QThreadPool Class Reference

\inmodule QtCore More...

#include <qthreadpool.h>

Inheritance diagram for QThreadPool:

Collaboration diagram for QThreadPool:

Public Member Functions
	QThreadPool (QObject *parent=nullptr)
	Constructs a thread pool with the given parent.
	~QThreadPool ()
	Destroys the QThreadPool.
void	start (QRunnable *runnable, int priority=0)
	Reserves a thread and uses it to run runnable, unless this thread will make the current thread count exceed maxThreadCount().
bool	tryStart (QRunnable *runnable)
	Attempts to reserve a thread to run runnable.
void	startOnReservedThread (QRunnable *runnable)
	Releases a thread previously reserved with reserveThread() and uses it to run runnable.
template<typename Callable , QRunnable::if_callable< Callable > = true>
void	start (Callable &&functionToRun, int priority=0)
template<typename Callable , QRunnable::if_callable< Callable > = true>
bool	tryStart (Callable &&functionToRun)
template<typename Callable , QRunnable::if_callable< Callable > = true>
void	startOnReservedThread (Callable &&functionToRun)
int	expiryTimeout () const
void	setExpiryTimeout (int expiryTimeout)
int	maxThreadCount () const
void	setMaxThreadCount (int maxThreadCount)
int	activeThreadCount () const
void	setStackSize (uint stackSize)
uint	stackSize () const
void	setThreadPriority (QThread::Priority priority)
QThread::Priority	threadPriority () const
void	reserveThread ()
	Reserves one thread, disregarding activeThreadCount() and maxThreadCount().
void	releaseThread ()
	Releases a thread previously reserved by a call to reserveThread().
bool	waitForDone (int msecs=-1)
	Waits up to msecs milliseconds for all threads to exit and removes all threads from the thread pool.
void	clear ()
bool	contains (const QThread *thread) const
bool	tryTake (QRunnable *runnable)
Public Member Functions inherited from QObject
Q_INVOKABLE	QObject (QObject *parent=nullptr)
	Constructs an object with parent object parent.
virtual	~QObject ()
	Destroys the object, deleting all its child objects.
virtual bool	event (QEvent *event)
	This virtual function receives events to an object and should return true if the event e was recognized and processed.
virtual bool	eventFilter (QObject *watched, QEvent *event)
	Filters events if this object has been installed as an event filter for the watched object.
QString	objectName () const
Q_WEAK_OVERLOAD void	setObjectName (const QString &name)
	Sets the object's name to name.
void	setObjectName (QAnyStringView name)
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
QBindable< QString >	bindableObjectName ()
bool	isWidgetType () const
	Returns true if the object is a widget; otherwise returns false.
bool	isWindowType () const
	Returns true if the object is a window; otherwise returns false.
bool	isQuickItemType () const
	Returns true if the object is a QQuickItem; otherwise returns false.
bool	signalsBlocked () const noexcept
	Returns true if signals are blocked; otherwise returns false.
bool	blockSignals (bool b) noexcept
	If block is true, signals emitted by this object are blocked (i.e., emitting a signal will not invoke anything connected to it).
QThread *	thread () const
	Returns the thread in which the object lives.
void	moveToThread (QThread *thread)
	Changes the thread affinity for this object and its children.
int	startTimer (int interval, Qt::TimerType timerType=Qt::CoarseTimer)
	This is an overloaded function that will start a timer of type timerType and a timeout of interval milliseconds.
int	startTimer (std::chrono::milliseconds time, Qt::TimerType timerType=Qt::CoarseTimer)
void	killTimer (int id)
	Kills the timer with timer identifier, id.
template<typename T >
T	findChild (const QString &aName=QString(), Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	Returns the child of this object that can be cast into type T and that is called name, or \nullptr if there is no such object.
template<typename T >
QList< T >	findChildren (const QString &aName, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	Returns all children of this object with the given name that can be cast to type T, or an empty list if there are no such objects.
template<typename T >
QList< T >	findChildren (Qt::FindChildOptions options=Qt::FindChildrenRecursively) const
	This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
const QObjectList &	children () const
	Returns a list of child objects.
void	setParent (QObject *parent)
	Makes the object a child of parent.
void	installEventFilter (QObject *filterObj)
	Installs an event filter filterObj on this object.
void	removeEventFilter (QObject *obj)
	Removes an event filter object obj from this object.
QMetaObject::Connection	connect (const QObject *sender, const char *signal, const char *member, Qt::ConnectionType type=Qt::AutoConnection) const
bool	disconnect (const char *signal=nullptr, const QObject *receiver=nullptr, const char *member=nullptr) const
bool	disconnect (const QObject *receiver, const char *member=nullptr) const
void	dumpObjectTree () const
	Dumps a tree of children to the debug output.
void	dumpObjectInfo () const
	Dumps information about signal connections, etc.
bool	setProperty (const char *name, const QVariant &value)
	Sets the value of the object's name property to value.
bool	setProperty (const char *name, QVariant &&value)
QVariant	property (const char *name) const
	Returns the value of the object's name property.
QList< QByteArray >	dynamicPropertyNames () const
QBindingStorage *	bindingStorage ()
const QBindingStorage *	bindingStorage () const
QObject *	parent () const
	Returns a pointer to the parent object.
bool	inherits (const char *classname) const
	Returns true if this object is an instance of a class that inherits className or a QObject subclass that inherits className; otherwise returns false.

