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Chapter 8.  Localization

Table of Contents

Locales
locale
Requirements
Design
Implementation
Interacting with "C" locales
Future
Facets
ctype
Implementation
Specializations
Future
codecvt
Requirements
Design
wchar_t Size
Support for Unicode
Other Issues
Implementation
Use
Future
messages
Requirements
Design
Implementation
Models
The GNU Model
Use
Future

Locales

locale

Describes the basic locale object, including nested classes id, facet, and the reference-counted implementation object, class _Impl.

Requirements

Class locale is non-templatized and has two distinct types nested inside of it:

class facet 22.1.1.1.2 Class locale::facet

Facets actually implement locale functionality. For instance, a facet called numpunct is the data object that can be used to query for the thousands separator in the locale.

Literally, a facet is strictly defined:

  • Containing the following public data member:

    static locale::id id;

  • Derived from another facet:

    class gnu_codecvt: public std::ctype<user-defined-type>

Of interest in this class are the memory management options explicitly specified as an argument to facet's constructor. Each constructor of a facet class takes a std::size_t __refs argument: if __refs == 0, the facet is deleted when the locale containing it is destroyed. If __refs == 1, the facet is not destroyed, even when it is no longer referenced.

class id 22.1.1.1.3 - Class locale::id

Provides an index for looking up specific facets.

Design

The major design challenge is fitting an object-orientated and non-global locale design on top of POSIX and other relevant standards, which include the Single Unix (nee X/Open.)

Because C and earlier versions of POSIX fall down so completely, portability is an issue.

Implementation

Interacting with "C" locales
  • locale -a displays available locales.

    af_ZA
    ar_AE
    ar_AE.utf8
    ar_BH
    ar_BH.utf8
    ar_DZ
    ar_DZ.utf8
    ar_EG
    ar_EG.utf8
    ar_IN
    ar_IQ
    ar_IQ.utf8
    ar_JO
    ar_JO.utf8
    ar_KW
    ar_KW.utf8
    ar_LB
    ar_LB.utf8
    ar_LY
    ar_LY.utf8
    ar_MA
    ar_MA.utf8
    ar_OM
    ar_OM.utf8
    ar_QA
    ar_QA.utf8
    ar_SA
    ar_SA.utf8
    ar_SD
    ar_SD.utf8
    ar_SY
    ar_SY.utf8
    ar_TN
    ar_TN.utf8
    ar_YE
    ar_YE.utf8
    be_BY
    be_BY.utf8
    bg_BG
    bg_BG.utf8
    br_FR
    bs_BA
    C
    ca_ES
    ca_ES@euro
    ca_ES.utf8
    ca_ES.utf8@euro
    cs_CZ
    cs_CZ.utf8
    cy_GB
    da_DK
    da_DK.iso885915
    da_DK.utf8
    de_AT
    de_AT@euro
    de_AT.utf8
    de_AT.utf8@euro
    de_BE
    de_BE@euro
    de_BE.utf8
    de_BE.utf8@euro
    de_CH
    de_CH.utf8
    de_DE
    de_DE@euro
    de_DE.utf8
    de_DE.utf8@euro
    de_LU
    de_LU@euro
    de_LU.utf8
    de_LU.utf8@euro
    el_GR
    el_GR.utf8
    en_AU
    en_AU.utf8
    en_BW
    en_BW.utf8
    en_CA
    en_CA.utf8
    en_DK
    en_DK.utf8
    en_GB
    en_GB.iso885915
    en_GB.utf8
    en_HK
    en_HK.utf8
    en_IE
    en_IE@euro
    en_IE.utf8
    en_IE.utf8@euro
    en_IN
    en_NZ
    en_NZ.utf8
    en_PH
    en_PH.utf8
    en_SG
    en_SG.utf8
    en_US
    en_US.iso885915
    en_US.utf8
    en_ZA
    en_ZA.utf8
    en_ZW
    en_ZW.utf8
    es_AR
    es_AR.utf8
    es_BO
    es_BO.utf8
    es_CL
    es_CL.utf8
    es_CO
    es_CO.utf8
    es_CR
    es_CR.utf8
    es_DO
    es_DO.utf8
    es_EC
    es_EC.utf8
    es_ES
    es_ES@euro
    es_ES.utf8
    es_ES.utf8@euro
    es_GT
    es_GT.utf8
    es_HN
    es_HN.utf8
    es_MX
    es_MX.utf8
    es_NI
    es_NI.utf8
    es_PA
    es_PA.utf8
    es_PE
    es_PE.utf8
    es_PR
    es_PR.utf8
    es_PY
    es_PY.utf8
    es_SV
    es_SV.utf8
    es_US
    es_US.utf8
    es_UY
    es_UY.utf8
    es_VE
    es_VE.utf8
    et_EE
    et_EE.utf8
    eu_ES
    eu_ES@euro
    eu_ES.utf8
    eu_ES.utf8@euro
    fa_IR
    fi_FI
    fi_FI@euro
    fi_FI.utf8
    fi_FI.utf8@euro
    fo_FO
    fo_FO.utf8
    fr_BE
    fr_BE@euro
    fr_BE.utf8
    fr_BE.utf8@euro
    fr_CA
    fr_CA.utf8
    fr_CH
    fr_CH.utf8
    fr_FR
    fr_FR@euro
    fr_FR.utf8
    fr_FR.utf8@euro
    fr_LU
    fr_LU@euro
    fr_LU.utf8
    fr_LU.utf8@euro
    ga_IE
    ga_IE@euro
    ga_IE.utf8
    ga_IE.utf8@euro
    gl_ES
    gl_ES@euro
    gl_ES.utf8
    gl_ES.utf8@euro
    gv_GB
    gv_GB.utf8
    he_IL
    he_IL.utf8
    hi_IN
    hr_HR
    hr_HR.utf8
    hu_HU
    hu_HU.utf8
    id_ID
    id_ID.utf8
    is_IS
    is_IS.utf8
    it_CH
    it_CH.utf8
    it_IT
    it_IT@euro
    it_IT.utf8
    it_IT.utf8@euro
    iw_IL
    iw_IL.utf8
    ja_JP.eucjp
    ja_JP.utf8
    ka_GE
    kl_GL
    kl_GL.utf8
    ko_KR.euckr
    ko_KR.utf8
    kw_GB
    kw_GB.utf8
    lt_LT
    lt_LT.utf8
    lv_LV
    lv_LV.utf8
    mi_NZ
    mk_MK
    mk_MK.utf8
    mr_IN
    ms_MY
    ms_MY.utf8
    mt_MT
    mt_MT.utf8
    nl_BE
    nl_BE@euro
    nl_BE.utf8
    nl_BE.utf8@euro
    nl_NL
    nl_NL@euro
    nl_NL.utf8
    nl_NL.utf8@euro
    nn_NO
    nn_NO.utf8
    no_NO
    no_NO.utf8
    oc_FR
    pl_PL
    pl_PL.utf8
    POSIX
    pt_BR
    pt_BR.utf8
    pt_PT
    pt_PT@euro
    pt_PT.utf8
    pt_PT.utf8@euro
    ro_RO
    ro_RO.utf8
    ru_RU
    ru_RU.koi8r
    ru_RU.utf8
    ru_UA
    ru_UA.utf8
    se_NO
    sk_SK
    sk_SK.utf8
    sl_SI
    sl_SI.utf8
    sq_AL
    sq_AL.utf8
    sr_YU
    sr_YU@cyrillic
    sr_YU.utf8
    sr_YU.utf8@cyrillic
    sv_FI
    sv_FI@euro
    sv_FI.utf8
    sv_FI.utf8@euro
    sv_SE
    sv_SE.iso885915
    sv_SE.utf8
    ta_IN
    te_IN
    tg_TJ
    th_TH
    th_TH.utf8
    tl_PH
    tr_TR
    tr_TR.utf8
    uk_UA
    uk_UA.utf8
    ur_PK
    uz_UZ
    vi_VN
    vi_VN.tcvn
    wa_BE
    wa_BE@euro
    yi_US
    zh_CN
    zh_CN.gb18030
    zh_CN.gbk
    zh_CN.utf8
    zh_HK
    zh_HK.utf8
    zh_TW
    zh_TW.euctw
    zh_TW.utf8
    
