the wording below would use a Mandates: element, but due to the still open issue LWG 3193 the wording below uses instead the more general "ill-formed" vocabulary.

> 1. Modify 32.9.7 [futures.unique.future] as indicated:

>    > >     namespace std {
>    > >       template<class R>
>    > >       class future {
>    > >         […]
>    > >       };
>    > >     }

>    > -?- If `is_array_v<R>` is `true` or `is_function_v<R>` is `true`, the program is ill-formed.

>    > -4- The implementation provides the template `future` and two specializations, `future<R&>` and `future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

> 2. Modify 32.9.8 [futures.shared.future] as indicated:

>    > >     namespace std {
>    > >       template<class R>
>    > >       class shared_future {
>    > >         […]
>    > >       };
>    > >     }

>    > -?- If `is_array_v<R>` is `true` or `is_function_v<R>` is `true`, the program is ill-formed.

>    > -4- The implementation provides the template `shared_future` and two specializations, `shared_future<R&>` and `shared_future<void>`. These differ only in the return type and return value of the member function `get`, as set out in its description, below.

[2020-08-02; Daniel comments]

I'm request to reopen the issue and to follow the recent wording update of LWG 3466 instead.

[2020-11-09 Resolved by acceptance of 3466. Status changed: Tentatively Resolved → Resolved.]

Proposed resolution:

Resolved by 3466