Static Public Member Functions
static QThreadPool *	globalInstance ()
	Returns the global QThreadPool instance.
Static Public Member Functions inherited from QObject
static QMetaObject::Connection	connect (const QObject *sender, const char *signal, const QObject *receiver, const char *member, Qt::ConnectionType=Qt::AutoConnection)
	\threadsafe
static QMetaObject::Connection	connect (const QObject *sender, const QMetaMethod &signal, const QObject *receiver, const QMetaMethod &method, Qt::ConnectionType type=Qt::AutoConnection)
template<typename Func1 , typename Func2 >
static QMetaObject::Connection	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const typename QtPrivate::ContextTypeForFunctor< Func2 >::ContextType *context, Func2 &&slot, Qt::ConnectionType type=Qt::AutoConnection)
template<typename Func1 , typename Func2 >
static QMetaObject::Connection	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 &&slot)
static bool	disconnect (const QObject *sender, const char *signal, const QObject *receiver, const char *member)
	\threadsafe
static bool	disconnect (const QObject *sender, const QMetaMethod &signal, const QObject *receiver, const QMetaMethod &member)
static bool	disconnect (const QMetaObject::Connection &)
	Disconnect a connection.
template<typename Func1 , typename Func2 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const typename QtPrivate::FunctionPointer< Func2 >::Object *receiver, Func2 slot)
template<typename Func1 >
static bool	disconnect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, const QObject *receiver, void **zero)

Properties
int	expiryTimeout
	the thread expiry timeout value in milliseconds.
int	maxThreadCount
	the maximum number of threads used by the thread pool.
int	activeThreadCount
	the number of active threads in the thread pool.
uint	stackSize
	the stack size for the thread pool worker threads.
QThread::Priority	threadPriority
	the thread priority for new worker threads.
Properties inherited from QObject
QString	objectName
	the name of this object

