locale/doc/using_localization_backends.txt

//
//  Copyright (c) 2009-2011 Artyom Beilis (Tonkikh)
//
//  Distributed under the Boost Software License, Version 1.0. (See
//  accompanying file LICENSE_1_0.txt or copy at
//  http://www.boost.org/LICENSE_1_0.txt)
//

// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4 filetype=cpp.doxygen
/*!
\page using_localization_backends Using Localization Backends

By default, Boost.Locale uses ICU for all localization and text manipulation tasks.
This is the most powerful library available, but sometimes we don't need
the full power of this library or we want to reduce dependencies from third-party
libraries, and ICU is by no means a small library.

Boost.Locale provides an option to use non-ICU based localization
backends. Although usually less powerful, these often provide all you need:
message formatting, currency, date, time, number formatting, basic collation and 
case manipulation. They are implemented using the standard OS API or a C or C++ library.

\section when_to_use_non_icu_backends When to use non-ICU backends

There are situations when using non-ICU based localization is appropriate:

- Embedded systems, where the ICU library is very hefty.
- Applications where only basic features like message, date, and time formatting and
  basic collation are required, and using a third-party library like ICU would be too
  complicated.
- Performance. ICU is a very powerful library, but it is generally slower than the standard
  library. Sometimes it's better to use a simpler but faster localization backend.


\section non_icu_backends Non-ICU Backends

All of the alternate backends have these limitations:

- Only the Gregorian calendar is supported and it is based
  on capabilites of mktime functionality (including dates range)
- No boundary analysis.
- Case handling is very simple and based on single codepoint conversions,
  though they still handle UTF-8 better than the standard library.
- Time zone specification is very limited: either local time or a time zone
  in the format "GMT+HH:MM".
- No percent formatting, no spellout or ordinal number formatting.
- Collation, with exception of the \c winapi backend, is limited to a single level,
  similar to what is done by \c strcoll.


\subsection std_backend std - The standard C++ library backend

This localization backend is based on the standard C++ library.

It is supported on all platforms, but is only actually useful on platforms where
the standard library supports locales besides "C" and "POSIX": on Linux with GCC
or Intel compilers, and under the MSVC compiler.

It works around some common standard library bugs like invalid UTF-8 generation for numeric
formatting, and it gives otherwise-absent POSIX locales names and UTF-8 support under MSVC.

It is very useful when the compiler and the library actually give fine localization
support, like GCC under Linux or MSVC under Windows.

\subsection posix_backend posix - POSIX 2008 C library

This backend is based on the latest POSIX 2008 standards, and uses POSIX api functions like
\c newlocale, \c freelocale, \c strftime_l  etc. It is available on the Linux and Mac OS X
platforms.

It gives you simple and ready-made localization support, most notably under Mac OS X where
GCC's \c libstdc++ does not support locales.

\note The POSIX backend only supports UTF-8, single-byte, and double-byte encodings.

\subsection winapi_backend winapi - Win32 API.

The Win32API-based localization backend provides decent UTF-8/UTF-16 locale support.
It is based on Windows API functions like \c GetLocaleInfoW, \c LCMapStringW, \c GetDateFormatW etc and
provides good localization support even on the MinGW and Cygwin platforms, which normally have
problems with this.

\note

- If you using GCC compiler under Windows you need GCC-4.x series to use it, GCC-3.4 is not supported
- Only UTF-8 as narrow locale encoding and UTF-16 as wide encoding are supported. 

\section supported_features_by_backends Supported Features

<table border="1" sellpadding="5" sellspacing="3">
<tr>
  <th>Backend</th>
  <th>icu</th><th>posix</th><th>winapi</th><th>std</th>
</tr>
<tr>
  <th>Message Formatting</th>
  <td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td>
</tr>
<tr>
  <th>Non UTF-8 encodings</th>
  <td>Yes</td><td>Yes</td><td>No</td><td>Yes</td>
</tr>
<tr>
  <th>Date/Time Formatting/Parsing</th>
  <td>Yes</td><td>Formatting Only</td><td>Formatting Only</td><td>Formatting Only</td>
</tr>
<tr>
  <th>Monetary Formatting/Parsing</th>
  <td>Yes</td><td>Formatting Only</td><td>Formatting Only</td><td>Yes</td>
</tr>
<tr>
  <th>Number Formatting/Parsing</th>
  <td>Yes</td><td>Yes</td><td>Yes</td><td>Yes</td>
</tr>
<tr>
  <th>Numbers as Percent, Spelled Out</th>
  <td>Yes</td><td>No</td><td>No</td><td>No</td>
</tr>
<tr>
  <th>Case Manipulation</th>
  <td>Yes</td><td>Basic</td><td>Basic</td><td>Basic</td>
</tr>
<tr>
  <th>Collation</th>
  <td>Full</td><td>Linux - 1 level<br>Mac OS X - broken</td><td>3 levels</td><td>1 level</td>
</tr>
<tr>
  <th>Calendar</th>
  <td>Yes</td><td>Gregorian Only</td><td>Gregorian Only</td><td>Gregorian Only</td>
</tr>
<tr>
  <th>Boundary Analysis</th>
  <td>Yes</td><td>No</td><td>No</td><td>No</td>
</tr>
<tr>
  <th>Unicode Normalization</th>
  <td>Yes</td><td>No</td><td>Vista and above</td><td>No</td>
</tr>
<tr>
  <th>C++0x characters</th>
  <td>Yes</td><td>No</td><td>No</td><td>Yes</td>
</tr>
<tr>
  <th>OS Support</th>
  <td>Any</td><td>Linux, Mac OS X</td><td>Windows, Cygwin</td><td>Any</td>
</tr>
<tr>
  <th>Useful on</th>
  <td>Any Platform</td><td>Linux and Mac OS X</td><td>Windows/MinGW/Cygwin</td><td>Linux with GCC or Intel<br>Windows with MSVC</td>
</tr>
</table>


\section using_localization_backends Using Localization Backends

Accessing a localization backend is done via the boost::locale::localization_backend_manager class.

You can create your own boost::locale::localization_backend_manager by starting with a global backend
via the boost::locale::localization_backend_manager::global static member function and modifying it.

For example:

\code
    localization_backend_manager my = localization_backend_manager::global(); 
    // Get global backend

    my.select("std"); 
    // select std backend as default

    generator gen(my); 
    // create a generator that uses this backend.

    localization_backend_manager::global(my);
    // set this backend globally

    generator gen2();
    // now this one would use the new global backend.
\endcode

You can also create a mixture of several backends, using
for example \c icu for one kind of operation and \c std
for all others:

\code
    localization_backend_manager my = localization_backend_manager::global(); 
    // Get global backend

    my.select("std"); 
    // select std backend as default for all categories
    my.select("icu",boundary_facet); 
    // select icu backend for boundary analysis (since it is not supported by \c std)
\endcode

*/