<QtPlugin> - Defining Plugins

Macro Documentation

Q_DECLARE_INTERFACE(ClassName, Identifier)

This macro associates the given Identifier (a string literal) to the interface class called ClassName. The Identifier must be unique. For example:

 #define BrushInterface_iid "org.qt-project.Qt.Examples.PlugAndPaint.BrushInterface/1.0"

 Q_DECLARE_INTERFACE(BrushInterface, BrushInterface_iid)

This macro is normally used right after the class definition for ClassName, in a header file. See the Plug & Paint example for details.

If you want to use Q_DECLARE_INTERFACE with interface classes declared in a namespace then you have to make sure the Q_DECLARE_INTERFACE is not inside a namespace though. For example:

 namespace Foo
 {
     struct MyInterface { ... };
 }

 Q_DECLARE_INTERFACE(Foo::MyInterface, "org.examples.MyInterface")

See also Q_INTERFACES() and How to Create Qt Plugins.

Q_IMPORT_PLUGIN(PluginName)

This macro imports the plugin named PluginName, which corresponds with the name of the class that declares metadata for the plugin with Q_PLUGIN_METADATA().

Inserting this macro into your application's source code will allow you to make use of a static plugin.

Example:

 Q_IMPORT_PLUGIN(qjpeg)

Static plugins must also be included by the linker when your application is built. For Qt's predefined plugins, you can use the QTPLUGIN to add the required plugins to your build. For example:

 TEMPLATE      = app
 QTPLUGIN     += qjpeg qgif    # image formats

See also Static Plugins, How to Create Qt Plugins, and Getting Started with qmake.

Q_PLUGIN_METADATA(...)

This macro is being used to declare meta data that is part of a plugin that instantiates this object.

The macro needs to declare the IID of the interface implemented through the object, and reference a file containing the meta data for the plugin.

There should be exactly one occurrence of this macro in the source code for a Qt plugin.

Example:

 class MyInstance : public QObject
 {
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QDummyPlugin" FILE "mymetadata.json")
 };

See the Plug & Paint example for details.

Note that the class this macro appears on must be default-constructible.

FILE is optional and points to a json file.

The json file must reside in one of the include directories specified by the build-system. moc exits with an error when it could not find the specified file.

This function was introduced in Qt 5.0.

See also Q_DECLARE_INTERFACE() and How to Create Qt Plugins.