31 Aug 2012

Objects and Threads in C++ and Qt (Part 2)

Last time I explained how plain C++ objects and their methods interact with threads. This time around, we will look at QObjects, and their “thread affinity”.

Now that we know object methods can be accessed by any thread at any time, we’ll consider the situation from the point of view of Qt.

What are QObjects?

Qt is a great framework, and at its heart are QObjects. Through Qt’s moc compiler extra functionality is seemlessly added to C++ objects. The two most notable additions are signals and slots for inter-object communication, and events. These allow great flexibility and modularity, but how do they work in the context of threads?

For example, if an event or slot is triggered in a QObject (which ultimately triggers a function), which thread is calling that function? The answer lies in thread affinity. But let’s back up a bit.

Qt threads and Event loops

Having the ability to asynchronously trigger functions, and raise/handle events means that Qt must have some kind of an event loop. An event loop will continually monitor a queue of events to be handled, and dispatch them accordingly. Indeed, every QThread has a built-in event loop that can be entered.

One way to see this directly is by inheriting from QThread:

class MyThread : public QThread {


    virtual void QThread::run() {
        // custom initialization code

        exec(); // start event loop!

The above is a good example for demonstration, but is rarely done in production. We will see a better way to run custom code on QThreads in the next section.

In particular the GUI thread (the main thread), also has an event loop which is launched by calling QApplication::exec(), which only returns after the user has quit the program.

int main(int argc, char **argv)
    QApplication app(argc, argv);
    QWidget myWidget;

    return app.exec(); // starts main event loop

So far the most important thing to remember is:

Threads in Qt handle asynchronous events, and thus all have an event-loop.

QObjects and QThreads

Now we come to the meat of this post- if C++ objects can be accessed by any thread, then what thread is handling the events of a particular QObject? The answer is that whenever a QObject is created, it is assigned a parent thread which handles all of it’s events and slot invocations- it has a thread affinity. Whichever thread it was created in, becomes it’s parent thread!

This is where the confusion came about for me. On the one hand C++ object methods can be called from any thread at any time, while QObjects (themselves C++ objects) have a parent thread which handles its events. As you can hopefully see, there is no conflict:

QObject methods can be called from any thread at any time, just like a C++ object. In addition, a parent thread is assigned to handle any asynchronous events and slot invocations.

So there are two ways for a function to be called on a QObject:

  • Directly from any thread
  • Indirectly by invoking a connected slot or raising an event. This posts an event onto the parent thread’s event loop, which eventually calls the function in question.

Two threads with an object assigned to each thread’s event loop. Each event loop handles events by invoking corresponding functions on the object. Notice how another object’s methods can still be called from another thread directly. (calling userFunction)

To complete this article, let’s look at running our code on other threads. As promised before we will not inherit from QThread for the job. If we can’t customize a thread, and QObjects are bound to the thread that created them, how can we achieve this? Qt allows users to move QObjects to other threads, thereby changing the thread affinity to the new thread:

// create a simple thread
QThread* newThread = new QThread();

// create your own object 
CustomQObject* obj = new CustomQObject();

// let the new thread handle its events

// start new thread's event loop

// continue to do other stuff on this thread

This is much simpler and easier to follow than subclassing a QThread each time you want to create a worker thread. Thanks to Jurily for suggesting this in a reddit comment.

I hope you enjoyed this simplified rundown of QObjects and threads! More in-depth documentation can be found on the Qt-project website.

  • c++
  • qt
  • threading