QPluginLoader Class

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

Header: #include <QPluginLoader>
CMake: find_package(Qt6 REQUIRED COMPONENTS Core)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Inherits: QObject

Note: All functions in this class are reentrant.


Public Functions

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

Static Public Members

QObjectList staticInstances()
QList<QStaticPlugin> staticPlugins()
void qRegisterStaticPluginFunction(QStaticPlugin plugin)

Detailed Description

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 happens, the root component will also be deleted.

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.

Property Documentation

fileName : QString

This property holds the file name of the plugin

We recommend omitting the file's suffix in the file name, since QPluginLoader will automatically look for the file with the appropriate suffix (see QLibrary::isLibrary()).

When loading the plugin, QPluginLoader searches in all plugin locations specified by QCoreApplication::libraryPaths(), unless the file name has an absolute path. After loading the plugin successfully, fileName() returns the fully-qualified file name of the plugin, including the full path to the plugin if one was given in the constructor or passed to setFileName().

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().

loadHints : QLibrary::LoadHints

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 since Qt 5.7, QLibrary::PreventUnloadHint is set.

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

Access functions:

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

See also QLibrary::loadHints.

Member Function Documentation

[explicit] QPluginLoader::QPluginLoader(QObject *parent = nullptr)

Constructs a plugin loader with the given parent.

[explicit] QPluginLoader::QPluginLoader(const QString &fileName, QObject *parent = nullptr)

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 macOS and iOS, and .dll on Windows. The suffix can be verified with QLibrary::isLibrary().

See also setFileName().

[virtual noexcept] 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.

QObject *QPluginLoader::instance()

Returns the root component object of the plugin. The plugin is loaded if necessary. The function returns nullptr 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().

QJsonObject QPluginLoader::metaData() const

Returns the meta data for this plugin. The meta data is data specified in a json format using the Q_PLUGIN_METADATA() macro when compiling the plugin.

The meta data can be queried in a fast and inexpensive way without actually loading the plugin. This makes it possible to e.g. store capabilities of the plugin in there, and make the decision whether to load the plugin dependent on this meta data.

[static] QObjectList QPluginLoader::staticInstances()

Returns a list of static plugin instances (root components) held by the plugin loader.

See also staticPlugins().

[static] QList<QStaticPlugin> QPluginLoader::staticPlugins()

Returns a list of QStaticPlugins held by the plugin loader. The function is similar to staticInstances() with the addition that a QStaticPlugin also contains meta data information.

See also staticInstances().

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 qRegisterStaticPluginFunction(QStaticPlugin plugin)

Registers the plugin specified with the plugin loader, and is used by Q_IMPORT_PLUGIN().

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