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### 1050. Clarify postconditions for `get_future()`

Section: 32.9.6 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 340 [CD1]

There is an implied postcondition for `get_future()` that the state of the `promise` is transferred into the `future` leaving the `promise` with no associated state. It should be spelled out.

[ Summit: ]

> Agree, move to Review.

[ 2009-04-03 Thomas J. Gritzan adds: ]

> `promise::get_future()` must not invalidate the state of the promise object.

> A promise is used like this:

> >     promise<int> p;
> >     unique_future<int> f = p.get_future();
> >     // post 'p' to a thread that calculates a value
> >     // use 'f' to retrieve the value.

> So `get_future()` must return an object that shares the same associated state with `*this`.

> But still, this function should throw an `future_already_retrieved` error when it is called twice.

> `packaged_task::get_future()` throws `std::bad_function_call` if its `future` was already retrieved. It should throw `future_error(future_already_retrieved)`, too.

> Suggested resolution:

> Replace p12/p13 32.9.6 [futures.promise]:

> > -12- Throws: `future_error` if `*this` has no associated state the `future` has already been retrieved.

> > -13- Error conditions: `future_already_retrieved` if `*this` has no associated state the `future` associated with the associated state has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated state.

> Replace p14 32.9.10 [futures.task]:

> > -14- Throws: `std::bad_function_call future_error` if the future associated with the task has already been retrieved.

> > Error conditions: `future_already_retrieved` if the `future` associated with the task has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated task.

[ Batavia (2009-05): ]

> Keep in Review status pending Detlef's forthcoming paper on futures.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

Add after p13 32.9.6 [futures.promise]:

>     unique_future<R> get_future();

> > -13- ...

> > Postcondition: `*this` has no associated state.