QThreadPool Class

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

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

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


  • 1 property inherited from QObject

Public Functions

QThreadPool(QObject *parent = Q_NULLPTR)
int activeThreadCount() const
void clear()
int expiryTimeout() const
int maxThreadCount() const
void releaseThread()
void reserveThread()
void setExpiryTimeout(int expiryTimeout)
void setMaxThreadCount(int maxThreadCount)
void start(QRunnable *runnable, int priority = 0)
bool tryStart(QRunnable *runnable)
bool tryTake(QRunnable *runnable)
bool waitForDone(int msecs = -1)
  • 32 public functions inherited from QObject

Static Public Members

QThreadPool *globalInstance()
  • 11 static public members inherited from QObject

Additional Inherited Members

  • 1 public slot inherited from QObject
  • 2 signals inherited from QObject
  • 9 protected functions inherited from QObject

Detailed Description

The QThreadPool class manages a collection of QThreads.

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()
        qDebug() << "Hello world from thread" << QThread::currentThread();

HelloWorldTask *hello = new HelloWorldTask();
// QThreadPool takes ownership and deletes 'hello' automatically

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.

Property Documentation

activeThreadCount : const int

This property represents 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.

Access functions:

int activeThreadCount() const

See also reserveThread() and releaseThread().

expiryTimeout : int

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().

Access functions:

int expiryTimeout() const
void setExpiryTimeout(int expiryTimeout)

maxThreadCount : int

This property represents the maximum number of threads used by the thread pool.

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().

Access functions:

int maxThreadCount() const
void setMaxThreadCount(int maxThreadCount)

Member Function Documentation

QThreadPool::QThreadPool(QObject *parent = Q_NULLPTR)

Constructs a thread pool with the given parent.


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().

[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.

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 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::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).

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