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# QPluginLoader Class Reference
[QtCore module]

The QPluginLoader class loads a plugin at run-time. More...

    #include <QPluginLoader>

Inherits QObject.

Note: All the functions in this class are reentrant.

- List of all members, including inherited members
- Obsolete members

### Properties

- fileName : QString

- 1 property inherited from QObject

### Public Functions

- QPluginLoader ( QObject * parent = 0 )
- QPluginLoader ( const QString & fileName, QObject * parent = 0 )
- ~QPluginLoader ()
- QString fileName () const
- QObject * instance ()
- bool isLoaded () const
- bool load ()
- void setFileName ( const QString & fileName )
- bool unload ()

- 28 public functions inherited from QObject

### Static Public Members

- QObjectList staticInstances ()

- 4 static public members inherited from QObject

### Macros

- Q_DECLARE_INTERFACE ( ClassName, Identifier )
- Q_EXPORT_PLUGIN2 ( PluginName, ClassName )
- Q_IMPORT_PLUGIN ( PluginName )

### Additional Inherited Members

- 1 public slot inherited from QObject
- 1 signal inherited from QObject
- 7 protected functions inherited from QObject

## Detailed Description

The QPluginLoader class loads a plugin at run-time.

QPluginLoader provides access to a Qt plugin. A Qt plugin is stored in a shared library (a DLL) and offers these benefits over shared libraries accessed using QLibrary:

- QPluginLoader checks that a plugin is linked against the same version of Qt as the application.
- QPluginLoader provides direct access to a root component object (instance()), instead of forcing you to resolve a C function manually.

An instance of a QPluginLoader object operates on a single shared library file, which we call a plugin. It provides access to the functionality in the plugin in a platform-independent way. To specify which plugin to load, either pass a file name in the constructor or set it with setFileName().

The most important functions are load() to dynamically load the plugin file, isLoaded() to check whether loading was successful, and instance() to access the root component in the plugin. The instance() function implicitly tries to load the plugin if it has not been loaded yet. Multiple instances of QPluginLoader can be used to access the same physical plugin.

Once loaded, plugins remain in memory until the application terminates. You can attempt to unload a plugin using unload(), but if other instances of QPluginLoader are using the same library, the call will fail, and unloading will only happen when every instance has called unload().

See How to Create Qt Plugins for more information about how to make your application extensible through plugins.

See also QLibrary.

## Property Documentation

### fileName : QString

This property holds the file name of the plugin.

To be loadable, the file's suffix must be a valid suffix for a loadable library in accordance with the platform, e.g. `.so` on Unix, - `.dylib` on Mac OS X, and `.dll` on Windows. The suffix can be verified with QLibrary::isLibrary().

Access functions:

- QString fileName () const
- void setFileName ( const QString & fileName )

See also load().

## Member Function Documentation

### QPluginLoader::QPluginLoader ( QObject * parent = 0 )

Constructs a plugin loader with the given parent.

### QPluginLoader::QPluginLoader ( const QString & fileName, QObject * parent = 0 )

Constructs a plugin loader with the given parent that will load the plugin specified by fileName.

To be loadable, the file's suffix must be a valid suffix for a loadable library in accordance with the platform, e.g. `.so` on Unix, - `.dylib` on Mac OS X, and `.dll` on Windows. The suffix can be verified with QLibrary::isLibrary().

See also setFileName().

### QPluginLoader::~QPluginLoader ()

Destroys the QPluginLoader object.

Unless unload() was called explicitly, the plugin stays in memory until the application terminates.

See also isLoaded() and unload().

### QObject * QPluginLoader::instance ()

Returns the root component object of the plugin. The plugin is loaded if necessary. The function returns 0 if the plugin could not be loaded or if the root component object could not be instantiated.

If the root component object was destroyed, calling this function creates a new instance.

The instance is not deleted when the QPluginLoader is destroyed.

The component object is a QObject. Use qobject_cast() to access interfaces you are interested in.

See also load().

### bool QPluginLoader::isLoaded () const

Returns true if the plugin is loaded; otherwise returns false.

See also load().

### bool QPluginLoader::load ()

Loads the plugin and returns true if the plugin was loaded successfully; otherwise returns false. Since instance() always calls this function before resolving any symbols it is not necessary to call it explicitly. In some situations you might want the plugin loaded in advance, in which case you would use this function.

On Mac OS X this function uses code from dlcompat, part of the OpenDarwin project.

Copyright (c) 2002 Jorge Acereda and Peter O'Gorman.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

See also unload().

### QObjectList QPluginLoader::staticInstances () `[static]`

Returns a list of static plugin instances held by the plugin loader.

### bool QPluginLoader::unload ()

Unloads the plugin and returns true if the plugin could be unloaded; otherwise returns false.

This happens automatically on application termination, so you shouldn't normally need to call this function.

If other instances of QPluginLoader are using the same plugin, the call will fail, and unloading will only happen when every instance has called unload().

See also instance() and load().

## 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:

        Q_DECLARE_INTERFACE(BrushInterface,
                            "com.trolltech.PlugAndPaint.BrushInterface/1.0")

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(), Q_EXPORT_PLUGIN2(), and How to Create Qt Plugins.

### Q_EXPORT_PLUGIN2 ( PluginName, ClassName )

This macro exports the plugin class ClassName with the name PluginName. There should be exactly one occurrence of this macro in a Qt plugin's source code (in an implementation file).

Example:

        Q_EXPORT_PLUGIN2(pnp_extrafilters, ExtraFiltersPlugin)

See the Plug & Paint example for details.

This function was introduced in Qt 4.1.

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

### Q_IMPORT_PLUGIN ( PluginName )

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

Example:

        Q_IMPORT_PLUGIN(qjpegplugin)

Static plugins must also be included by the linker when your application is built. With qmake, you can use `QTPLUGIN` to add the required plugins to your build. For example:

        TEMPLATE      = app
        QTPLUGIN     += qjpeg qgif qmng    # image formats
        QTPLUGIN     += cn tw           # codecs

Depending on your build, Qt provides the following static plugins:

Plugin name | Type | Description | QTPLUGIN entry
--- | --- | --- | ---
`qjpeg` | Image format | The JPEG image format | qjpeg
`qgif` | Image format | The GIF image format | qgif
`qmng` | Image format | The MNG image format | qmng

You should also ensure that the linker will use the correct linker path by adding the following line to your application's project file:

        QMAKE_LFLAGS += -L$$[QT_INSTALL_PLUGINS]/imageformats

See also How to Create Qt Plugins and Using qmake.