QThreadPool Class

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

Header:	#include <QThreadPool>
qmake:	QT += core
Since:	Qt 4.4
Inherits:	QObject

This class was introduced in Qt 4.4.

• List of all members, including inherited members
• Obsolete members

Note: All functions in this class are thread-safe.

Properties

• activeThreadCount : const int
• expiryTimeout : int
• maxThreadCount : int
• stackSize : uint

Public Functions

	QThreadPool(QObject *parent = nullptr)
virtual	~QThreadPool()
int	activeThreadCount() const
void	clear()
bool	contains(const QThread *thread) const
int	expiryTimeout() const
int	maxThreadCount() const
void	releaseThread()
void	reserveThread()
void	setExpiryTimeout(int expiryTimeout)
void	setMaxThreadCount(int maxThreadCount)
void	setStackSize(uint stackSize)
uint	stackSize() const
void	start(QRunnable *runnable, int priority = 0)
void	start(std::function<void ()> functionToRun, int priority = 0)
bool	tryStart(QRunnable *runnable)
bool	tryStart(std::function<void ()> functionToRun)
bool	tryTake(QRunnable *runnable)
bool	waitForDone(int msecs = -1)

Static Public Members

QThreadPool *

globalInstance()

Detailed Description

QThreadPool manages and recyles 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.

Member Function Documentation

QThreadPool::QThreadPool(QObject *parent = nullptr)

Constructs a thread pool with the given parent.

[virtual] QThreadPool::~QThreadPool()

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

void QThreadPool::clear()

Removes the runnables that are not yet started from the queue. The runnables for which runnable->autoDelete() returns true are deleted.

This function was introduced in Qt 5.2.

See also start().

bool QThreadPool::contains(const QThread *thread) const

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

This function was introduced in Qt 6.0.

[static] QThreadPool *QThreadPool::globalInstance()

Returns the global QThreadPool instance.

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().

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: This function will always increase the 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().

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 runnable->autoDelete() returns true, and the runnable will be deleted automatically by the thread pool after the runnable->run() returns. If 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.

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

This is an overloaded function.

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.

This function was introduced in Qt 5.15.

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 runnable->autoDelete() returns true, and the runnable will be deleted automatically by the thread pool after the runnable->run() returns. If 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.

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

This is an overloaded function.

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.

This function was introduced in Qt 5.15.

bool QThreadPool::tryTake(QRunnable *runnable)

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 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.

This function was introduced in Qt 5.9.

See also start() and QRunnable::autoDelete().

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).