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| 18 | <td valign=center>
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| 19 | <a href="index.html">
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| 20 | <font color="#004faf">Home</font></a>
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| 21 | | <a href="classes.html">
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| 22 | <font color="#004faf">All Classes</font></a>
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| 23 | | <a href="mainclasses.html">
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| 24 | <font color="#004faf">Main Classes</font></a>
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| 25 | | <a href="annotated.html">
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| 26 | <font color="#004faf">Annotated</font></a>
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| 27 | | <a href="groups.html">
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| 28 | <font color="#004faf">Grouped Classes</font></a>
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| 29 | | <a href="functions.html">
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| 30 | <font color="#004faf">Functions</font></a>
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| 31 | </td>
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| 32 | <td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Qt Plugins HOWTO</h1>
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| 33 |
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| 34 |
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| 35 |
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| 36 | <p> Qt provides a simple plugin interface which makes it easy to create
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| 37 | custom database drivers, image formats, text codecs, styles and
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| 38 | widgets as stand-alone components.
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| 39 | <a href="#footnote1"><sup>(1)</sup></a><a name="footnote-call1"></a>
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| 40 | <p> Writing a plugin is achieved by subclassing the appropriate plugin
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| 41 | base clase, implementing a few functions, and adding a macro.
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| 42 | <p> There are five plugin base classes. Derived plugins are stored
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| 43 | by default in the standard plugin directory.
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| 44 | <p> <center><table cellpadding="4" cellspacing="2" border="0">
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| 45 | <tr bgcolor="#a2c511">
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| 46 | <th valign="top">Base Class
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| 47 | <th valign="top">Default Path
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| 48 | <tr bgcolor="#f0f0f0">
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| 49 | <td valign="top"><a href="qimageformatplugin.html">QImageFormatPlugin</a>
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| 50 | <td valign="top"><tt>pluginsbase/imageformats</tt> <sup>*</sup>
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| 51 | <tr bgcolor="#d0d0d0">
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| 52 | <td valign="top"><a href="qsqldriverplugin.html">QSqlDriverPlugin</a>
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| 53 | <td valign="top"><tt>pluginsbase/sqldrivers</tt> <sup>*</sup>
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| 54 | <tr bgcolor="#f0f0f0">
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| 55 | <td valign="top"><a href="qstyleplugin.html">QStylePlugin</a>
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| 56 | <td valign="top"><tt>pluginsbase/styles</tt> <sup>*</sup>
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| 57 | <tr bgcolor="#d0d0d0">
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| 58 | <td valign="top"><a href="qtextcodecplugin.html">QTextCodecPlugin</a>
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| 59 | <td valign="top"><tt>pluginsbase/codecs</tt> <sup>*</sup>
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| 60 | <tr bgcolor="#f0f0f0">
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| 61 | <td valign="top"><a href="qwidgetplugin.html">QWidgetPlugin</a>
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| 62 | <td valign="top"><tt>pluginsbase/designer</tt> <sup>*</sup>
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| 63 | </table></center>
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| 64 | <p> But where is the <tt>pluginsbase</tt> directory? When the application is
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| 65 | run, Qt will first treat the application's executable directory as the
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| 66 | <tt>pluginsbase</tt>. For example if the application is in <tt>C:\Program Files\MyApp</tt> and has a style plugin, Qt will look in <tt>C:\Program Files\MyApp\styles</tt>. (See <a href="qapplication.html#applicationDirPath">QApplication::applicationDirPath</a>() for
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| 67 | how to find out where the application's executable is.) Qt will also
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| 68 | look in the directory given by <tt>qInstallPathPlugins()</tt>. If you want
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| 69 | Qt to look in additional places you can add as many paths as you need
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| 70 | with calls to <a href="qapplication.html#addLibraryPath">QApplication::addLibraryPath</a>(). And if you want to
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| 71 | set your own path or paths you can use
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| 72 | <a href="qapplication.html#setLibraryPaths">QApplication::setLibraryPaths</a>().
