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3118. fpos equality comparison unspecified

Section: 31.5.3.3 [fpos.operations] Status: C++23 Submitter: Jonathan Wakely Opened: 2018-06-04 Last modified: 2023-11-22

Priority: 4

View all other issues in [fpos.operations].

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Discussion:

The fpos requirements do not give any idea what is compared by operator== (even after Daniel's P0759R1 paper). I'd like something to make it clear that return true; is not a valid implementation of operator==(const fpos<T>&, const fpos<T>&). Maybe in the P(o) row state that "p == P(o)" and "p != P(o + 1)", i.e. two fpos objects constructed from the same streamoff values are equal, and two fpos objects constructed from two different streamoff values are not equal.

[2018-06-23 after reflector discussion]

Priority set to 4

[2022-05-01; Daniel comments and provides wording]

The proposed wording does intentionally not use a form involving addition or subtraction to prevent the need for extra wording that ensures that this computed value is well-defined. According to 31.2.2 [stream.types], streamoff is a signed basic integral type, so we know what equality means for such values.

Previous resolution [SUPERSEDED]:

This wording is relative to N4910.

  • Modify in 31.5.3.3 [fpos.operations] as indicated:

    [Drafting note: The return type specification of operator== should be resolved in sync with D2167R2; see also LWG 2114(i).]

    1. (1.1) — […]

    2. […]

    3. (1.5) — o and o2 refers to a values of type streamoff or const streamoff.

    Table 119: Position type requirements [tab:fpos.operations]
    Expression Return type Operational
    semantics    		 Assertion/note
    pre-/post-condition
    …
    P p(o);
    P p = o;    		 Effects: Value-initializes the
    state object.
    Postconditions: p == P(o) is true.
    …
    O(p) streamoff converts to offset P(O(p)) == p
    p == q convertible to bool Remarks: For any two values o and o2, if
    p is obtained from o converted to P or from a copy
    of such P value and if q is obtained from o2
    converted to P or from a copy of such P value, then
    bool(p == q) is true only if o == o2 is true.
    p != q convertible to bool !(p == q)
    …

    • [2022-11-02; Daniel comments and improves wording]

    LWG discussion of P2167R2 has shown preference to require that the equality operations of fpos should be specified to have type bool instead of being specified as "convertible to bool". This has been reflected in the most recent paper revision P2167R3. The below wording changes follow that direction to reduce the wording mismatch to a minimum.

    [2022-11-02; LWG telecon]

    Moved to Ready. For: 6, Against: 0, Neutral: 0

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

    Proposed resolution:

    This wording is relative to N4917.

  • Modify in 31.5.3.3 [fpos.operations] as indicated:

    1. (1.1) — […]

    2. […]

    3. (1.5) — o and o2 refers to a values of type streamoff or const streamoff.

    Table 124: Position type requirements [tab:fpos.operations]
    Expression Return type Operational
    semantics    		 Assertion/note
    pre-/post-condition
    …
    P p(o);
    P p = o;    		 Effects: Value-initializes the
    state object.
    Postconditions: p == P(o) is true.
    …
    O(p) streamoff converts to offset P(O(p)) == p
    p == q bool Remarks: For any two values o and o2, if
    p is obtained from o converted to P or from a copy
    of such P value and if q is obtained from o2
    converted to P or from a copy of such P value, then
    p == q is true only if o == o2 is true.
    p != q convertible to bool !(p == q)
    …