- Qt 6.8
- Qt Core
- C++ Classes
- QThreadPool
- Qt 6.8.1 Reference Documentation

# QThreadPool Class

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

Header: | `#include <QThreadPool>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`
Inherits: | QObject

- List of all members, including inherited members

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

## Properties

- activeThreadCount : const int
- expiryTimeout : int
- maxThreadCount : int

- stackSize : uint
- `(since 6.2)` threadPriority : QThread::Priority

## Public Functions

| QThreadPool(QObject *parent = nullptr)
virtual | ~QThreadPool()
int | activeThreadCount() const
void | clear()
`(since 6.0)` 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)
void | setThreadPriority(QThread::Priority priority)
uint | stackSize() const
void | start(QRunnable *runnable, int priority = 0)
void | start(Callable &&callableToRun, int priority = 0)
`(since 6.3)` void | startOnReservedThread(QRunnable *runnable)
`(since 6.3)` void | startOnReservedThread(Callable &&callableToRun)
QThread::Priority | threadPriority() const
bool | tryStart(QRunnable *runnable)
bool | tryStart(Callable &&callableToRun)
bool | tryTake(QRunnable *runnable)
`(since 6.8)` bool | waitForDone(QDeadlineTimer deadline = QDeadlineTimer::Forever)
bool | waitForDone(int msecs)

## Static Public Members

QThreadPool * | globalInstance()

## Detailed Description

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.

## Property Documentation

### `[read-only]` activeThreadCount : const int

This property holds 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

This property holds 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().

Access functions:

int | expiryTimeout() const
void | setExpiryTimeout(int expiryTimeout)

### maxThreadCount : int

This property holds 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().

Access functions:

int | maxThreadCount() const
void | setMaxThreadCount(int maxThreadCount)

### stackSize : uint

This property holds 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.

Access functions:

uint | stackSize() const
void | setStackSize(uint stackSize)

### `[since 6.2]` threadPriority : QThread::Priority

This property holds 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.

This property was introduced in Qt 6.2.

Access functions:

QThread::Priority | threadPriority() const
void | setThreadPriority(QThread::Priority priority)

See also QThread::Priority.

## Member Function Documentation

### QThreadPool::QThreadPool(QObject *parent = nullptr)

Constructs a thread pool with the given parent.

### `[virtual noexcept]` 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.

See also start().

### `[since 6.0]` 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: Even if reserving maxThreadCount() threads or more, the thread pool will still allow a minimum of one thread.

Note: 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().

### 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.

### template <typename Callable, QRunnable::if_callable<Callable> = true> void QThreadPool::start(Callable &&callableToRun, int priority = 0)

This is an overloaded function.

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

Note: This function participates in overload resolution only if `Callable` is a function or function object which can be called with zero arguments.

Note: In Qt version prior to 6.6, this function took std::function<void()>, and therefore couldn't handle move-only callables.

### `[since 6.3]` 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 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.

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

This function was introduced in Qt 6.3.

See also reserveThread() and start().

### `[since 6.3]` template <typename Callable, QRunnable::if_callable<Callable> = true> void QThreadPool::startOnReservedThread(Callable &&callableToRun)

This is an overloaded function.

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

Note: This function participates in overload resolution only if `Callable` is a function or function object which can be called with zero arguments.

Note: In Qt version prior to 6.6, this function took std::function<void()>, and therefore couldn't handle move-only callables.

This function was introduced in Qt 6.3.

### 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.

### template <typename Callable, QRunnable::if_callable<Callable> = true> bool QThreadPool::tryStart(Callable &&callableToRun)

This is an overloaded function.

Attempts to reserve a thread to run callableToRun.

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

Note: This function participates in overload resolution only if `Callable` is a function or function object which can be called with zero arguments.

Note: In Qt version prior to 6.6, this function took std::function<void()>, and therefore couldn't handle move-only callables.

### 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.

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

### `[since 6.8]` bool QThreadPool::waitForDone(QDeadlineTimer deadline = QDeadlineTimer::Forever)

Waits until deadline expires for all threads to exit and removes all threads from the thread pool. Returns `true` if all threads were removed; otherwise it returns `false`.

This function was introduced in Qt 6.8.

### bool QThreadPool::waitForDone(int msecs)

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, this function waits for the last thread to exit.