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().
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.
See also QLibrary and Plug & Paint Example.
Property Documentation
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.
Access functions:
- QString fileName () const
- void setFileName ( const QString & fileName )
See also load().
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().
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.
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().
Warning: The root component of a plugin, returned by the instance() function, becomes invalid once the plugin is unloaded. Delete the root component before unloading the plugin. Attempting to access members of invalid root components will in most cases result in a segmentation fault.
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.