Friends
class	QFutureInterfaceBase

Additional Inherited Members
Public Slots inherited from QObject
void	deleteLater ()
	\threadsafe
Signals inherited from QObject
void	destroyed (QObject *=nullptr)
	This signal is emitted immediately before the object obj is destroyed, after any instances of QPointer have been notified, and cannot be blocked.
void	objectNameChanged (const QString &objectName, QPrivateSignal)
	This signal is emitted after the object's name has been changed.
Protected Member Functions inherited from QObject
QObject *	sender () const
	Returns a pointer to the object that sent the signal, if called in a slot activated by a signal; otherwise it returns \nullptr.
int	senderSignalIndex () const
int	receivers (const char *signal) const
	Returns the number of receivers connected to the signal.
bool	isSignalConnected (const QMetaMethod &signal) const
virtual void	timerEvent (QTimerEvent *event)
	This event handler can be reimplemented in a subclass to receive timer events for the object.
virtual void	childEvent (QChildEvent *event)
	This event handler can be reimplemented in a subclass to receive child events.
virtual void	customEvent (QEvent *event)
	This event handler can be reimplemented in a subclass to receive custom events.
virtual void	connectNotify (const QMetaMethod &signal)
virtual void	disconnectNotify (const QMetaMethod &signal)
	QObject (QObjectPrivate &dd, QObject *parent=nullptr)
Protected Attributes inherited from QObject
QScopedPointer< QObjectData >	d_ptr
Related Symbols inherited from QObject
template< class T > T	qobject_cast (const QObject *object)
	Returns the given object cast to type T if the object is of type T (or of a subclass); otherwise returns \nullptr.
template< typename T > T	qFindChildqFindChildren (const QObject *obj, const QString &name)()
template< typename T > QList< T >	qFindChildrenqFindChildren (const QObject *obj, const QString &name)()
	QObjectList
	\macro Q_CLASSINFO(Name, Value)

Detailed Description

\inmodule QtCore

The QThreadPool class manages a collection of QThreads.

Since

4.4 \threadsafe

QThreadPool manages and recycles individual QThread objects to help reduce thread creation costs in programs that use threads. Each Qt application has one global QThreadPool object, which can be accessed by calling globalInstance().

To use one of the QThreadPool threads, subclass QRunnable and implement the run() virtual function. Then create an object of that class and pass it to QThreadPool::start().

class HelloWorldTask : public QRunnable
{
    void run() override
    {
        qDebug() << "Hello world from thread" << QThread::currentThread();
    }
};
 
HelloWorldTask *hello = new HelloWorldTask();
// QThreadPool takes ownership and deletes 'hello' automatically
QThreadPool::globalInstance()->start(hello);

QThreadPool deletes the QRunnable automatically by default. Use QRunnable::setAutoDelete() to change the auto-deletion flag.

QThreadPool supports executing the same QRunnable more than once by calling tryStart(this) from within QRunnable::run(). If autoDelete is enabled the QRunnable will be deleted when the last thread exits the run function. Calling start() multiple times with the same QRunnable when autoDelete is enabled creates a race condition and is not recommended.

Threads that are unused for a certain amount of time will expire. The default expiry timeout is 30000 milliseconds (30 seconds). This can be changed using setExpiryTimeout(). Setting a negative expiry timeout disables the expiry mechanism.

Call maxThreadCount() to query the maximum number of threads to be used. If needed, you can change the limit with setMaxThreadCount(). The default maxThreadCount() is QThread::idealThreadCount(). The activeThreadCount() function returns the number of threads currently doing work.

The reserveThread() function reserves a thread for external use. Use releaseThread() when your are done with the thread, so that it may be reused. Essentially, these functions temporarily increase or reduce the active thread count and are useful when implementing time-consuming operations that are not visible to the QThreadPool.

Note that QThreadPool is a low-level class for managing threads, see the Qt Concurrent module for higher level alternatives.

See also

QRunnable

Definition at line 19 of file qthreadpool.h.

Constructor & Destructor Documentation

QThreadPool()

QThreadPool::QThreadPool

(

QObject *

parent = nullptr

)

Constructs a thread pool with the given parent.

Definition at line 440 of file qthreadpool.cpp.

References QObject::connect(), d, and QObject::objectNameChanged().

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~QThreadPool()

QThreadPool::~QThreadPool

(

)

Destroys the QThreadPool.

This function will block until all runnables have been completed.

Definition at line 455 of file qthreadpool.cpp.

References d, Q_ASSERT, and waitForDone().

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Member Function Documentation

activeThreadCount()

int QThreadPool::activeThreadCount

(

)

const

Definition at line 654 of file qthreadpool.cpp.

References d.

clear()

void QThreadPool::clear

(

)

Since

5.2

Removes the runnables that are not yet started from the queue. The runnables for which \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns true are deleted.

