How to Create Qt Plugins

The Higher-Level API: Writing Qt Extensions

Writing a plugin that extends Qt itself is achieved by subclassing the appropriate plugin base class, implementing a few functions, and adding a macro.

There are several plugin base classes. Derived plugins are stored by default in sub-directories of the standard plugin directory. Qt will not find plugins if they are not stored in the right directory.

But where is the plugins directory? When the application is run, Qt will first treat the application's executable directory as the pluginsbase. For example if the application is in C:\Program Files\MyApp and has a style plugin, Qt will look in C:\Program Files\MyApp\styles. (See QCoreApplication::applicationDirPath() for how to find out where the application's executable is.) Qt will also look in the directory specified by QLibraryInfo::location(QLibraryInfo::PluginsPath), which typically is located in QTDIR/plugins (where QTDIR is the directory where Qt is installed). If you want Qt to look in additional places you can add as many paths as you need with calls to QCoreApplication::addLibraryPath(). And if you want to set your own path or paths you can use QCoreApplication::setLibraryPaths(). You can also use a qt.conf file to override the hard-coded paths that are compiled into the Qt library. For more information, see the Using qt.conf documentation. Yet another possibility is to set the QT_PLUGIN_PATH environment variable before running the application. If set, Qt will look for plugins in the paths (separated by the system path separator) specified in the variable.

Suppose that you have a new style class called MyStyle that you want to make available as a plugin. The required code is straightforward, here is the class definition (mystyleplugin.h):

 class MyStylePlugin : public QStylePlugin
 {
 public:
     QStringList keys() const;
     QStyle *create(const QString &key);
 };

Ensure that the class implementation is located in a .cpp file (including the class definition):

 #include "mystyleplugin.h"

 QStringList MyStylePlugin::keys() const
 {
     return QStringList() << "MyStyle";
 }

 QStyle *MyStylePlugin::create(const QString &key)
 {
     if (key.toLower() == "mystyle")
         return new MyStyle;
     return 0;
 }

 Q_EXPORT_PLUGIN2(pnp_mystyleplugin, MyStylePlugin)

(Note that QStylePlugin is case insensitive, and the lower-case version of the key is used in our create() implementation; most other plugins are case sensitive.)

For database drivers, image formats, text codecs, and most other plugin types, no explicit object creation is required. Qt will find and create them as required. Styles are an exception, since you might want to set a style explicitly in code. To apply a style, use code like this:

 QApplication::setStyle(QStyleFactory::create("MyStyle"));

Some plugin classes require additional functions to be implemented. See the class documentation for details of the virtual functions that must be reimplemented for each type of plugin.

Qt applications automatically know which plugins are available, because plugins are stored in the standard plugin subdirectories. Because of this applications don't require any code to find and load plugins, since Qt handles them automatically.

The default directory for plugins is QTDIR/plugins (where QTDIR is the directory where Qt is installed), with each type of plugin in a subdirectory for that type, e.g. styles. If you want your applications to use plugins and you don't want to use the standard plugins path, have your installation process determine the path you want to use for the plugins, and save the path, e.g. using QSettings, for the application to read when it runs. The application can then call QCoreApplication::addLibraryPath() with this path and your plugins will be available to the application. Note that the final part of the path (e.g., styles) cannot be changed.

The normal way to include a plugin with an application is either to compile it in with the application or to compile it into a dynamic library and use it like any other library. If you want the plugin to be loadable then one approach is to create a subdirectory under the application and place the plugin in that directory. For more information about deployment, see the Deploying Qt Applications documentation.

The Style Plugin Example shows how to implement a plugin that extends the QStylePlugin base class.

 class FilterInterface
 {
 public:
     virtual ~FilterInterface() {}

     virtual QStringList filters() const = 0;
     virtual QImage filterImage(const QString &filter, const QImage &image,
                                QWidget *parent) = 0;
 };

 #include <QObject>
 #include <QStringList>
 #include <QImage>

 #include <plugandpaint/interfaces.h>

 class ExtraFiltersPlugin : public QObject, public FilterInterface
 {
     Q_OBJECT
     Q_INTERFACES(FilterInterface)

 public:
     QStringList filters() const;
     QImage filterImage(const QString &filter, const QImage &image,
                        QWidget *parent);
 };

 CONFIG += release

 #include <QApplication>
 #include <QtPlugin>

 Q_IMPORT_PLUGIN(qjpeg)
 Q_IMPORT_PLUGIN(qgif)
 Q_IMPORT_PLUGIN(qkrcodecs)

 int main(int argc, char *argv[])
 {
     QApplication app(argc, argv);
     ...
     return app.exec();
 }

 QTPLUGIN     += qjpeg \
                 qgif \
                 qkrcodecs

 HKEY_CURRENT_USER\Software\Trolltech\OrganizationDefaults\Qt Plugin Cache 4.2.debug
 HKEY_CURRENT_USER\Software\Trolltech\OrganizationDefaults\Qt Plugin Cache 4.2.false