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| 73 | <p> Suppose that you have a new style class called 'MyStyle' that you want
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| 74 | to make available as a plugin. The required code is straightforward:
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| 75 | <pre>
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| 76 | class MyStylePlugin : public <a href="qstyleplugin.html">QStylePlugin</a>
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| 77 | {
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| 78 | public:
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| 79 | MyStylePlugin() {}
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| 80 | ~MyStylePlugin() {}
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| 81 |
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| 82 | <a href="qstringlist.html">QStringList</a> keys() const {
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| 83 | return QStringList() << "mystyle";
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| 84 | }
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| 85 |
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| 86 | <a href="qstyle.html">QStyle</a>* create( const <a href="qstring.html">QString</a>& key ) {
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| 87 | if ( key == "mystyle" )
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| 88 | return new MyStyle;
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| 89 | return 0;
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| 90 | }
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| 91 | };
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| 92 |
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| 93 | Q_EXPORT_PLUGIN( MyStylePlugin )
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| 94 | </pre>
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| 95 |
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| 96 | <p> (Note that <a href="qstylefactory.html">QStyleFactory</a> is case-insensitive, and the lower case
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| 97 | version of the key is used; other factories, e.g. <a href="qwidgetfactory.html">QWidgetFactory</a>, are
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| 98 | case sensitive.)
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| 99 | <p> The constructor and destructor do not need to do anything, so are left
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| 100 | empty. There are only two virtual functions that must be implemented.
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| 101 | The first is keys() which returns a string list of the classes
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| 102 | implemented in the plugin. (We've just implemented one class in the
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| 103 | example above.) The second is a function that returns an object of the
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| 104 | required class (or 0 if the plugin is asked to create an object of a
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| 105 | class that it doesn't implement). For <a href="qstyleplugin.html">QStylePlugin</a>, this second
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| 106 | function is called create().
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| 107 | <p> It is possible to implement any number of plugin subclasses in a
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| 108 | single plugin, providing they are all derived from the same base
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| 109 | class, e.g. QStylePlugin.
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| 110 | <p> For database drivers, image formats, custom widgets and text codecs,
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| 111 | no explicit object creation is required. Qt will find and create them
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| 112 | as required. Styles are an exception, since you might want to set a
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| 113 | style explicitly in code. To apply a style, use code like this:
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| 114 | <pre>
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| 115 | QApplication::<a href="qapplication.html#setStyle">setStyle</a>( QStyleFactory::<a href="qstylefactory.html#create">create</a>( "MyStyle" ) );
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| 116 | </pre>
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| 117 |
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| 118 | <p> Some plugin classes require additional functions to be implemented.
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| 119 | See the <a href="designer-manual.html">Qt Designer manual's</a>,
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| 120 | 'Creating Custom Widgets' section in the 'Creating Custom Widgets'
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| 121 | chapter, for a complete example of a <a href="qwidgetplugin.html">QWidgetPlugin</a>, which implements
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| 122 | extra functions to integrate the plugin into <em>Qt Designer</em>. The
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| 123 | <a href="qwidgetfactory.html">QWidgetFactory</a> class provides additional information on
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| 124 | QWidgetPlugins.
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| 125 | <p> See the class documentation for details of the virtual functions that
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| 126 | must be reimplemented for each type of plugin.
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| 127 | <p> Qt applications automatically know which plugins are available,
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| 128 | because plugins are stored in the standard plugin subdirectories.
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| 129 | Because of this applications don't require any code to find and load
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| 130 | plugins, since Qt handles them automatically.
