The QThreadPool class manages a collection of QThreads. More...

#include <qthreadpool.h>

Inheritance diagram for QThreadPool:

Collaboration diagram for QThreadPool:

Public Member Functions
	QThreadPool (QObject *parent=nullptr)

	~QThreadPool ()

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

bool	tryStart (QRunnable *runnable)

void	start (std::function< void()> functionToRun, int priority=0)

bool	tryStart (std::function< void()> functionToRun)

void	startOnReservedThread (QRunnable *runnable)

void	startOnReservedThread (std::function< void()> 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 ()

void	releaseThread ()

bool	waitForDone (int msecs=-1)

void	clear ()

bool	contains (const QThread *thread) const

bool	tryTake (QRunnable *runnable)

Public Member Functions inherited from QObject
Q_INVOKABLE	QObject (QObject *parent=nullptr)

virtual	~QObject ()

virtual bool	event (QEvent *event)

virtual bool	eventFilter (QObject watched, QEvent event)

QString	objectName () const

void	setObjectName (const QString &name)

QBindable< QString >	bindableObjectName ()

bool	isWidgetType () const

bool	isWindowType () const

bool	signalsBlocked () const noexcept

bool	blockSignals (bool b) noexcept

QThread *	thread () const

void	moveToThread (QThread *thread)

int	startTimer (int interval, Qt::TimerType timerType=Qt::CoarseTimer)

void	killTimer (int id)

template<typename T >
T	findChild (const QString &aName=QString(), Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

template<typename T >
QList< T >	findChildren (const QString &aName, Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

template<typename T >
QList< T >	findChildren (Qt::FindChildOptions options=Qt::FindChildrenRecursively) const

const QObjectList &	children () const

void	setParent (QObject *parent)

void	installEventFilter (QObject *filterObj)

void	removeEventFilter (QObject *obj)

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

void	dumpObjectInfo () const

bool	setProperty (const char *name, const QVariant &value)

QVariant	property (const char *name) const

QList< QByteArray >	dynamicPropertyNames () const

QBindingStorage *	bindingStorage ()

const QBindingStorage *	bindingStorage () const

QObject *	parent () const

bool	inherits (const char *classname) const

Static Public Member Functions
static QThreadPool *	globalInstance ()

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)

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::FunctionPointer< Func2 >::Object receiver, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

static QMetaObject::Connection ::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 slot)

static QMetaObject::Connection ::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const QObject context, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

template<typename Func1 , typename Func2 >
static std::enable_if< QtPrivate::FunctionPointer< Func2 >::ArgumentCount==-1 &&!std::is_convertible_v< Func2, const char * >, QMetaObject::Connection >::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object *sender, Func1 signal, Func2 slot)

template<typename Func1 , typename Func2 >
static std::enable_if< QtPrivate::FunctionPointer< Func2 >::ArgumentCount==-1 &&!std::is_convertible_v< Func2, const char * >, QMetaObject::Connection >::type	connect (const typename QtPrivate::FunctionPointer< Func1 >::Object sender, Func1 signal, const QObject context, Func2 slot, Qt::ConnectionType type=Qt::AutoConnection)

static bool	disconnect (const QObject sender, const char signal, const QObject receiver, const char member)

static bool	disconnect (const QObject sender, const QMetaMethod &signal, const QObject receiver, const QMetaMethod &member)

static bool	disconnect (const QMetaObject::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. More...

int	maxThreadCount
	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. More...

int	activeThreadCount
	the number of active threads in the thread pool. More...

uint	stackSize
	the stack size for the thread pool worker threads. More...

QThread::Priority	threadPriority
	the thread priority for new worker threads. More...

Properties inherited from QObject
QString	objectName
	the name of this object More...

Friends
class	QFutureInterfaceBase

Additional Inherited Members
Public Slots inherited from QObject
void	deleteLater ()

Signals inherited from QObject
void	destroyed (QObject *=nullptr)

void	objectNameChanged (const QString &objectName, QPrivateSignal)

Protected Member Functions inherited from QObject
QObject *	sender () const

int	senderSignalIndex () const

int	receivers (const char *signal) const

bool	isSignalConnected (const QMetaMethod &signal) const

virtual void	timerEvent (QTimerEvent *event)

virtual void	childEvent (QChildEvent *event)

virtual void	customEvent (QEvent *event)

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 Functions inherited from QObject
template< class T > T	qobject_cast (const QObject *object)

template< typename T > T	qFindChildqFindChildren (const QObject *obj, const QString &name)()

template< typename T > QList< T >	qFindChildrenqFindChildren (const QObject *obj, const QString &name)()

	QObjectList

Detailed Description

The QThreadPool class manages a collection of QThreads.

\inmodule QtCore

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().

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 55 of file qthreadpool.h.

