How to Create Qt Plugins

Qt provides two APIs for creating plugins:

• A high-level API for writing extensions to Qt itself: custom database drivers, image formats, text codecs, custom styles, etc.
• A low-level API for extending Qt applications.

For example, if you want to write a custom QStyle subclass and have Qt applications load it dynamically, you would use the higher-level API.

Since the higher-level API is built on top of the lower-level API, some issues are common to both.

If you want to provide plugins for use with Qt Designer, see the Qt Designer module documentation.

Topics:

The High-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 appropriate directory.

The following table summarizes the plugin base classes. Some of the classes are private, and are therefore not documented. You can use them, but there is no compatibility promise with later Qt versions.

If you have a new style class called MyStyle that you want to make available as a plugin, the class needs to be defined as follows (mystyleplugin.h):

class MyStylePlugin : public QStylePlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QStyleFactoryInterface" FILE "mystyleplugin.json")
public:
    QStyle *create(const QString &key);
};

Ensure that the class implementation is located in a .cpp file:

#include "mystyleplugin.h"

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

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

In addition, a json file (mystyleplugin.json) containing meta data describing the plugin is required for most plugins. For style plugins it simply contains a list of styles that can be created by the plugin:

{ "Keys": [ "mystyleplugin" ] }

The type of information that needs to be provided in the json file is plugin dependent, please see the class documentation for details on the information that needs to be contained in the file.

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.

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

The Low-Level API: Extending Qt Applications

Not only Qt itself but also Qt application can be extended through plugins. This requires the application to detect and load plugins using QPluginLoader. In that context, plugins may provide arbitrary functionality and are not limited to database drivers, image formats, text codecs, styles, and the other types of plugin that extend Qt's functionality.

Making an application extensible through plugins involves the following steps:

1. Define a set of interfaces (classes with only pure virtual functions) used to talk to the plugins.
2. Use the Q_DECLARE_INTERFACE() macro to tell Qt's meta-object system about the interface.
3. Use QPluginLoader in the application to load the plugins.
4. Use qobject_cast() to test whether a plugin implements a given interface.

Writing a plugin involves these steps:

1. Declare a plugin class that inherits from QObject and from the interfaces that the plugin wants to provide.
2. Use the Q_INTERFACES() macro to tell Qt's meta-object system about the interfaces.
3. Export the plugin using the Q_PLUGIN_METADATA() macro.
4. Build the plugin using a suitable .pro file.

For example, here's the definition of an interface class:

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

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

Here's the definition of a plugin class that implements that interface:

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

#include <plugandpaint/interfaces.h>

class ExtraFiltersPlugin : public QObject, public FilterInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.Examples.PlugAndPaint.FilterInterface" FILE "extrafilters.json")
    Q_INTERFACES(FilterInterface)

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

The Plug & Paint example documentation explains this process in detail. See also Creating Custom Widgets for Qt Designer for information about issues that are specific to Qt Designer. You can also take a look at the Echo Plugin Example which is a more trivial example on how to implement a plugin that extends Qt applications. Please note that a QCoreApplication must have been initialized before plugins can be loaded.

Locating Plugins

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.

During development, the 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, for example, 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, for example, by 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 (for example, styles) cannot be changed.

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. If you distribute any of the plugins that come with Qt (the ones located in the plugins directory), you must copy the subdirectory under plugins where the plugin is located to your applications root folder (i.e., do not include the plugins directory).

For more information about deployment, see the Deploying Qt Applications and Deploying Plugins documentation.

Static Plugins

The normal and most flexible way to include a plugin with an application is to compile it into a dynamic library that is shipped separately, and detected and loaded at runtime.

Plugins can be linked statically into your application. If you build the static version of Qt, this is the only option for including Qt's predefined plugins. Using static plugins makes the deployment less error-prone, but has the disadvantage that no functionality from plugins can be added without a complete rebuild and redistribution of the application.

To link plugins statically, you need to add the required plugins to your build using QTPLUGIN.

In the .pro file for your application, you need the following entry:

QTPLUGIN     += qjpeg \
                qgif \
                qkrcodecs

qmake automatically adds the plugins to QTPLUGIN that are typically needed by the Qt modules used (see QT), while more specialized plugins need to be added manually. The default list of automatically added plugins can be overridden per type. For example, to link the minimal plugin instead of the default Qt platform adaptation plugin, use:

QTPLUGIN.platforms = qminimal

If you want neither the default, nor the minimal QPA plugin to be linked automatically, use:

QTPLUGIN.platforms = -

The defaults are tuned towards an optimal out-of-the-box experience, but may unnecessarily bloat the application. It is recommended to inspect the linker command line built by qmake and eliminate unnecessary plugins.

Details of Linking Static Plugins

To cause static plugins actually being linked and instantiated, Q_IMPORT_PLUGIN() macros are also needed in application code, but those are automatically generated by qmake and added to your application project.

If you do not want all plugins added to QTPLUGIN to be automatically linked, remove import_plugins from the CONFIG variable:

CONFIG -= import_plugins

Creating Static Plugins

It is also possible to create your own static plugins, by following these steps:

1. Add CONFIG += static to your plugin's .pro file.
2. Use the Q_IMPORT_PLUGIN() macro in your application.
3. Use the Q_INIT_RESOURCE() macro in your application if the plugin ships qrc files.
4. Link your application with your plugin library using LIBS in the .pro file.

See the Plug & Paint example and the associated Basic Tools plugin for details on how to do this.

Note: If you are not using qmake to build your plugin you need to make sure that the QT_STATICPLUGIN preprocessor macro is defined.

Deploying and Debugging Plugins

The Deploying Plugins document covers the process of deploying plugins with applications and debugging them when problems arise.