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| <div class="title">Introduction to C++ Standard Library localization support </div> </div> |
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| <div class="textblock"><h2><a class="anchor" id="std_locales_basics"></a> |
| Getting familiar with standard C++ Locales</h2> |
| <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> |
| <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> |
| <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> |
| <div class="fragment"><pre class="fragment">std::ctype<char> <span class="keyword">const</span> &ctype_facet = std::use_facet<std::ctype<char> >(some_locale); |
| <span class="keywordtype">char</span> upper_a = ctype_facet.toupper(<span class="charliteral">'a'</span>); |
| </pre></div><p>A locale object can be imbued into an <code>iostream</code> so it would format information according to the locale:</p> |
| <div class="fragment"><pre class="fragment">cout.imbue(std::locale(<span class="stringliteral">"en_US.UTF-8"</span>)); |
| cout << 1345.45 << endl; |
| cout.imbue(std::locale(<span class="stringliteral">"ru_RU.UTF-8"</span>)); |
| cout << 1345.45 << endl; |
| </pre></div><p>Would display:</p> |
| <div class="fragment"><pre class="fragment"> |
| 1,345.45 1.345,45 |
| </pre></div><p>You can also create your own facets and install them into existing locale objects. For example:</p> |
| <div class="fragment"><pre class="fragment"> <span class="keyword">class </span>measure : <span class="keyword">public</span> std::locale::facet { |
| <span class="keyword">public</span>: |
| <span class="keyword">typedef</span> <span class="keyword">enum</span> { inches, ... } measure_type; |
| measure(measure_type m,<span class="keywordtype">size_t</span> refs=0) |
| double from_metric(<span class="keywordtype">double</span> value) const; |
| std::<span class="keywordtype">string</span> name() const; |
| ... |
| }; |
| </pre></div><p> And now you can simply provide this information to a locale:</p> |
| <div class="fragment"><pre class="fragment"> std::locale::global(std::locale(std::locale(<span class="stringliteral">"en_US.UTF-8"</span>),<span class="keyword">new</span> measure(measure::inches))); |
| <span class="comment">/// Create default locale built from en_US locale and add paper size facet.</span> |
| </pre></div><p>Now you can print a distance according to the correct locale:</p> |
| <div class="fragment"><pre class="fragment"> <span class="keywordtype">void</span> print_distance(std::ostream &out,<span class="keywordtype">double</span> value) |
| { |
| measure <span class="keyword">const</span> &m = std::use_facet<measure>(out.getloc()); |
| <span class="comment">// Fetch locale information from stream</span> |
| out << m.from_metric(value) << <span class="stringliteral">" "</span> << m.name(); |
| } |
| </pre></div><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> |
| <h2><a class="anchor" id="std_locales_common"></a> |
| Common Critical Problems with the Standard Library</h2> |
| <p>There are numerous issues in the standard library that prevent the use of its full power, and there are several additional issues:</p> |
| <ul> |
| <li>Setting the global locale has bad side effects. <br/> |
| Consider following code: <br/> |
| <div class="fragment"><pre class="fragment"> <span class="keywordtype">int</span> main() |
| { |
| std::locale::global(std::locale(<span class="stringliteral">""</span>)); |
| <span class="comment">// Set system's default locale as global</span> |
| std::ofstream csv(<span class="stringliteral">"test.csv"</span>); |
| csv << 1.1 << <span class="stringliteral">","</span> << 1.3 << std::endl; |
| } |
| </pre></div> <br/> |
| 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/> |
| 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/> |
| 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/> |
| </li> |
| <li>Number formatting is broken on some locales. <br/> |
| 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/> |
| </li> |
| <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/> |
| More than that, MSVC does not support UTF-8 locales at all. <br/> |
| </li> |
| <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> |
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