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

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 .