Issue 3118 - WG21 LWG Issues
Title
fpos equality comparison unspecified
Status
new
Section
[fpos.operations]
Submitter
Jonathan Wakely

Created on 2018-06-04.00:00:00 last changed 3 weeks ago

Messages

msg12444 (view)
Date: 2022-05-01.17:48:13

Proposed resolution:

This wording is relative to N4910.

  • Modify in [fpos.operations] as indicated:

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

    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)
    …
    • msg12443 (view)
    Date: 2022-05-15.00:00:00

    [ 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 [stream.types], streamoff is a signed basic integral type, so we know what equality means for such values.

    msg9984 (view)
    Date: 2018-06-23.00:00:00

    [ 2018-06-23 after reflector discussion ]

    Priority set to 4
    msg9917 (view)
    Date: 2018-06-04.00:00:00

    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.

    History
    Date User Action Args
    2022-05-01 17:17:39adminsetmessages: + msg12444
    2022-05-01 17:17:39adminsetmessages: + msg12443
    2018-06-25 00:47:25adminsetmessages: + msg9984
    2018-06-04 00:00:00admincreate