What Are Signals and Slots in Qt?

In Qt, signals and slots are used for communication between objects. The signal/slot mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.

Signals and slots are very powerful and enable the objects to communicate with each other in a loosely coupled way. They also allow you to dynamically connect and disconnect objects at runtime.

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A signal is emitted when a particular event occurs. For example, a button would emit a clicked() signal when it is clicked.

A slot is a function that is called in response to a particular signal. So, in the above example, we could connect the clicked() signal from the button to a slot that performs some action, like quitting the application.

Signals and slots are usually used together, but they can also be used independently. Signals can be connected to any number of slots, and they can be emitted even if there are no slots connected to them.

This enables you to do things like log messages whenever a signal is emitted, even if there is no slot that specifically handles that signal.

Slots can also be connected to signals from other objects. So, in our example, we could connect the clicked() signal from the button to a slot on some other object, like a main window, and have that slot perform some action when the button is clicked.

The power of signals and slots lies in their flexibility: you can connect any signal to any slot, regardless of whether the slot makes sense for that particular signal or not. Of course, this means that you have to be careful when using signals and slots; if you connect a signal to a slot that doesn’t make sense for that signal, you’ll probably get unexpected results.

In general, signals and slots are used for two different purposes: communication between objects (which we’ve already seen), and communication with the outside world (which we’ll see in the next section).