Issue 2556: Wide contract for future::share() (original) (raw)

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2556. Wide contract for future::share()

Section: 32.10.7 [futures.unique.future] Status: C++17 Submitter: Agustín K-ballo Bergé Opened: 2015-11-05 Last modified: 2021-06-06

Priority: 3

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Discussion:

future::share() is not noexcept, it has a narrow contact requiring valid() as per the blanket wording in [futures.unique_future] p3. Its effects, however, are return shared_future<R>(std::move(*this)), which is noexcept as it has a wide contract. If the source future isn't valid then the targetshared_future simply won't be valid either. There appears to be no technical reason preventing future::share()from having a wide contract, and thus being noexcept.

[2016-08-03 Chicago]

Fri AM: Moved to Tentatively Ready

Proposed resolution:

This wording is relative to N4567.

1. Change [futures.unique_future] as indicated:

   -3- The effect of calling any member function other than the destructor, the move-assignment operator, share,or valid on a future object for which valid() == false is undefined. [Note: Implementations are encouraged to detect this case and throw an object of type future_error with an error condition offuture_errc::no_state. — _end note_]

   namespace std {
   template
   class future {
   public:
   […]
   shared_future share() noexcept;
   […]
   };
   }

   […]

   shared_future share() noexcept;

   -12- Returns: shared_future<R>(std::move(*this)).

   -13- Postcondition: valid() == false.

   […]