1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 2<html xmlns="http://www.w3.org/1999/xhtml"> 3<head> 4<meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/> 5<meta http-equiv="X-UA-Compatible" content="IE=9"/> 6<meta name="generator" content="Doxygen 1.8.6"/> 7<title>Boost.Locale: Introduction to C++ Standard Library localization support</title> 8<link href="tabs.css" rel="stylesheet" type="text/css"/> 9<script type="text/javascript" src="jquery.js"></script> 10<script type="text/javascript" src="dynsections.js"></script> 11<link href="navtree.css" rel="stylesheet" type="text/css"/> 12<script type="text/javascript" src="resize.js"></script> 13<script type="text/javascript" src="navtree.js"></script> 14<script type="text/javascript"> 15 $(document).ready(initResizable); 16 $(window).load(resizeHeight); 17</script> 18<link href="doxygen.css" rel="stylesheet" type="text/css" /> 19</head> 20<body> 21<div id="top"><!-- do not remove this div, it is closed by doxygen! --> 22<div id="titlearea"> 23<table cellspacing="0" cellpadding="0"> 24 <tbody> 25 <tr style="height: 56px;"> 26 <td id="projectlogo"><img alt="Logo" src="boost-small.png"/></td> 27 <td style="padding-left: 0.5em;"> 28 <div id="projectname">Boost.Locale 29 </div> 30 </td> 31 </tr> 32 </tbody> 33</table> 34</div> 35<!-- end header part --> 36<!-- Generated by Doxygen 1.8.6 --> 37 <div id="navrow1" class="tabs"> 38 <ul class="tablist"> 39 <li><a href="index.html"><span>Main Page</span></a></li> 40 <li class="current"><a href="pages.html"><span>Related Pages</span></a></li> 41 <li><a href="modules.html"><span>Modules</span></a></li> 42 <li><a href="namespaces.html"><span>Namespaces</span></a></li> 43 <li><a href="annotated.html"><span>Classes</span></a></li> 44 <li><a href="files.html"><span>Files</span></a></li> 45 <li><a href="examples.html"><span>Examples</span></a></li> 46 </ul> 47 </div> 48</div><!-- top --> 49<div id="side-nav" class="ui-resizable side-nav-resizable"> 50 <div id="nav-tree"> 51 <div id="nav-tree-contents"> 52 <div id="nav-sync" class="sync"></div> 53 </div> 54 </div> 55 <div id="splitbar" style="-moz-user-select:none;" 56 class="ui-resizable-handle"> 57 </div> 58</div> 59<script type="text/javascript"> 60$(document).ready(function(){initNavTree('std_locales.html','');}); 61</script> 62<div id="doc-content"> 63<div class="header"> 64 <div class="headertitle"> 65<div class="title">Introduction to C++ Standard Library localization support </div> </div> 66</div><!--header--> 67<div class="contents"> 68<div class="textblock"><h1><a class="anchor" id="std_locales_basics"></a> 69Getting familiar with standard C++ Locales</h1> 70<p>The C++ standard library offers a simple and powerful way to provide locale-specific information. It is done via the <code>std::locale</code> class, the container that holds all the required information about a specific culture, such as number formatting patterns, date and time formatting, currency, case conversion etc.</p> 71<p>All this information is provided by facets, special classes derived from the <code>std::locale::facet</code> base class. Such facets are packed into the <code>std::locale</code> class and allow you to provide arbitrary information about the locale. The <code>std::locale</code> class keeps reference counters on installed facets and can be efficiently copied.</p> 72<p>Each facet that was installed into the <code>std::locale</code> object can be fetched using the <code>std::use_facet</code> function. For example, the <code>std::ctype<Char></code> facet provides rules for case conversion, so you can convert a character to upper-case like this:</p> 73<div class="fragment"><div class="line">std::ctype<char> <span class="keyword">const</span> &ctype_facet = std::use_facet<std::ctype<char> >(some_locale);</div> 74<div class="line"><span class="keywordtype">char</span> upper_a = ctype_facet.toupper(<span class="charliteral">'a'</span>);</div> 75</div><!-- fragment --><p>A locale object can be imbued into an <code>iostream</code> so it would format information according to the locale:</p> 76<div class="fragment"><div class="line">cout.imbue(std::locale(<span class="stringliteral">"en_US.UTF-8"</span>));</div> 77<div class="line">cout << 1345.45 << endl;</div> 78<div class="line">cout.imbue(std::locale(<span class="stringliteral">"ru_RU.UTF-8"</span>));</div> 79<div class="line">cout << 1345.45 << endl;</div> 80</div><!