Comment on library issue 1050. "Response to UK 340"

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.

Proposed resolution:

Replace p12/p13 30.6.6 [futures.promise]:

Throws: future_error if the future has already been retrieved.

Error conditions: future_already_retrieved if the future associated with
the associated state has already been retrieved.

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

Replace p14 30.6.8 [futures.task]:

Throws: future_error if the future 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.

--
Thomas J. Gritzan

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