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| 131 | <p> The default directory for plugins is <tt>QTDIR/plugins</tt><sup>*</sup>,
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| 132 | with each type of plugin in a subdirectory for that type, e.g. <tt>styles</tt>. If you want your applications to use plugins and you don't
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| 133 | want to use the standard plugins path, have your installation process
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| 134 | determine the path you want to use for the plugins, and save the path,
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| 135 | e.g. using <a href="qsettings.html">QSettings</a>, for the application to read when it runs. The
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| 136 | application can then call <a href="qapplication.html#addLibraryPath">QApplication::addLibraryPath</a>() with this
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| 137 | path and your plugins will be available to the application. Note that
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| 138 | the final part of the path, i.e. <tt>styles</tt>, <tt>widgets</tt>, etc., cannot
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| 139 | be changed.
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| 140 | <p> The normal way to include a plugin with an application is either to
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| 141 | compile it in with the application, or to compile it into a <tt>DLL</tt> (or
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| 142 | <tt>so</tt> or other platform specific library type) and use it like any
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| 143 | other library. If you want the plugin to be loadable then one approach
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| 144 | is to create a subdirectory under the application, e.g. <tt>appdir/plugins/designer</tt>, and place the plugin in that directory.
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| 145 | <p> For <a href="designer-manual.html">Qt Designer</a>, you may need to
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| 146 | call QApplication::addLibraryPath("QTDIR/plugins/designer") to load
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| 147 | your <a href="designer-manual.html">Qt Designer</a> plugins.
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| 148 | <p> <sup>*</sup><small> All references to <tt>QTDIR</tt> refer to the path
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| 149 | where Qt was installed. </small>
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| 150 | <p> <h2> Loading and Verifying Plugins
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| 151 | </h2>
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| 152 | <a name="1"></a><p> When loading plugins, the Qt library does some sanity checking to
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| 153 | determine whether or not the plugin can be loaded and used. This
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| 154 | provides the ability to have multiple versions and configurations of
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| 155 | the Qt library installed side by side.
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| 156 | <ul>
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| 157 | <li> Plugins linked with a Qt library that has a higher major and/or
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| 158 | minor version number will not be loaded by a library with a lower
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| 159 | major and/or minor version number.
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| 160 | <p> <em>Rationale</em>:
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| 161 | <p> A plugin linked against a newer Qt library may use new
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| 162 | features that are not available in older versions. Trolltech
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| 163 | has a policy of adding new features and APIs only between minor
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| 164 | releases, which is why this test only looks at the major and minor
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| 165 | version numbers, and not at the patchlevel version number.
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| 166 | <p> <li> Plugins linked against a Qt library <em>with</em> thread support can only be
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| 167 | loaded by libraries that are built <em>with</em> thread support.
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| 168 | <p> <em>Rationale</em>:
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| 169 | <p> The threaded and non-threaded Qt libraries have different names.
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| 170 | A library <em>with</em> thread support that loads a plugin linked against a
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| 171 | Qt library <em>without</em> thread support will cause two versions of the same
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| 172 | library to be in memory at the same time. On UNIX systems, this
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| 173 | causes the non-threaded Qt library to be loaded. When this
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| 174 | happens, the constructors for all static objects in the Qt library
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| 175 | will be called a second time, but they will operate on the objects
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| 176 | already in memory. There is no way to work around this, as this is
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| 177 | a feature of the object binary format: the static symbols already
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| 178 | defined by the threaded Qt library cannot be replaced or copied
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| 179 | when the non-threaded Qt library is loaded.
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| 180 | <p> <li> Plugins linked against a Qt library <em>without</em> thread support can only
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| 181 | be loaded by libraries that are built <em>without</em> thread support.
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| 182 | <p> <em>Rationale</em>:
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| 183 | <p> See the Rationale above.
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| 184 | <p> <li> Starting with Qt 3.0.5, both the Qt library and all plugins are
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| 185 | built using a <em>build key</em>. The build key in the Qt library is
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| 186 | examined against the build key in the plugin, and if they match,
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| 187 | the plugin is loaded. If the build keys do not match, then the Qt
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| 188 | library refuses to load the plugin.
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| 189 | <p> <em>Rationale</em>:
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| 190 | <p> See the Rationale for the build key below.
