Qt5 Tutorial QThreads Introduction - 2020

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QThreads

In this tutorial, we will learn about QThreads.

GUI Thread and Worker Thread

Each program has one thread when it is started. This thread is called the main thread or GUI thread in Qt applications. The Qt GUI must run in this thread. All widgets and several related classes, for example QPixmap, don't work in secondary threads. A secondary thread is commonly referred to as a worker thread because it is used to offload processing work from the main thread.

Simultaneous Access to Data

If two threads have a pointer to the same object, it is possible that both threads will access that object at the same time and this can potentially destroy the object's integrity. Simultaneous execution of operations by different threads on one object must be prevented.

Use Cases

There are basically two use cases for threads:

Make processing faster by making use of multicore processors.
Keep the GUI thread or other time critical threads responsive by offloading long lasting processing or blocking calls to other threads.

When to Use Alternatives to Threads

Before creating threads to solve certain problems, possible alternatives should be considered.

The following table is borrowed from Threading Basics.

Alternative	Description
QEventLoop::processEvents()	Calling QEventLoop::processEvents() repeatedly during a time-consuming calculation prevents GUI blocking. However, this solution doesn't scale well because the call to processEvents() may occur too often, or not often enough, depending on hardware.
QSocketNotifier QNetworkAccessManager QIODevice::readyRead()	This is an alternative to having one or multiple threads, each with a blocking read on a slow network connection. As long as the calculation in response to a chunk of network data can be executed quickly, this reactive design is better than synchronous waiting in threads. Reactive design is less error prone and energy efficient than threading. In many cases there are also performance benefits.
QTimer	Background processing can sometimes be done conveniently using a timer to schedule execution of a slot at some point in the future. A timer with an interval of 0 will time out as soon as there are no more events to process.

Which Qt Thread?

The following table is borrowed from Threading Basics.

Thread Lifetime	Development task	Solution
One call	Run one method within another thread and quit the thread when the method is finished.	Qt provides different solutions: Write a function and run it with QtConcurrent::run() Derive a class from QRunnable and run it in the global thread pool with QThreadPool::globalInstance()->start() Derive a class from QThread, reimplement the QThread::run() method and use QThread::start() to run it.
One call	A long running operation has to be put in another thread. During the course of processing, status information should be sent to the GUI thread.	Use QThread, reimplement run and emit signals as needed. Connect the signals to the GUI thread's slots using queued signal/slot connections.
One call	Operations are to be performed on all items of a container. Processing should be performed using all available cores. A common example is to produce thumbnails from a list of images.	QtConcurrent provides the map() function for applying operations on every container element, filter() for selecting container elements, and the option of specifying a reduce function for combining the remaining elements.
Permanent	Have an object living in another thread and let it perform different tasks upon request. This means communication to and from the worker thread is required.	Derive a class from QObject and implement the necessary slots and signals, move the object to a thread with a running event loop and communicate with the object over queued signal/slot connections.
Permanent	Have an object living in another thread, let the object perform repeated tasks such as polling a port and enable communication with the GUI thread.	Same as above but also use a timer in the worker thread to implement polling. However, the best solution for polling is to avoid it completely. Sometimes using QSocketNotifier is an alternative.