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<h3><a name="toc">Contents</a></h3>
<ul>
<li class="level1"><a href="#the-basics">The Basics</a></li>
<li class="level1"><a href="#defining-a-translation-context">Defining a Translation Context</a></li>
<li class="level1"><a href="#using-tr-to-obtain-a-translation">Using tr() to Obtain a Translation</a></li>
<li class="level1"><a href="#translator-comments">Translator Comments</a></li>
<li class="level1"><a href="#adding-meta-data-to-strings">Adding Meta-Data to Strings</a></li>
<li class="level1"><a href="#disambiguation">Disambiguation</a></li>
<li class="level1"><a href="#character-encodings">Character Encodings</a></li>
<li class="level1"><a href="#handling-plurals">Handling Plurals</a></li>
<li class="level1"><a href="#enabling-translation">Enabling Translation</a></li>
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<h1 class="title">Writing Source Code for Translation</h1>
<span class="subtitle"></span>
<!-- $$$i18n-source-translation.html-description -->
<div class="descr"> <a name="details"></a>
<a name="the-basics"></a>
<h2>The Basics</h2>
<p>Developers use the <a href="qobject.html#tr">tr()</a> function to obtain translated text for their classes, typically for display purposes. This function is also used to indicate which text strings in an application are translatable.</p>
<p>Qt indexes each translatable string by the <i>translation context</i> it is associated with; this is generally the name of the <a href="qobject.html">QObject</a> subclass it is used in.</p>
<p>Translation contexts are defined for new <a href="qobject.html">QObject</a>-based classes by the use of the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro in each new class definition.</p>
<p>When tr() is called, it looks up the translatable string using a <a href="qtranslator.html">QTranslator</a> object. For translation to work, one or more of these must have been installed on the application object in the way described in the <a href="#enabling-translation">Enabling Translation</a> section below.</p>
<a name="defining-a-translation-context"></a>
<h2>Defining a Translation Context</h2>
<p>The translation context for <a href="qobject.html">QObject</a> and each <a href="qobject.html">QObject</a> subclass is the class name itself. Developers subclassing <a href="qobject.html">QObject</a> must use the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro in their class definition to override the translation context. This macro sets the context to the name of the subclass.</p>
<p>For example, the following class definition includes the <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro, implementing a new tr() that uses the <tt>MainWindow</tt> context:</p>
<pre class="cpp"> <span class="keyword">class</span> MainWindow : <span class="keyword">public</span> <span class="type"><a href="qmainwindow.html">QMainWindow</a></span>
{
Q_OBJECT
<span class="keyword">public</span>:
MainWindow();
...</pre>
<p>If <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> is not used in a class definition, the context will be inherited from the base class. For example, since all <a href="qobject.html">QObject</a>-based classes in Qt provide a context, a new <a href="qwidget.html">QWidget</a> subclass defined without a <a href="qobject.html#Q_OBJECT">Q_OBJECT</a> macro will use the <tt>QWidget</tt> context if its tr() function is invoked.</p>
<a name="using-tr-to-obtain-a-translation"></a>
<h2>Using tr() to Obtain a Translation</h2>
<p>The following example shows how a translation is obtained for the class shown in the previous section:</p>
<pre class="cpp"> <span class="type">void</span> MainWindow<span class="operator">::</span>createMenus()
{
fileMenu <span class="operator">=</span> menuBar()<span class="operator">-</span><span class="operator">></span>addMenu(tr(<span class="string">"&File"</span>));
...</pre>
<p>Here, the translation context is <tt>MainWindow</tt> because it is the <tt>MainWindow::tr()</tt> function that is invoked. The text returned by the tr() function is a translation of "&File" obtained from the <tt>MainWindow</tt> context.</p>
<p>When Qt's translation tool, <a href="linguist-manager.html#lupdate">lupdate</a>, is used to process a set of source files, the text wrapped in tr() calls is stored in a section of the translation file that corresponds to its translation context.</p>
<p>In some situations, it is useful to give a translation context explicitly by fully qualifying the call to tr(); for example:</p>
<pre class="cpp"> <span class="type"><a href="qstring.html">QString</a></span> text <span class="operator">=</span> <span class="type"><a href="qscrollbar.html">QScrollBar</a></span><span class="operator">::</span>tr(<span class="string">"Page up"</span>);</pre>