  • `locale` displays environmental variables that impact how locale("") will be deduced.

    LANG=en_US
    LC_CTYPE="en_US"
    LC_NUMERIC="en_US"
    LC_TIME="en_US"
    LC_COLLATE="en_US"
    LC_MONETARY="en_US"
    LC_MESSAGES="en_US"
    LC_PAPER="en_US"
    LC_NAME="en_US"
    LC_ADDRESS="en_US"
    LC_TELEPHONE="en_US"
    LC_MEASUREMENT="en_US"
    LC_IDENTIFICATION="en_US"
    LC_ALL=
    

From Josuttis, p. 697-698, which says, that "there is only *one* relation (of the C++ locale mechanism) to the C locale mechanism: the global C locale is modified if a named C++ locale object is set as the global locale" (emphasis Paolo), that is:

std::locale::global(std::locale(""));

affects the C functions as if the following call was made:

std::setlocale(LC_ALL, "");

On the other hand, there is *no* vice versa, that is, calling setlocale has *no* whatsoever on the C++ locale mechanism, in particular on the working of locale(""), which constructs the locale object from the environment of the running program, that is, in practice, the set of LC_ALL, LANG, etc. variable of the shell.

Future

  • Locale initialization: at what point does _S_classic, _S_global get initialized? Can named locales assume this initialization has already taken place?

  • Document how named locales error check when filling data members. I.e., a fr_FR locale that doesn't have numpunct::truename(): does it use "true"? Or is it a blank string? What's the convention?

  • Explain how locale aliasing happens. When does "de_DE" use "de" information? What is the rule for locales composed of just an ISO language code (say, "de") and locales with both an ISO language code and ISO country code (say, "de_DE").

  • What should non-required facet instantiations do? If the generic implementation is provided, then how to end-users provide specializations?

Bibliography

The GNU C Library . Roland McGrath. Ulrich Drepper. Copyright © 2007 FSF. Chapters 6 Character Set Handling and 7 Locales and Internationalization .

Correspondence . Ulrich Drepper. Copyright © 2002 .

ISO/IEC 14882:1998 Programming languages - C++ . Copyright © 1998 ISO.

ISO/IEC 9899:1999 Programming languages - C . Copyright © 1999 ISO.

System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008) . Copyright © 2008 The Open Group/The Institute of Electrical and Electronics Engineers, Inc. .

The C++ Programming Language, Special Edition . Bjarne Stroustrup. Copyright © 2000 Addison Wesley, Inc.. Appendix D. Addison Wesley .

Standard C++ IOStreams and Locales . Advanced Programmer's Guide and Reference . Angelika Langer. Klaus Kreft. Copyright © 2000 Addison Wesley Longman, Inc.. Addison Wesley Longman .