QThread Class

class Worker : public QObject
{
    Q_OBJECT

public slots:
    void doWork(const QString &parameter) {
        QString result;
        /* ... here is the expensive or blocking operation ... */
        emit resultReady(result);
    }

signals:
    void resultReady(const QString &result);
};

class Controller : public QObject
{
    Q_OBJECT
    QThread workerThread;
public:
    Controller() {
        Worker *worker = new Worker;
        worker->moveToThread(&workerThread);
        connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
        connect(this, &Controller::operate, worker, &Worker::doWork);
        connect(worker, &Worker::resultReady, this, &Controller::handleResults);
        workerThread.start();
    }
    ~Controller() {
        workerThread.quit();
        workerThread.wait();
    }
public slots:
    void handleResults(const QString &);
signals:
    void operate(const QString &);
};

class WorkerThread : public QThread
{
    Q_OBJECT
    void run() override {
        QString result;
        /* ... here is the expensive or blocking operation ... */
        emit resultReady(result);
    }
signals:
    void resultReady(const QString &s);
};

void MyObject::startWorkInAThread()
{
    WorkerThread *workerThread = new WorkerThread(this);
    connect(workerThread, &WorkerThread::resultReady, this, &MyObject::handleResults);
    connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
    workerThread->start();
}

Member Function Documentation

QThread::QThread(QObject *parent = nullptr)

Constructs a new QThread to manage a new thread. The parent takes ownership of the QThread. The thread does not begin executing until start() is called.

See also start().

[virtual] QThread::~QThread()

Destroys the QThread.

Note that deleting a QThread object will not stop the execution of the thread it manages. Deleting a running QThread (i.e. isFinished() returns false) will result in a program crash. Wait for the finished() signal before deleting the QThread.

[static] QThread *QThread::create(Function &&f, Args &&... args)

Creates a new QThread object that will execute the function f with the arguments args.

The new thread is not started -- it must be started by an explicit call to start(). This allows you to connect to its signals, move QObjects to the thread, choose the new thread's priority and so on. The function f will be called in the new thread.

Returns the newly created QThread instance.

Note: the caller acquires ownership of the returned QThread instance.

Note: this function is only available when using C++17.

Warning: do not call start() on the returned QThread instance more than once; doing so will result in undefined behavior.

This function was introduced in Qt 5.10.

See also start().

[static] QThread *QThread::create(Function &&f)

Creates a new QThread object that will execute the function f.

The new thread is not started -- it must be started by an explicit call to start(). This allows you to connect to its signals, move QObjects to the thread, choose the new thread's priority and so on. The function f will be called in the new thread.

Returns the newly created QThread instance.

Note: the caller acquires ownership of the returned QThread instance.

Warning: do not call start() on the returned QThread instance more than once; doing so will result in undefined behavior.

This function was introduced in Qt 5.10.

See also start().

[static] QThread *QThread::currentThread()

Returns a pointer to a QThread which manages the currently executing thread.

[static] Qt::HANDLE QThread::currentThreadId()

Returns the thread handle of the currently executing thread.

Warning: The handle returned by this function is used for internal purposes and should not be used in any application code.

Warning: On Windows, the returned value is a pseudo-handle for the current thread. It can't be used for numerical comparison. i.e., this function returns the DWORD (Windows-Thread ID) returned by the Win32 function getCurrentThreadId(), not the HANDLE (Windows-Thread HANDLE) returned by the Win32 function getCurrentThread().

[override virtual] bool QThread::event(QEvent *event)

Reimplemented from QObject::event().

QAbstractEventDispatcher *QThread::eventDispatcher() const

Returns a pointer to the event dispatcher object for the thread. If no event dispatcher exists for the thread, this function returns 0.

This function was introduced in Qt 5.0.

See also setEventDispatcher().

[protected] int QThread::exec()

Enters the event loop and waits until exit() is called, returning the value that was passed to exit(). The value returned is 0 if exit() is called via quit().

This function is meant to be called from within run(). It is necessary to call this function to start event handling.

See also quit() and exit().

void QThread::exit(int returnCode = 0)

Tells the thread's event loop to exit with a return code.

After calling this function, the thread leaves the event loop and returns from the call to QEventLoop::exec(). The QEventLoop::exec() function returns returnCode.

By convention, a returnCode of 0 means success, any non-zero value indicates an error.

Note that unlike the C library function of the same name, this function does return to the caller -- it is event processing that stops.

No QEventLoops will be started anymore in this thread until QThread::exec() has been called again. If the eventloop in QThread::exec() is not running then the next call to QThread::exec() will also return immediately.

See also quit() and QEventLoop.

[signal] void QThread::finished()

This signal is emitted from the associated thread right before it finishes executing.

When this signal is emitted, the event loop has already stopped running. No more events will be processed in the thread, except for deferred deletion events. This signal can be connected to QObject::deleteLater(), to free objects in that thread.

Note: If the associated thread was terminated using terminate(), it is undefined from which thread this signal is emitted.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also started().

[static] int QThread::idealThreadCount()

Returns the ideal number of threads that can be run on the system. This is done querying the number of processor cores, both real and logical, in the system. This function returns 1 if the number of processor cores could not be detected.

bool QThread::isFinished() const

Returns true if the thread is finished; otherwise returns false.

See also isRunning().

bool QThread::isInterruptionRequested() const

Return true if the task running on this thread should be stopped. An interruption can be requested by requestInterruption().

This function can be used to make long running tasks cleanly interruptible. Never checking or acting on the value returned by this function is safe, however it is advisable do so regularly in long running functions. Take care not to call it too often, to keep the overhead low.

void long_task() {
     forever {
        if ( QThread::currentThread()->isInterruptionRequested() ) {
            return;
        }
    }
}

This function was introduced in Qt 5.2.