Constructor & Destructor Documentation

◆ QThreadPool()

QThreadPool::QThreadPool ( QObject * parent = nullptr )

Constructs a thread pool with the given parent.

Definition at line 474 of file qthreadpool.cpp.

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◆ ~QThreadPool()

QThreadPool::~QThreadPool ( )

Destroys the QThreadPool. This function will block until all runnables have been completed.

Definition at line 489 of file qthreadpool.cpp.

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Member Function Documentation

◆ activeThreadCount()

int QThreadPool::activeThreadCount ( ) const

Definition at line 678 of file qthreadpool.cpp.

◆ 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 849 of file qthreadpool.cpp.

◆ 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 860 of file qthreadpool.cpp.

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◆ expiryTimeout()

int QThreadPool::expiryTimeout ( ) const

Definition at line 624 of file qthreadpool.cpp.

◆ globalInstance()

QThreadPool * QThreadPool::globalInstance ( )

static

Returns the global QThreadPool instance.

Definition at line 500 of file qthreadpool.cpp.

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◆ maxThreadCount()

int QThreadPool::maxThreadCount ( ) const

Definition at line 650 of file qthreadpool.cpp.

◆ 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 769 of file qthreadpool.cpp.

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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 700 of file qthreadpool.cpp.

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◆ setExpiryTimeout()

void QThreadPool::setExpiryTimeout ( int expiryTimeout )

Definition at line 630 of file qthreadpool.cpp.

◆ setMaxThreadCount()

void QThreadPool::setMaxThreadCount ( int maxThreadCount )

Definition at line 656 of file qthreadpool.cpp.

◆ setStackSize()

void QThreadPool::setStackSize ( uint stackSize )

Definition at line 719 of file qthreadpool.cpp.

◆ setThreadPriority()

void QThreadPool::setThreadPriority ( QThread::Priority priority )

Definition at line 745 of file qthreadpool.cpp.

◆ stackSize()

uint QThreadPool::stackSize ( ) const

Definition at line 725 of file qthreadpool.cpp.

◆ start() [1/2]

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 526 of file qthreadpool.cpp.

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◆ start() [2/2]

void QThreadPool::start	(	std::function< void()>	functionToRun,
		int	priority = `0`
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Since: 5.15

Reserves a thread and uses it to run functionToRun, unless this thread will make the current thread count exceed maxThreadCount(). In that case, functionToRun is added to a run queue instead. The priority argument can be used to control the run queue's order of execution.

Definition at line 547 of file qthreadpool.cpp.

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◆ startOnReservedThread() [1/2]

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 796 of file qthreadpool.cpp.

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◆ startOnReservedThread() [2/2]

void QThreadPool::startOnReservedThread ( std::function< void()> functionToRun )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Since: 6.3

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

Definition at line 820 of file qthreadpool.cpp.

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◆ threadPriority()

QThread::Priority QThreadPool::threadPriority ( ) const

Definition at line 751 of file qthreadpool.cpp.

◆ tryStart() [1/2]

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 570 of file qthreadpool.cpp.

◆ tryStart() [2/2]

bool QThreadPool::tryStart ( std::function< void()> functionToRun )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Since: 5.15 Attempts to reserve a thread to run functionToRun.

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

Definition at line 592 of file qthreadpool.cpp.

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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 377 of file qthreadpool.cpp.

◆ 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 834 of file qthreadpool.cpp.

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Friends And Related Function Documentation

◆ QFutureInterfaceBase

friend class QFutureInterfaceBase

friend

Definition at line 64 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 107 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 107 of file qthreadpool.h.

◆ 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 107 of file qthreadpool.h.

◆ 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 107 of file qthreadpool.h.

◆ 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 107 of file qthreadpool.h.

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

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

Public Member Functions

Static Public Member Functions

Properties

Friends

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ QThreadPool()

◆ ~QThreadPool()

Member Function Documentation

◆ activeThreadCount()

◆ clear()

◆ contains()

◆ expiryTimeout()

◆ globalInstance()

◆ maxThreadCount()

◆ releaseThread()

◆ reserveThread()

◆ setExpiryTimeout()

◆ setMaxThreadCount()

◆ setStackSize()

◆ setThreadPriority()

◆ stackSize()

◆ start() [1/2]

◆ start() [2/2]

◆ startOnReservedThread() [1/2]

◆ startOnReservedThread() [2/2]

◆ threadPriority()

◆ tryStart() [1/2]

◆ tryStart() [2/2]

◆ tryTake()

◆ waitForDone()

Friends And Related Function Documentation

◆ QFutureInterfaceBase

Property Documentation

◆ activeThreadCount

◆ expiryTimeout

◆ maxThreadCount

◆ stackSize

◆ threadPriority