Title
thread::id spaceship may be inconsistent with equality
Status
new
Section
[thread.thread.id]
Submitter
Casey Carter

Created on 2021-01-26.00:00:00 last changed 1 month ago

Messages

Date: 2021-03-12.15:07:58

Proposed resolution:

This wording is relative to N4878.

  1. Modify [thread.thread.id] as indicated:

    strong_ordering operator<=>(thread::id x, thread::id y) noexcept;
    

    -6- Let P(x, y) be an unspecifieda total ordering over thread::id as described in [alg.sorting], unspecified except that P(x, y) is false when x and y both represent the same thread of execution, or when neither represents a thread of execution.

    -7- Returns: strong_ordering::less if P(x, y) is true. Otherwise, strong_ordering::greater if P(y, x) is true. Otherwise, strong_ordering::equal.

Date: 2021-03-15.00:00:00

[ 2021-03-12; Reflector poll ]

Set priority to 3 following reflector poll.

Date: 2021-01-26.00:00:00

[thread.thread.id]/5-7 specify the behavior of == and <=> for std::thread::id:

bool operator==(thread::id x, thread::id y) noexcept;

-5- Returns: true only if x and y represent the same thread of execution or neither x nor y represents a thread of execution.

strong_ordering operator<=>(thread::id x, thread::id y) noexcept;

-6- Let P(x, y) be an unspecified total ordering over thread::id as described in 25.8.

-7- Returns: strong_ordering::less if P(x, y) is true. Otherwise, strong_ordering::greater if P(y, x) is true. Otherwise, strong_ordering::equal.

"Unspecified total ordering" provides too much freedom, since it does not require that !P(x, y) holds when x and y both represent the same thread of execution or both represent no thread of execution. A conforming implementation could return strong_ordering::equal from <=> for a pair of thread::id values for which == returns false. We should guarantee consistency of == and <=> for thread::id to preserve sanity of the programming model.

History
Date User Action Args
2021-03-12 15:07:58adminsetmessages: + msg11726
2021-01-31 13:40:19adminsetmessages: + msg11670
2021-01-26 00:00:00admincreate