See also

start()

Definition at line 829 of file qthreadpool.cpp.

References d.

contains()

bool QThreadPool::contains

(

const QThread *

thread

)

const

Since

6.0

Returns true if thread is a thread managed by this thread pool.

Definition at line 840 of file qthreadpool.cpp.

References d, and QObject::thread().

Referenced by QImage::applyColorTransform(), convert_generic_inplace(), convert_generic_inplace_over_rgb64(), and multithread_pixels_function().

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expiryTimeout()

int QThreadPool::expiryTimeout

(

)

const

Definition at line 597 of file qthreadpool.cpp.

References d.

globalInstance()

QThreadPool * QThreadPool::globalInstance ( )

static

Returns the global QThreadPool instance.

Definition at line 466 of file qthreadpool.cpp.

References QCoreApplication::closingDown().

Referenced by QCoreApplication::~QCoreApplication(), QtConcurrent::blockingFiltered(), QtConcurrent::blockingFiltered(), QtConcurrent::blockingMap(), QtConcurrent::blockingMap(), QtConcurrent::blockingMapped(), QtConcurrent::blockingMapped(), QtConcurrent::filter(), QtConcurrent::filtered(), QtConcurrent::filtered(), QtConcurrent::filteredReduced(), QtConcurrent::filteredReduced(), QtConcurrent::filteredReduced(), QtConcurrent::filteredReduced(), QmlLsp::QQmlCodeModel::indexNeedsUpdate(), QtConcurrent::map(), QtConcurrent::map(), QtConcurrent::mapped(), QtConcurrent::mapped(), QtConcurrent::mappedReduced(), QtConcurrent::mappedReduced(), QtConcurrent::mappedReduced(), QtConcurrent::mappedReduced(), QmlLsp::QQmlCodeModel::openNeedUpdate(), QGeoCodeJsonParser::parse(), QGeoRouteXmlParser::parse(), QFutureInterfaceBasePrivate::pool(), QtConcurrent::run(), and QtPrivate::AsyncContinuation< Function, ResultType, ParentResultType >::runImpl().

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maxThreadCount()

int QThreadPool::maxThreadCount

(

)

const

Definition at line 625 of file qthreadpool.cpp.

References d.

releaseThread()

void QThreadPool::releaseThread

(

)

Releases a thread previously reserved by a call to reserveThread().

Note

Calling this function without previously reserving a thread temporarily increases maxThreadCount(). This is useful when a thread goes to sleep waiting for more work, allowing other threads to continue. Be sure to call reserveThread() when done waiting, so that the thread pool can correctly maintain the activeThreadCount().

See also

reserveThread()

Definition at line 749 of file qthreadpool.cpp.

References d.

Referenced by startOnReservedThread().

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reserveThread()

void QThreadPool::reserveThread

(

)

Reserves one thread, disregarding activeThreadCount() and maxThreadCount().

Once you are done with the thread, call releaseThread() to allow it to be reused.

Note

Even if reserving maxThreadCount() threads or more, the thread pool will still allow a minimum of one thread.

This function will increase the reported number of active threads. This means that by using this function, it is possible for activeThreadCount() to return a value greater than maxThreadCount() .

See also

releaseThread()

Definition at line 676 of file qthreadpool.cpp.

References d.

setExpiryTimeout()

void QThreadPool::setExpiryTimeout

(

int

expiryTimeout

)

Definition at line 604 of file qthreadpool.cpp.

References d, and expiryTimeout.

setMaxThreadCount()

void QThreadPool::setMaxThreadCount

(

int

maxThreadCount

)

Definition at line 632 of file qthreadpool.cpp.

References d, and maxThreadCount.

Referenced by QQuickShapeGenericRenderer::endSync(), and Q_LOGGING_CATEGORY().

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setStackSize()

void QThreadPool::setStackSize

(

uint

stackSize

)

Definition at line 695 of file qthreadpool.cpp.

