1106. Multiple exceptions from connected shared_future::get()?

Section: 33.6.8 [futures.shared_future] Status: Resolved Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

It is not clear, if multiple threads are waiting in a shared_future::get() call, if each will rethrow the stored exception.

Paragraph 9 reads:

Throws: the stored exception, if an exception was stored and not retrieved before.

The "not retrieved before" suggests that only one exception is thrown, but one exception for each call to get() is needed, and multiple calls to get() even on the same shared_future object seem to be allowed.

I suggest removing "and not retrieved before" from the Throws paragraph. I recommend adding a note that explains that multiple calls on get() are allowed, and each call would result in an exception if an exception was stored.

[ Batavia (2009-05): ]

We note there is a pending paper by Detlef on such future-related issues; we would like to wait for his paper before proceeding.

Alisdair suggests we may want language to clarify that this get() function can be called from several threads with no need for explicit locking.

Move to Open.

[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]

Rationale:

Resolved by paper N2997.

Proposed resolution:

Change 33.6.8 [futures.shared_future]:

const R& shared_future::get() const; 
R& shared_future<R&>::get() const; 
void shared_future<void>::get() const;

...

-9- Throws: the stored exception, if an exception was stored and not retrieved before. [Note: Multiple calls on get() are allowed, and each call would result in an exception if an exception was stored. — end note]