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3767. codecvt<charN_t, char8_t, mbstate_t> incorrectly added to locale

Section: 30.3.1.2.1 [locale.category], 30.4.2.5.1 [locale.codecvt.general] Status: Open Submitter: Victor Zverovich Opened: 2022-09-05 Last modified: 2023-02-19

Priority: 3

View all other issues in [locale.category].

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Discussion:

Table [tab:locale.category.facets] includes the following two facets:

However, neither of those actually has anything to do with a locale and therefore it doesn't make sense to dynamically register them with std::locale. Instead they provide conversions between fixed encodings (UTF-8, UTF-16, UTF-32) that are unrelated to locale encodings other than they may happen to coincide with encodings of some locales by accident.

The issue was introduced when adding codecvt<char[16|32]_t, char, mbstate_t> in N2035 which gave no design rationale for using codecvt in the first place. Likely it was trying to do a minimal amount of changes and copied the wording for codecvt<wchar_t, char, mbstate_t> but unfortunately didn't consider encoding implications.

P0482 changed char to char8_t in these facets which made the issue more glaring but unfortunately, despite the breaking change, it failed to address it.

Apart from an obvious design mistake this also adds a small overhead for every locale construction because the implementation has to copy these pseudo-facets for no good reason violating "don't pay for what you don't use" principle.

A simple fix is to remove the two facets from table [tab:locale.category.facets] and make them directly constructible.

[2022-09-23; Reflector poll]

Set priority to 3 after reflector poll. Send to SG16 (then maybe LEWG).

[2022-09-28; SG16 responds]

SG16 agrees that the codecvt facets mentioned in LWG3767 "codecvt<charN_t, char8_t, mbstate_t> incorrectly added to locale" are intended to be invariant with respect to locale. Unanimously in favor.

[Issaquah 2023-02-10; LWG issue processing]

Removing these breaks most code using them today, because the most obvious way to use them is via use_facet on a locale, which would throw if they're removed (and because they were guaranteed to be present, code using them might have not bothered to check for them using has_facet). Instead of removing them, deprecate the guarantee that they're always present (so move them to D.28 [depr.locale.category]). Don't bother changing the destructor. Victor to update wording.

Previous resolution [SUPERSEDED]:

This wording is relative to N4917.

  1. Modify 30.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — as indicated:

    Table 105: Locale category facets [tab:locale.category.facets]
    Category Includes facets
    …
    ctype ctype<char>, ctype<wchar_t>
    codecvt<char, char, mbstate_t>
    codecvt<char16_t, char8_t, mbstate_t>
    codecvt<char32_t, char8_t, mbstate_t>
    codecvt<wchar_t, char, mbstate_t>
    …

  2. Modify 30.4.2.5.1 [locale.codecvt.general] as indicated:

    namespace std {
  […]
  template<class internT, class externT, class stateT>
    class codecvt : public locale::facet, public codecvt_base {
    public:
      using intern_type = internT;
      using extern_type = externT;
      using state_type = stateT;

      explicit codecvt(size_t refs = 0);
      ~codecvt();

      […]
    protected:
      ~codecvt();
      […]
    };
}

    […]

    -3- The specializations required in Table 105 [tab:locale.category.facets]106 [tab:locale.spec] (30.3.1.2.1 [locale.category]) convert the implementation-defined native character set. codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all. The specialization codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms. codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters. Specializations on mbstate_t perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined stateT type. Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.

[2023-02-10; Victor Zverovich comments and provides improved wording]

Per today's LWG discussion the following changes have been implemented in revised wording:

Proposed resolution:

This wording is relative to N4928.

  1. Modify 30.3.1.2.1 [locale.category], Table 105 ([tab:locale.category.facets]) — "Locale category facets" — and Table 106 ([tab:locale.spec]) "Required specializations" as indicated:

    Table 105: Locale category facets [tab:locale.category.facets]
    Category Includes facets
    …
    ctype ctype<char>, ctype<wchar_t>
    codecvt<char, char, mbstate_t>
    codecvt<char16_t, char8_t, mbstate_t>
    codecvt<char32_t, char8_t, mbstate_t>
    codecvt<wchar_t, char, mbstate_t>
    …
    […]
    Table 106: Required specializations [tab:locale.spec]
    Category Includes facets
    …
    ctype ctype_byname<char>, ctype_byname<wchar_t>
    codecvt_byname<char, char, mbstate_t>
    codecvt_byname<char16_t, char8_t, mbstate_t>
    codecvt_byname<char32_t, char8_t, mbstate_t>
    codecvt_byname<wchar_t, char, mbstate_t>
    …

  2. Modify 30.4.2.5.1 [locale.codecvt.general] as indicated:

    […]

    -3- The specializations required in Table 105 (30.3.1.2.1 [locale.category]) convert the implementation-defined native character set. codecvt<char, char, mbstate_t> implements a degenerate conversion; it does not convert at all. The specialization codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specialization codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms. codecvt<wchar_t, char, mbstate_t> converts between the native character sets for ordinary and wide characters. Specializations on mbstate_t perform conversion between encodings known to the library implementer. Other encodings can be converted by specializing on a program-defined stateT type. Objects of type stateT can contain any state that is useful to communicate to or from the specialized do_in or do_out members.

  3. Modify D.28 [depr.locale.category] (Deprecated locale category facets) in Annex D as indicated:

    -1- The ctype locale category includes the following facets as if they were specified in table Table 105 [tab:locale.category.facets] of 30.4.2.5.1 [locale.codecvt.general]. 

    codecvt<char16_t, char, mbstate_t>
codecvt<char32_t, char, mbstate_t>
codecvt<char16_t, char8_t, mbstate_t>
codecvt<char32_t, char8_t, mbstate_t>

    -1- The ctype locale category includes the following facets as if they were specified in table Table 106 [tab:locale.spec] of 30.4.2.5.1 [locale.codecvt.general]. 

    codecvt_byname<char16_t, char, mbstate_t>
codecvt_byname<char32_t, char, mbstate_t>
codecvt_byname<char16_t, char8_t, mbstate_t>
codecvt_byname<char32_t, char8_t, mbstate_t>

    -3- The following class template specializations are required in addition to those specified in 30.4.2.5 [locale.codecvt]. The specializations codecvt<char16_t, char, mbstate_t> and codecvt<char16_t, char8_t, mbstate_t> converts between the UTF-16 and UTF-8 encoding forms, and the specializations codecvt<char32_t, char, mbstate_t> and codecvt<char32_t, char8_t, mbstate_t> converts between the UTF-32 and UTF-8 encoding forms.