References d, and stackSize.

setThreadPriority()

void QThreadPool::setThreadPriority

(

QThread::Priority

priority

)

Definition at line 723 of file qthreadpool.cpp.

References d.

stackSize()

uint QThreadPool::stackSize

(

)

const

Definition at line 702 of file qthreadpool.cpp.

References d.

start() [1/2]

template<typename Callable , QRunnable::if_callable< Callable > >

void QThreadPool::start	(	Callable &&	functionToRun,
		int	priority = 0
	)

Definition at line 83 of file qthreadpool.h.

References QRunnable::create().

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start() [2/2]

void QThreadPool::start	(	QRunnable *	runnable,
		int	priority = 0
	)

Reserves a thread and uses it to run runnable, unless this thread will make the current thread count exceed maxThreadCount().

In that case, runnable is added to a run queue instead. The priority argument can be used to control the run queue's order of execution.

Note that the thread pool takes ownership of the runnable if \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns true, and the runnable will be deleted automatically by the thread pool after the \l{QRunnable::run()}{runnable->run()} returns. If \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns false, ownership of runnable remains with the caller. Note that changing the auto-deletion on runnable after calling this functions results in undefined behavior.

Definition at line 507 of file qthreadpool.cpp.

References d.

Referenced by QImage::applyColorTransform(), convert_generic(), convert_generic_inplace(), convert_generic_inplace_over_rgb64(), convert_generic_over_rgb64(), QQuickShapeGenericRenderer::endSync(), QmlLsp::QQmlCodeModel::indexNeedsUpdate(), multithread_pixels_function(), QmlLsp::QQmlCodeModel::openNeedUpdate(), QGeoCodeJsonParser::parse(), QGeoRouteXmlParser::parse(), QtPrivate::AsyncContinuation< Function, ResultType, ParentResultType >::runImpl(), QtConcurrent::RunFunctionTaskBase< T >::start(), and QtConcurrent::ThreadEngine< T >::startAsynchronously().

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startOnReservedThread() [1/2]

template<typename Callable , QRunnable::if_callable< Callable > >

void QThreadPool::startOnReservedThread

(

Callable &&

functionToRun

)

Definition at line 99 of file qthreadpool.h.

References QRunnable::create(), and startOnReservedThread().

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startOnReservedThread() [2/2]

void QThreadPool::startOnReservedThread

(

QRunnable *

runnable

)

Releases a thread previously reserved with reserveThread() and uses it to run runnable.

Note that the thread pool takes ownership of the runnable if \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns true, and the runnable will be deleted automatically by the thread pool after the \l{QRunnable::run()}{runnable->run()} returns. If \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns false, ownership of runnable remains with the caller. Note that changing the auto-deletion on runnable after calling this functions results in undefined behavior.

Note

Calling this when no threads are reserved results in undefined behavior.

Since

6.3

See also

reserveThread(), start()

Definition at line 776 of file qthreadpool.cpp.

References d, Q_ASSERT, and releaseThread().

Referenced by startOnReservedThread().

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threadPriority()

QThread::Priority QThreadPool::threadPriority

(

)

const

Definition at line 730 of file qthreadpool.cpp.

References d.

tryStart() [1/2]

template<typename Callable , QRunnable::if_callable< Callable > >

bool QThreadPool::tryStart

(

Callable &&

functionToRun

)

Definition at line 89 of file qthreadpool.h.

References QRunnable::create(), and tryStart().

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tryStart() [2/2]

bool QThreadPool::tryStart

(

QRunnable *

runnable

)

Attempts to reserve a thread to run runnable.

If no threads are available at the time of calling, then this function does nothing and returns false. Otherwise, runnable is run immediately using one available thread and this function returns true.

Note that on success the thread pool takes ownership of the runnable if \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns true, and the runnable will be deleted automatically by the thread pool after the \l{QRunnable::run()}{runnable->run()} returns. If \l{QRunnable::autoDelete()}{runnable->autoDelete()} returns false, ownership of runnable remains with the caller. Note that changing the auto-deletion on runnable after calling this function results in undefined behavior.

