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

Section: 33.10.7 [futures.unique.future], 33.10.8 [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Previous resolution [SUPERSEDED]: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 33.10.7 [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 33.10.8 [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