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| 191 | </ul>
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| 192 | <p> <h2> The Build Key
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| 193 | </h2>
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| 194 | <a name="2"></a><p> The build key contains the following information:
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| 195 | <ul>
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| 196 | <li> Architecture, operating system and compiler.
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| 197 | <p> <em>Rationale</em>:
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| 198 | <p> In cases where different versions of the same compiler do not
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| 199 | produce binary compatible code, the version of the compiler is
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| 200 | also present in the build key.
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| 201 | <p> <li> Configuration of the Qt library. The configuration is a list
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| 202 | of the missing features that affect the available API in the
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| 203 | library.
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| 204 | <p> <em>Rationale</em>:
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| 205 | <p> Two different configurations of the same version of
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| 206 | the Qt library are not binary compatible. The Qt library that
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| 207 | loads the plugin uses the list of (missing) features to
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| 208 | determine if the plugin is binary compatible.
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| 209 | <p> <em>Note</em>: There are cases where a plugin can use features that are
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| 210 | available in two different configurations. However, the
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| 211 | developer writing plugins would need to know which features are
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| 212 | in use, both in their plugin and internally by the utility
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| 213 | classes in Qt. The Qt library would require complex feature
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| 214 | and dependency queries and verification when loading plugins.
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| 215 | Requiring this would place an unnecessary burden on the developer, and
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| 216 | increase the overhead of loading a plugin. To reduce both
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| 217 | development time and application runtime costs, a simple string
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| 218 | comparision of the build keys is used.
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| 219 | <p> <li> Optionally, an extra string may be specified on the configure
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| 220 | script command line.
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| 221 | <p> <em>Rationale</em>:
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| 222 | <p> When distributing binaries of the Qt library with an
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| 223 | application, this provides a way for developers to write
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| 224 | plugins that can only be loaded by the library with which the
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| 225 | plugins were linked.
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| 226 | </ul>
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| 227 | <p> <h2> Plugins and Threaded Applications
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| 228 | </h2>
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| 229 | <a name="3"></a><p> If you want to build a plugin which you want to use with a threaded Qt
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| 230 | library (whether or not the plugin itself uses threads) you must use a
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| 231 | threaded environment. Specifically, you must link the plugin with a
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| 232 | threaded Qt library, and you must build <a href="designer-manual.html">Qt
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| 233 | Designer</a> with that library. Your <tt>.pro</tt> file for your plugin
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| 234 | must include the line:
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| 235 | <pre>
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| 236 | CONFIG += thread
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| 237 | </pre>
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| 238 |
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| 239 | <p> <b>Warning:</b> Do not mix the normal Qt library and the threaded Qt library in
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| 240 | an application. If your application uses the threaded Qt library, you
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| 241 | should not link your plugin with the normal Qt library. Nor should you
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| 242 | dynamically load the normal Qt library or dynamically load another library,
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| 243 | e.g. a plugin, that depends on the normal Qt library. On some systems,
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| 244 | mixing threaded and non-threaded libraries or plugins will corrupt the
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| 245 | static data used in the Qt library.
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| 246 | <p>
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| 247 | <hr>
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| 248 | <ol> <li><a name="footnote1"></a>
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| 249 | Qt 3.0.5 introduces changes into some aspects of plugins, in
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| 250 | particular regarding loading, path handling and library versions. As
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| 251 | a result of this change, <b><em>no</em></b> plugins compiled with Qt 3.0.4 and
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| 252 | earlier will work with Qt 3.0.5 and later: they must be recompiled.
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| 253 | <a href="#footnote-call1">Back...</a></ol>
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| 254 | </hr>
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| 255 | <!-- eof -->
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| 256 | <p><address><hr><div align=center>
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| 257 | <table width=100% cellspacing=0 border=0><tr>
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| 258 | <td>Copyright © 2007
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| 259 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 260 | <td align=right><div align=right>Qt 3.3.8</div>
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| 261 | </table></div></address></body>
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| 262 | </html>
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