Definition at line 552 of file qthreadpool.cpp.

References d.

Referenced by tryStart().

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tryTake()

bool QThreadPool::tryTake

(

QRunnable *

runnable

)

Since

5.9

Attempts to remove the specified runnable from the queue if it is not yet started. If the runnable had not been started, returns true, and ownership of runnable is transferred to the caller (even when {runnable->autoDelete() == true}). Otherwise returns false.

Note

If {runnable->autoDelete() == true}, this function may remove the wrong runnable. This is known as the \l{https://en.wikipedia.org/wiki/ABA_problem}{ABA problem}: the original runnable may already have executed and has since been deleted. The memory is re-used for another runnable, which then gets removed instead of the intended one. For this reason, we recommend calling this function only for runnables that are not auto-deleting.

See also

start(), QRunnable::autoDelete()

Definition at line 343 of file qthreadpool.cpp.

References d, and page.

waitForDone()

bool QThreadPool::waitForDone

(

int

msecs = -1

)

Waits up to msecs milliseconds for all threads to exit and removes all threads from the thread pool.

Returns true if all threads were removed; otherwise it returns false. If msecs is -1 (the default), the timeout is ignored (waits for the last thread to exit).

Definition at line 814 of file qthreadpool.cpp.

References d.

Referenced by QCoreApplication::~QCoreApplication(), and ~QThreadPool().

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Friends And Related Symbol Documentation

QFutureInterfaceBase

friend class QFutureInterfaceBase

friend

Definition at line 28 of file qthreadpool.h.

Property Documentation

activeThreadCount

QThreadPool::activeThreadCount

read

the number of active threads in the thread pool.

Note

It is possible for this function to return a value that is greater than maxThreadCount(). See reserveThread() for more details.

See also

reserveThread(), releaseThread()

Definition at line 104 of file qthreadpool.h.

expiryTimeout

QThreadPool::expiryTimeout

readwrite

the thread expiry timeout value in milliseconds.

Threads that are unused for expiryTimeout milliseconds are considered to have expired and will exit. Such threads will be restarted as needed. The default expiryTimeout is 30000 milliseconds (30 seconds). If expiryTimeout is negative, newly created threads will not expire, e.g., they will not exit until the thread pool is destroyed.

Note that setting expiryTimeout has no effect on already running threads. Only newly created threads will use the new expiryTimeout. We recommend setting the expiryTimeout immediately after creating the thread pool, but before calling start().

Definition at line 104 of file qthreadpool.h.

Referenced by setExpiryTimeout().

maxThreadCount

QThreadPool::maxThreadCount

readwrite

the maximum number of threads used by the thread pool.

This property will default to the value of QThread::idealThreadCount() at the moment the QThreadPool object is created.

Note

The thread pool will always use at least 1 thread, even if maxThreadCount limit is zero or negative.

The default maxThreadCount is QThread::idealThreadCount().

Definition at line 104 of file qthreadpool.h.

Referenced by setMaxThreadCount().

stackSize

QThreadPool::stackSize

readwrite

the stack size for the thread pool worker threads.

The value of the property is only used when the thread pool creates new threads. Changing it has no effect for already created or running threads.

The default value is 0, which makes QThread use the operating system default stack size.

Since

5.10

Definition at line 104 of file qthreadpool.h.

Referenced by setStackSize().

threadPriority

QThreadPool::threadPriority

readwrite

the thread priority for new worker threads.

The value of the property is only used when the thread pool starts new threads. Changing it has no effect for already running threads.

The default value is QThread::InheritPriority, which makes QThread use the same priority as the one the QThreadPool object lives in.

See also

QThread::Priority

Since

6.2

Definition at line 104 of file qthreadpool.h.

---

The documentation for this class was generated from the following files:

• qtbase/src/corelib/thread/qthreadpool.h
• qtbase/src/corelib/doc/snippets/code/src_corelib_concurrent_qthreadpool.cpp
• qtbase/src/corelib/thread/qthreadpool.cpp