See also currentThread() and requestInterruption().

bool QThread::isRunning() const

Returns true if the thread is running; otherwise returns false.

See also isFinished().

int QThread::loopLevel() const

Returns the current event loop level for the thread.

Note: This can only be called within the thread itself, i.e. when it is the current thread.

This function was introduced in Qt 5.5.

[static] void QThread::msleep(unsigned long msecs)

Forces the current thread to sleep for msecs milliseconds.

See also sleep() and usleep().

QThread::Priority QThread::priority() const

Returns the priority for a running thread. If the thread is not running, this function returns InheritPriority.

This function was introduced in Qt 4.1.

See also Priority, setPriority(), and start().

[slot] void QThread::quit()

Tells the thread's event loop to exit with return code 0 (success). Equivalent to calling QThread::exit(0).

This function does nothing if the thread does not have an event loop.

See also exit() and QEventLoop.

void QThread::requestInterruption()

Request the interruption of the thread. That request is advisory and it is up to code running on the thread to decide if and how it should act upon such request. This function does not stop any event loop running on the thread and does not terminate it in any way.

This function was introduced in Qt 5.2.

See also isInterruptionRequested().

[virtual protected] void QThread::run()

The starting point for the thread. After calling start(), the newly created thread calls this function. The default implementation simply calls exec().

You can reimplement this function to facilitate advanced thread management. Returning from this method will end the execution of the thread.

See also start() and wait().

void QThread::setEventDispatcher(QAbstractEventDispatcher *eventDispatcher)

Sets the event dispatcher for the thread to eventDispatcher. This is only possible as long as there is no event dispatcher installed for the thread yet. That is, before the thread has been started with start() or, in case of the main thread, before QCoreApplication has been instantiated. This method takes ownership of the object.

This function was introduced in Qt 5.0.

See also eventDispatcher().

void QThread::setPriority(QThread::Priority priority)

This function sets the priority for a running thread. If the thread is not running, this function does nothing and returns immediately. Use start() to start a thread with a specific priority.

The priority argument can be any value in the QThread::Priority enum except for InheritPriorty.

The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see http://linux.die.net/man/2/sched_setscheduler for more details).

This function was introduced in Qt 4.1.

See also Priority, priority(), and start().

void QThread::setStackSize(uint stackSize)

Sets the maximum stack size for the thread to stackSize. If stackSize is greater than zero, the maximum stack size is set to stackSize bytes, otherwise the maximum stack size is automatically determined by the operating system.

Warning: Most operating systems place minimum and maximum limits on thread stack sizes. The thread will fail to start if the stack size is outside these limits.

See also stackSize().

[static protected] void QThread::setTerminationEnabled(bool enabled = true)

Enables or disables termination of the current thread based on the enabled parameter. The thread must have been started by QThread.

When enabled is false, termination is disabled. Future calls to QThread::terminate() will return immediately without effect. Instead, the termination is deferred until termination is enabled.

When enabled is true, termination is enabled. Future calls to QThread::terminate() will terminate the thread normally. If termination has been deferred (i.e. QThread::terminate() was called with termination disabled), this function will terminate the calling thread immediately. Note that this function will not return in this case.

See also terminate().

[static] void QThread::sleep(unsigned long secs)

Forces the current thread to sleep for secs seconds.

See also msleep() and usleep().

uint QThread::stackSize() const

Returns the maximum stack size for the thread (if set with setStackSize()); otherwise returns zero.

See also setStackSize().

[slot] void QThread::start(QThread::Priority priority = InheritPriority)

Begins execution of the thread by calling run(). The operating system will schedule the thread according to the priority parameter. If the thread is already running, this function does nothing.

The effect of the priority parameter is dependent on the operating system's scheduling policy. In particular, the priority will be ignored on systems that do not support thread priorities (such as on Linux, see the sched_setscheduler documentation for more details).

See also run() and terminate().

[signal] void QThread::started()

This signal is emitted from the associated thread when it starts executing, before the run() function is called.

Note: This is a private signal. It can be used in signal connections but cannot be emitted by the user.

See also finished().

[slot] void QThread::terminate()

Terminates the execution of the thread. The thread may or may not be terminated immediately, depending on the operating system's scheduling policies. Use QThread::wait() after terminate(), to be sure.

When the thread is terminated, all threads waiting for the thread to finish will be woken up.

Warning: This function is dangerous and its use is discouraged. The thread can be terminated at any point in its code path. Threads can be terminated while modifying data. There is no chance for the thread to clean up after itself, unlock any held mutexes, etc. In short, use this function only if absolutely necessary.

Termination can be explicitly enabled or disabled by calling QThread::setTerminationEnabled(). Calling this function while termination is disabled results in the termination being deferred, until termination is re-enabled. See the documentation of QThread::setTerminationEnabled() for more information.

See also setTerminationEnabled().

[static] void QThread::usleep(unsigned long usecs)

Forces the current thread to sleep for usecs microseconds.

See also sleep() and msleep().

bool QThread::wait(unsigned long time = ...)

Blocks the thread until either of these conditions is met:

• The thread associated with this QThread object has finished execution (i.e. when it returns from run()). This function will return true if the thread has finished. It also returns true if the thread has not been started yet.
• time milliseconds has elapsed. If time is ULONG_MAX (the default), then the wait will never timeout (the thread must return from run()). This function will return false if the wait timed out.

This provides similar functionality to the POSIX pthread_join() function.

See also sleep() and terminate().

[static] void QThread::yieldCurrentThread()

Yields execution of the current thread to another runnable thread, if any. Note that the operating system decides to which thread to switch.