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3795. Self-move-assignment of std::future and std::shared_future have unimplementable postconditions

Section: 32.10.7 [futures.unique.future], 32.10.8 [futures.shared.future] Status: C++23 Submitter: Jiang An Opened: 2022-10-19 Last modified: 2023-11-22

Priority: 3

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Discussion:

The move assignment operators of std::future and std::shared_future have their postconditions specified as below:

Postconditions:

• valid() returns the same value as rhs.valid() returned prior to the assignment.

• rhs.valid() == false.

It can be found that when *this and rhs is the same object and this->valid() is true before the assignment, the postconditions can't be achieved.

Mainstream implementations (libc++, libstdc++, msvc stl) currently implement such self-move-assignment as no-op, which doesn't meet the requirements in the Effects: element. As discussed in LWG 3788⁽ⁱ⁾, I think we should say self-move-assignment has no effects for these types.

[2022-11-01; Reflector poll]

Set priority to 3 after reflector poll.

[2022-11-01; Jonathan provides wording]

[2022-11-07; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 32.10.7 [futures.unique.future] as indicated:

   future& operator=(future&& rhs) noexcept;
   

   -11- Effects: If addressof(rhs) == this is true, there are no effects. Otherwise:

   1. (11.1) — Releases any shared state (32.10.5 [futures.state]).
   2. (11.2) — move assigns the contents of rhs to *this.

   -12- Postconditions:

   1. (12.1) — valid() returns the same value as rhs.valid() prior to the assignment.
   2. (12.2) — If addressof(rhs) == this is false, rhs.valid() == false.

2. Modify 32.10.8 [futures.shared.future] as indicated:

   shared_future& operator=(shared_future&& rhs) noexcept;
   

   -13- Effects: If addressof(rhs) == this is true, there are no effects. Otherwise:

   1. (13.1) — Releases any shared state (32.10.5 [futures.state]).
   2. (13.2) — move assigns the contents of rhs to *this.

   -14- Postconditions:

   1. (14.1) — valid() returns the same value as rhs.valid() returned prior to the assignment.
   2. (14.2) — If addressof(rhs) == this is false, rhs.valid() == false.

   shared_future& operator=(const shared_future& rhs) noexcept;
   

   -15- Effects: If addressof(rhs) == this is true, there are no effects. Otherwise:

   1. (15.1) — Releases any shared state (32.10.5 [futures.state]).
   2. (15.2) — assigns the contents of rhs to *this.

      [Note 3: As a result, *this refers to the same shared state as rhs (if any). — end note]

   -16- Postconditions: valid() == rhs.valid().