-- fragment --><p>Would display:</p> 81<pre class="fragment"> 1,345.45 1.345,45 82</pre><p>You can also create your own facets and install them into existing locale objects. For example:</p> 83<div class="fragment"><div class="line"><span class="keyword">class </span>measure : <span class="keyword">public</span> std::locale::facet {</div> 84<div class="line"><span class="keyword">public</span>:</div> 85<div class="line"> <span class="keyword">typedef</span> <span class="keyword">enum</span> { inches, ... } measure_type;</div> 86<div class="line"> measure(measure_type m,<span class="keywordtype">size_t</span> refs=0) </div> 87<div class="line"> double from_metric(<span class="keywordtype">double</span> value) const;</div> 88<div class="line"> std::<span class="keywordtype">string</span> name() const;</div> 89<div class="line"> ...</div> 90<div class="line">};</div> 91</div><!-- fragment --><p> And now you can simply provide this information to a locale:</p> 92<div class="fragment"><div class="line">std::locale::global(std::locale(std::locale(<span class="stringliteral">"en_US.UTF-8"</span>),<span class="keyword">new</span> measure(measure::inches)));</div> 93<div class="line"><span class="comment">/// Create default locale built from en_US locale and add paper size facet.</span></div> 94</div><!-- fragment --><p>Now you can print a distance according to the correct locale:</p> 95<div class="fragment"><div class="line"><span class="keywordtype">void</span> print_distance(std::ostream &out,<span class="keywordtype">double</span> value)</div> 96<div class="line">{</div> 97<div class="line"> measure <span class="keyword">const</span> &m = std::use_facet<measure>(out.getloc());</div> 98<div class="line"> <span class="comment">// Fetch locale information from stream</span></div> 99<div class="line"> out << m.from_metric(value) << <span class="stringliteral">" "</span> << m.name();</div> 100<div class="line">}</div> 101</div><!-- fragment --><p>This technique was adopted by the Boost.Locale library in order to provide powerful and correct localization. Instead of using the very limited C++ standard library facets, it uses ICU under the hood to create its own much more powerful ones.</p> 102<h1><a class="anchor" id="std_locales_common"></a> 103Common Critical Problems with the Standard Library</h1> 104<p>There are numerous issues in the standard library that prevent the use of its full power, and there are several additional issues:</p> 105<ul> 106<li>Setting the global locale has bad side effects. <br/> 107 Consider following code: <br/> 108<div class="fragment"><div class="line"><span class="keywordtype">int</span> main()</div> 109<div class="line">{</div> 110<div class="line"> std::locale::global(std::locale(<span class="stringliteral">""</span>)); </div> 111<div class="line"> <span class="comment">// Set system's default locale as global</span></div> 112<div class="line"> std::ofstream csv(<span class="stringliteral">"test.csv"</span>);</div> 113<div class="line"> csv << 1.1 << <span class="stringliteral">","</span> << 1.3 << std::endl;</div> 114<div class="line">}</div> 115</div><!-- fragment --> <br/> 116 What would be the content of <code>test.csv</code> ? It may be "1.1,1.3" or it may be "1,1,1,3" rather than what you had expected. <br/> 117 More than that it affects even <code>printf</code> and libraries like <code>boost::lexical_cast</code> giving incorrect or unexpected formatting. In fact many third-party libraries are broken in such a situation. <br/> 118 Unlike the standard localization library, Boost.Locale never changes the basic number formatting, even when it uses <code>std</code> based localization backends, so by default, numbers are always formatted using C-style locale. Localized number formatting requires specific flags. <br/> 119</li> 120<li>Number formatting is broken on some locales. <br/> 121 Some locales use the non-breakable space u00A0 character for thousands separator, thus in <code>ru_RU.UTF-8</code> locale number 1024 should be displayed as "1 024" where the space is a Unicode character with codepoint u00A0. Unfortunately many libraries don't handle this correctly, for example GCC and SunStudio display a "\xC2" character instead of the first character in the UTF-8 sequence "\xC2\xA0" that represents this code point, and actually generate invalid UTF-8. <br/> 122</li> 123<li>Locale names are not standardized. For example, under MSVC you need to provide the name <code>en-US</code> or <code>English_USA.1252</code> , when on POSIX platforms it would be <code>en_US.UTF-8</code> or <code>en_US.ISO-8859-1</code> <br/> 124 More than that, MSVC does not support UTF-8 locales at all. <br/> 125</li> 126<li>Many standard libraries provide only the C and POSIX locales, thus GCC supports localization only under Linux. On all other platforms, attempting to create locales other than "C" or "POSIX" would fail. </li> 127</ul> 128</div></div><!-- contents --> 129</div><!-- doc-content --> 130 131 <li class="footer"> 132© Copyright 2009-2012 Artyom Beilis, Distributed under the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License</a>, Version 1.0. 133 </li> 134 </ul> 135 </div> 136</body> 137</html> 138
