2098. Minor Inconsistency between promise::set_value and promise::set_value_at_thread_exit

Section: 32.10.6 [futures.promise] Status: C++14 Submitter: Pete Becker Opened: 2011-11-14 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

32.10.6 [futures.promise]/16 says that promise::set_value(const R&) throws any exceptions thrown by R's copy constructor, and that promise_set_value(R&&) throws any exceptions thrown by R's move constructor.

32.10.6 [futures.promise]/22 is the Throws: clause for promise::set_value_at_thread_exit. It has no corresponding requirements, only that these functions throw "future_error if an error condition occurs."

Daniel suggests wording to fix this: The approach is a bit more ambitious and also attempts to fix wording glitches of 32.10.6 [futures.promise]/16, because it would be beyond acceptable efforts of implementations to determine whether a constructor call of a user-defined type will indeed call a copy constructor or move constructor (in the first case it might be a template constructor, in the second case it might also be a copy-constructor, if the type has no move constructor).

[2012, Portland: move to Review]

Moved to Review by the concurrency working group, with no further comments.

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to the FDIS.

1. Change 32.10.6 [futures.promise]/16 as indicated:

   void promise::set_value(const R& r);
   void promise::set_value(R&& r);
   void promise<R&>::set_value(R& r);
   void promise<void>::set_value();
   

   […]

   -16- Throws:

   • future_error if its shared state already has a stored value or exception, or
   • for the first version, any exception thrown by the copy constructor ofconstructor selected to copy an object of R, or
   • for the second version, any exception thrown by the move constructor ofconstructor selected to move an object of R.

2. Change 32.10.6 [futures.promise]/22 as indicated:

   void promise::set_value_at_thread_exit(const R& r);
   void promise::set_value_at_thread_exit(R&& r);
   void promise<R&>::set_value_at_thread_exit(R& r);
   void promise<void>::set_value_at_thread_exit();
   

   […]

   -16- Throws: future_error if an error condition occurs.

   • future_error if its shared state already has a stored value or exception, or
   • for the first version, any exception thrown by the constructor selected to copy an object of R, or
   • for the second version, any exception thrown by the constructor selected to move an object of R.