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4287. §[locale.codecvt.virtuals] do_in and do_out could do with better specification

Section: 28.3.4.2.5.3 [locale.codecvt.virtuals] Status: New Submitter: S. B. Tam Opened: 2025-06-18 Last modified: 2025-10-17

Priority: 3

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Discussion:

Background: https://github.com/cplusplus/draft/pull/7347

The specification of codecvt::do_in and codecvt::do_out is unclear, and possibly incorrect:

  1. the meaning of noconv is specified twice (once in paragraph 3, once in Table 91 [tab:locale.codecvt.inout]);

  2. the effect on from_next is not specified;

  3. the specification talks about "the input sequence [from, from_next)", but from_next is supposed to be an out parameter. I think it should say "[from, from_end)" instead.

[2025-10-17; Reflector poll.]

Set priority to 3 after reflector poll.

"first element of the input that was not converted" doesn't make sense if there is no such element; similarly for the "first unchanged element".

Proposed resolution:

This wording is relative to N5008.

[Drafting note: This is modified from Jonathan Wakely's suggestion in https://github.com/cplusplus/draft/pull/7347#issuecomment]

  1. In 28.3.4.2.5.3 [locale.codecvt.virtuals] remove Table 91 [tab:locale.codecvt.inout] in its entirety:

    Table 91 — do_in/do_out result values [tab:locale.codecvt.inout]
    Value Meaning
    ok completed the conversion
    partial not all source characters converted
    error encountered a character in [from, from_end) that cannot be converted
    noconv internT and externT are the same type, and input sequence is identical to converted sequence

  2. Modify 28.3.4.2.5.3 [locale.codecvt.virtuals] as indicated:

    result do_out(
  stateT& state,
  const internT* from, const internT* from_end, const internT*& from_next,
  externT* to, externT* to_end, externT*& to_next) const;

result do_in(
  stateT& state,
  const externT* from, const externT* from_end, const externT*& from_next,
  internT* to, internT* to_end, internT*& to_next) const;

    -1- Preconditions: […]

    -2- Effects: Translates characters in the source range [from, from_end), placing the results in sequential positions starting at destination to. Converts no more than (from_end - from) source elements, and stores no more than (to_end - to) destination elements.

    -3- Stops if it encounters a character it cannot convert. It always leaves the from_next and to_next pointers pointing one beyond the last element successfully converted. If it returns noconv, internT and externT are the same type, and the converted sequence is identical to the input sequence [from, from_next), to_next is set equal to to, the value of state is unchanged, and there are no changes to the values in [to, to_end).

    -4- A codecvt facet that is used by basic_filebuf […]

    -5- Returns: An enumeration value, as summarized in Table 91 .