QThreadPool Class

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);

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().

[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).