Qt 4.8: QPluginLoader Class Reference

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# QPluginLoader Class Reference

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

     #include <QPluginLoader>

Inherits: QObject.

Note: All functions in this class are reentrant.

- List of all members, including inherited members

## Properties

- fileName : QString
- loadHints : QLibrary::LoadHints

- 1 property inherited from QObject

## Public Functions

| QPluginLoader ( QObject * parent = 0 )
| QPluginLoader ( const QString & fileName, QObject * parent = 0 )
| ~QPluginLoader ()
QString | errorString () const
QString | fileName () const
QObject * | instance ()
bool | isLoaded () const
bool | load ()
QLibrary::LoadHints | loadHints () const
void | setFileName ( const QString & fileName )
void | setLoadHints ( QLibrary::LoadHints loadHints )
bool | unload ()

- 29 public functions inherited from QObject

## Static Public Members

QObjectList | staticInstances ()

- 7 static public members inherited from QObject

## Related Non-Members

void | qRegisterStaticPluginInstanceFunction ( QtPluginInstanceFunction function )

### Additional Inherited Members

- 1 public slot inherited from QObject
- 1 signal inherited from QObject
- 8 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 all instances of QPluginLoader has been unloaded, or 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(). Right before the unloading happen, the root component will also be deleted.

In order to speed up loading and validation of plugins, some of the information that is collected during loading is cached in persistent memory (through QSettings). For instance, the result of a load operation (e.g. succeeded or failed) is stored in the cache, so that subsequent load operations don't try to load an invalid plugin. However, if the "last modified" timestamp of a plugin has changed, the plugin's cache entry is invalidated and the plugin is reloaded regardless of the values in the cache entry. The cache entry is then updated with the new result of the load operation.

This also means that the timestamp must be updated each time the plugin or any dependent resources (such as a shared library) is updated, since the dependent resources might influence the result of loading a plugin.

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

Note that the QPluginLoader cannot be used if your application is statically linked against Qt. In this case, you will also have to link to plugins statically. You can use QLibrary if you need to load dynamic libraries in a statically linked application.

Note: In Symbian the plugin stub files must be used whenever a path to plugin is needed. For the purposes of loading plugins, the stubs can be considered to have the same name as the actual plugin binary. In practice they have ".qtplugin" extension instead of ".dll", but this difference is handled transparently by QPluginLoader and QLibrary to avoid need for Symbian specific plugin handling in most Qt applications. Plugin stubs are needed because Symbian Platform Security denies all access to the directory where the actual plugin binaries are located.

See also QLibrary and Plug & Paint Example.

## 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().

If the file name does not exist, it will not be set. This property will then contain an empty string.

By default, this property contains an empty string.

Note: In Symbian the fileName must point to plugin stub file.

Access functions:

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

See also load().

### loadHints : QLibrary::LoadHints

This property holds give the load() function some hints on how it should behave.

You can give hints on how the symbols in the plugin are resolved. By default, none of the hints are set.

See the documentation of QLibrary::loadHints for a complete description of how this property works.

This property was introduced in Qt 4.4.

Access functions:

QLibrary::LoadHints | loadHints () const
void | setLoadHints ( QLibrary::LoadHints loadHints )

See also QLibrary::loadHints.

## 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().

Note: In Symbian the fileName must point to plugin stub file.

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().

### QString QPluginLoader::errorString () const

Returns a text string with the description of the last error that occurred.

This function was introduced in Qt 4.2.

### 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 root component, returned by this function, is not deleted when the QPluginLoader is destroyed. If you want to ensure that the root component is deleted, you should call unload() as soon you don't need to access the core component anymore. When the library is finally unloaded, the root component will automatically be deleted.

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.

See also unload().

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

Returns a list of static plugin instances (root components) 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().

Don't try to delete the root component. Instead rely on that unload() will automatically delete it when needed.

See also instance() and load().

## Related Non-Members

### void qRegisterStaticPluginInstanceFunction ( QtPluginInstanceFunction function )

Registers the given function with the plugin loader.

This function was introduced in Qt 4.4.