<p>This call obtains the translated text for "Page up" from the <tt>QScrollBar</tt> context. Developers can also use the <a href="qcoreapplication.html#translate">QCoreApplication::translate</a>() function to obtain a translation for a particular translation context.</p>
<a name="translator-comments"></a>
<h2>Translator Comments</h2>
<p>Developers can include information about each translatable string to help translators with the translation process. These are extracted when <a href="linguist-manager.html#lupdate">lupdate</a> is used to process the source files. The recommended way to add comments is to annotate the tr() calls in your code with comments of the form:</p>
<p><tt>//: ...</tt></p>
<p>or</p>
<p><tt><tt>/*</tt>: ... <tt>*/</tt></tt></p>
<p>Examples:</p>
<pre class="cpp"> <span class="comment">//: This name refers to a host name.</span>
hostNameLabel<span class="operator">-</span><span class="operator">></span>setText(tr(<span class="string">"Name:"</span>));
<span class="comment">/*: This text refers to a C++ code example. */</span>
<span class="type"><a href="qstring.html">QString</a></span> example <span class="operator">=</span> tr(<span class="string">"Example"</span>);</pre>
<p>In these examples, the comments will be associated with the strings passed to tr() in the context of each call.</p>
<a name="adding-meta-data-to-strings"></a>
<h2>Adding Meta-Data to Strings</h2>
<p>Additional data can be attached to each translatable message. These are extracted when <a href="linguist-manager.html#lupdate">lupdate</a> is used to process the source files. The recommended way to add meta-data is to annotate the tr() calls in your code with comments of the form:</p>
<p><tt>//= <id></tt></p>
<p>This can be used to give the message a unique identifier to support tools which need it.</p>
<p>An alternative way to attach meta-data is to use the following syntax:</p>
<p><tt>//~ <field name> <field contents></tt></p>
<p>This can be used to attach meta-data to the message. The field name should consist of a domain prefix (possibly the conventional file extension of the file format the field is inspired by), a hyphen and the actual field name in underscore-delimited notation. For storage in TS files, the field name together with the prefix "extra-" will form an XML element name. The field contents will be XML-escaped, but otherwise appear verbatim as the element's contents. Any number of unique fields can be added to each message.</p>
<p>Example:</p>
<pre class="cpp"> <span class="comment">//: This is a comment for the translator.</span>
<span class="comment">//= qtn_foo_bar</span>
<span class="comment">//~ loc-layout_id foo_dialog</span>
<span class="comment">//~ loc-blank False</span>
<span class="comment">//~ magic-stuff This might mean something magic.</span>
<span class="type"><a href="qstring.html">QString</a></span> text <span class="operator">=</span> MyMagicClass<span class="operator">::</span>tr(<span class="string">"Sim sala bim."</span>);</pre>
<p>Meta-data appearing right in front of a magic TRANSLATOR comment applies to the whole TS file.</p>
<a name="disambiguation"></a>
<h2>Disambiguation</h2>
<p>If the same translatable string is used in different roles within the same translation context, an additional identifying string may be passed in the call to <a href="qobject.html#tr">tr()</a>. This optional disambiguation argument is used to distinguish between otherwise identical strings.</p>
<p>Example:</p>
<pre class="cpp"> MyWindow<span class="operator">::</span>MyWindow()
{
<span class="type"><a href="qlabel.html">QLabel</a></span> <span class="operator">*</span>senderLabel <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qlabel.html">QLabel</a></span>(tr(<span class="string">"Name:"</span>));
<span class="type"><a href="qlabel.html">QLabel</a></span> <span class="operator">*</span>recipientLabel <span class="operator">=</span> <span class="keyword">new</span> <span class="type"><a href="qlabel.html">QLabel</a></span>(tr(<span class="string">"Name:"</span><span class="operator">,</span> <span class="string">"recipient"</span>));
...</pre>
<p>In Qt 4.4 and earlier, this disambiguation parameter was the preferred way to specify comments to translators.</p>
<a name="character-encodings"></a>
<h2>Character Encodings</h2>
<p>You can set the encoding for the source text by calling <a href="qtextcodec.html#setCodecForTr">QTextCodec::setCodecForTr</a>(). By default, the source text is assumed to be in Latin-1 encoding.</p>
<a name="handling-plurals"></a>
<h2>Handling Plurals</h2>
<p>Some translatable strings contain placeholders for integer values and need to be translated differently depending on the values in use.</p>
<p>To help with this problem, developers pass an additional integer argument to the <a href="qobject.html#tr">tr()</a> function, and typically use a special notation for plurals in each translatable string.</p>
