Qt events and signal/slots

In Qt, signals and events are both implementations of the Observer pattern. They are used in different situations because they have different strengths and weaknesses.

First of all let's define what we mean by 'Qt event' exactly: a virtual function in a Qt class, which you're expected to reimplement in a base class of yours if you want to handle the event. It's related to the Template Method pattern.

Note how I used the word "handle". Indeed, here's a basic difference between the intent of signals and events:

• You "handle" events
• You "get notified of" signal emissions

The difference is that when you "handle" the event, you take on the responsibility to "respond" with a behavior that is useful outside the class. For example, consider an app that has a button with a number on it. The app needs to let the user focus the button and change the number by pressing the "up" and "down" keyboard keys. Otherwise the button should function like a normal QPushButton (it can be clicked, etc). In Qt this is done by creating your own little reusable "component" (subclass of QPushButton), which reimplements QWidget::keyPressEvent. Pseudocode:

class NumericButton extends QPushButton
    private void addToNumber(int value):
        // ...

    reimplement base.keyPressEvent(QKeyEvent event):
        if(event.key == up)
            this.addToNumber(1)
        else if(event.key == down)
            this.addToNumber(-1)
        else
            base.keyPressEvent(event)

See? This code presents a new abstraction: a widget that acts like a button, but with some extra functionality. We added this functionality very conveniently:

• Since we reimplemented a virtual, our implementation automatically became encapsulated in our class. If Qt's designers had made keyPressEvent a signal, we would need to decide whether to inherit QPushButton or just externally connect to the signal. But that would be stupid, since in Qt you're always expected to inherit when writing a widget with a custom behavior (for good reason - reusability/modularity). So by making keyPressEvent an event, they convey their intent that keyPressEvent is just a basic building block of functionality. If it were a signal, it'd look like a user-facing thing, when it's not intended to be.
• Since the base-class-implementation of the function is available, we easily implement the Chain-of-responsibility pattern by handling our special cases (up&down keys) and leaving the rest to the base class. You can see this would be nearly impossible if keyPressEvent were a signal.

The design of Qt is well thought out - they made us fall into the pit of success by making it easy to do the right thing and hard to do the wrong thing (by making keyPressEvent an event).

On the other hand, consider the simplest usage of QPushButton - just instantiating it and getting notified when it's clicked:

button = new QPushButton(this)
connect(button, SIGNAL(clicked()), SLOT(sayHello())

This is clearly meant to be done by the user of the class:

• if we had to subclass QPushButton every time we want some button to notify us of a click, that would require a lot of subclasses for no good reason! A widget that always shows a "Hello world" messagebox when clicked is useful only in a single case - so it's totally not reusable. Again, we have no choice but to do the right thing - by connecting to it externally.
• we may want to connect several slots to clicked() - or connect several signals to sayHello(). With signals there is no fuss. With subclassing you would have to sit down and ponder some class diagrams until you decide on an appropriate design.

Note that one of the places QPushButton emits clicked() is in its mousePressEvent() implementation. That doesn't mean clicked() and mousePressEvent() are interchangable - just that they're related.

So signals and events have different purposes (but are related in that both let you "subscribe" to a notification of something happening).

I don’t like the answers so far. – Let me concentrate on this part of the question:

Are events an abstraction of signal/slots?

Short answer: no. The long answer raises a “better” question: How are signals and events related?

An idle main loop (Qt’s for example) is usually “stuck” in a select() call of the operating system. That call makes the application “sleep”, while it passes a bunch of sockets or files or whatever to the kernel asking for: if something changes on these, let the select() call return. – And the kernel, as the master of the world, knows when that happens.

The result of that select() call could be: new data on the socket connect to X11, a packet to a UDP port we listen on came in, etc. – That stuff is neither a Qt signal, nor a Qt event, and the Qt main loop decides itself if it turns the fresh data into the one, the other or ignores it.

Qt could call a method (or several) like keyPressEvent(), effectively turning it into a Qt event. Or Qt emits a signal, which in effect looks up all functions registered for that signal, and calls them one after the other.

One difference of those two concepts is visible here: a slot has no vote on whether other slots registered to that signal will get called or not. – Events are more like a chain, and the event handler decides if it interrupts that chain or not. Signals look like a star or tree in this respect.

An event can trigger or be entirely turned into a signal (just emit one, and don’t call “super()”). A signal can be turned into an event (call an event handler).

What abstracts what depends on the case: the clicked()-signal abstracts mouse events (a button goes down and up again without too much moving around). Keyboard events are abstractions from lower levels (things like 果 or é are several key strokes on my system).

Maybe the focusInEvent() is an example of the opposite: it could use (and thus abstract) the clicked() signal, but I don’t know if it actually does.

The Qt documentation probably explains it best:

In Qt, events are objects, derived from the abstract QEvent class, that represent things that have happened either within an application or as a result of outside activity that the application needs to know about. Events can be received and handled by any instance of a QObject subclass, but they are especially relevant to widgets. This document describes how events are delivered and handled in a typical application.

So events and signal/slots are two parallel mechanisms accomplishing the same things. In general, an event will be generated by an outside entity (for example, keyboard or mouse wheel) and will be delivered through the event loop in QApplication. In general, unless you set up the code, you will not be generating events. You might filter them through QObject::installEventFilter() or handle events in subclassed object by overriding the appropriate functions.

Signals and Slots are much easier to generate and receive and you can connect any two QObject subclasses. They are handled through the Metaclass (have a look at your moc_classname.cpp file for more), but most of the interclass communication that you will produce will probably use signals and slots. Signals can get delivered immediately or deferred via a queue (if you are using threads).

A signal can be generated.

Events are dispatched by the event loop. Each GUI program needs an event loop, whatever you write it Windows or Linux, using Qt, Win32 or any other GUI library. As well each thread has its own event loop. In Qt "GUI Event Loop" (which is the main loop of all Qt applications) is hidden, but you start it calling:

QApplication a(argc, argv);
return a.exec();

Messages OS and other applications send to your program are dispatched as events.

Signals and slots are Qt mechanisms. In the process of compilations using moc (meta-object compiler), they are changed to callback functions.

Event should have one receiver, which should dispatch it. No one else should get that event.

All slots connected to the emitted signal will be executed.

You shouldn't think of Signals as events, because as you can read in the Qt documentation:

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop.

When you send an event, it must wait for some time until the event loop dispatches all events that came earlier. Because of this, execution of the code after sending event or signal is different. Code following sending an event will be run immediately. With the signals and slots mechanisms it depends on the connection type. Normally it will be executed after all slots. Using Qt::QueuedConnection, it will be executed immediately, just like events. Check all connection types in the Qt documentation.

There is an article that discusses event processing in some detail: http://www.packtpub.com/article/events-and-signals

It discussions the difference between events and signals here:

Events and signals are two parallel mechanisms used to accomplish the same thing. As a general difference, signals are useful when using a widget, whereas events are useful when implementing the widget. For example, when we are using a widget like QPushButton, we are more interested in its clicked() signal than in the low-level mouse press or key press events that caused the signal to be emitted. But if we are implementing the QPushButton class, we are more interested in the implementation of code for mouse and key events. Also, we usually handle events but get notified by signal emissions.

This seems to be a common way of talking about it, as the accepted answer uses some of the same phrases.

Note, please see helpful comments below on this answer from Kuba Ober, that make me wonder if it might be a bit simplistic.