1163. shared_future is inconsistent with shared_ptr

Section: 33.6.8 [futures.shared_future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2016-02-10

Priority: Not Prioritized

View all other issues in [futures.shared_future].

View all issues with Resolved status.

Discussion:

Addresses UK 338

Description

shared_future is currently CopyConstructible, but not CopyAssignable. This is inconsistent with shared_ptr, and will surprise users. Users will then write work-arounds to provide this behaviour. We should provide it simply and efficiently as part of shared_future. Note that since the shared_future member functions for accessing the state are all declared const, the original usage of an immutable shared_future value that can be freely copied by multiple threads can be retained by declaring such an instance as "const shared_future".

Suggestion

Remove "=delete" from the copy-assignment operator of shared_future. Add a move-constructor shared_future(shared_future&& rhs), and a move-assignment operator shared_future& operator=(shared_future&& rhs). The postcondition for the copy-assignment operator is that *this has the same associated state as rhs. The postcondition for the move-constructor and move assignment is that *this has the same associated as rhs had before the constructor/assignment call and that rhs has no associated state.

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

NAD EditorialResolved. Adressed by N2997.

Proposed resolution: