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

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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 target shared_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.

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 of future_errc::no_state. — end note]
```
namespace std {
  template <class R>
  class future {
  public:
    […]
    shared_future<R> share() noexcept;
    […]
  };
}
```
[…]
```
shared_future<R> share() noexcept;
```
-12- Returns: shared_future<R>(std::move(*this)).
-13- Postcondition: valid() == false.

[…]

2556. Wide contract for future::share()

2556. Wide contract for `future::share()`