<p>If this argument is equal or greater than zero, all occurrences of <tt>%n</tt> in the resulting string are replaced with a decimal representation of the value supplied. In addition, the translation used will adapt to the value according to the rules for each language.</p>
<p>Example:</p>
<pre class="cpp"> <span class="type">int</span> n <span class="operator">=</span> messages<span class="operator">.</span>count();
showMessage(tr(<span class="string">"%n message(s) saved"</span><span class="operator">,</span> <span class="number">0</span><span class="operator">,</span> n));</pre>
<p>The table below shows what string is returned depending on the active translation:</p>
<table class="generic">
<thead><tr class="qt-style"><th ></th><th colspan="3">Active Translation</th></tr>
<tr class="qt-style"><th ><i>n</i></th><th >No Translation</th><th >French</th><th >English</th></tr></thead>
<tr valign="top" class="odd"><td >0</td><td >"0 message(s) saved"</td><td >"0 message sauvegardé"</td><td >"0 message<b>s</b> saved"</td></tr>
<tr valign="top" class="even"><td >1</td><td >"1 message(s) saved"</td><td >"1 message sauvegardé"</td><td >"1 message saved"</td></tr>
<tr valign="top" class="odd"><td >2</td><td >"2 message(s) saved"</td><td >"2 message<b>s</b> sauvegardé<b>s</b>"</td><td >"2 message<b>s</b> saved"</td></tr>
<tr valign="top" class="even"><td >37</td><td >"37 message(s) saved"</td><td >"37 message<b>s</b> sauvegardé<b>s</b>"</td><td >"37 message<b>s</b> saved"</td></tr>
</table>
<p>This idiom is more flexible than the traditional approach; e.g.,</p>
<pre class="cpp"> n <span class="operator">=</span><span class="operator">=</span> <span class="number">1</span> <span class="operator">?</span> tr(<span class="string">"%n message saved"</span>) : tr(<span class="string">"%n messages saved"</span>)</pre>
<p>because it also works with target languages that have several plural forms (e.g., Irish has a special "dual" form that should be used when <tt>n</tt> is 2), and it handles the <i>n</i> == 0 case correctly for languages such as French that require the singular. See the <a href="linguist-manual.html">Qt Linguist Manual</a> for details.</p>
<p>Instead of <tt>%n</tt>, you can use <tt>%Ln</tt> to produce a localized representation of <i>n</i>. The conversion uses the default locale, set using <a href="qlocale.html#setDefault">QLocale::setDefault</a>(). (If no default locale was specified, the "C" locale is used.)</p>
<p>A summary of supported languages and the rules used to translate strings containing plurals can be obtained by running <tt>lupdate -list-languages</tt>.</p>
<a name="enabling-translation"></a>
<h2>Enabling Translation</h2>
<p>Typically, your application's <tt>main()</tt> function will look like this:</p>
<pre class="cpp"> <span class="type">int</span> main(<span class="type">int</span> argc<span class="operator">,</span> <span class="type">char</span> <span class="operator">*</span>argv<span class="operator">[</span><span class="operator">]</span>)
{
<span class="type"><a href="qapplication.html">QApplication</a></span> app(argc<span class="operator">,</span> argv);
<span class="type"><a href="qtranslator.html">QTranslator</a></span> qtTranslator;
qtTranslator<span class="operator">.</span>load(<span class="string">"qt_"</span> <span class="operator">+</span> <span class="type"><a href="qlocale.html">QLocale</a></span><span class="operator">::</span>system()<span class="operator">.</span>name()<span class="operator">,</span>
<span class="type"><a href="qlibraryinfo.html">QLibraryInfo</a></span><span class="operator">::</span>location(<span class="type"><a href="qlibraryinfo.html">QLibraryInfo</a></span><span class="operator">::</span>TranslationsPath));
app<span class="operator">.</span>installTranslator(<span class="operator">&</span>qtTranslator);
<span class="type"><a href="qtranslator.html">QTranslator</a></span> myappTranslator;
myappTranslator<span class="operator">.</span>load(<span class="string">"myapp_"</span> <span class="operator">+</span> <span class="type"><a href="qlocale.html">QLocale</a></span><span class="operator">::</span>system()<span class="operator">.</span>name());
app<span class="operator">.</span>installTranslator(<span class="operator">&</span>myappTranslator);
<span class="operator">.</span><span class="operator">.</span><span class="operator">.</span>
<span class="keyword">return</span> app<span class="operator">.</span>exec();
}</pre>
<p>Note the use of <a href="qlibraryinfo.html#location">QLibraryInfo::location</a>() to locate the Qt translations. Developers should request the path to the translations at run-time by passing <a href="qlibraryinfo.html#LibraryLocation-enum">QLibraryInfo::TranslationsPath</a> to this function instead of using the <tt>QTDIR</tt> environment variable in their applications.</p>
<a name="further-reading"></a>
<h2>Further Reading</h2>
<p><a href="linguist-manual.html">Qt Linguist Manual</a>, <a href="linguist-hellotr.html">Hello tr</a>() Example</p>
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