How Qt Signals and Slots Work?

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 other toolkits.

Signals and slots are very powerful and they are core to how Qt works. But, at the same time, they are easy to use. Let’s start with a very simple example:

We have a button with the text “Click me” and we want to connect it to a slot that will be called when the button is clicked. In code, this looks like this:

button->setText(“Click me”);
connect(button, SIGNAL(clicked()), this, SLOT(buttonClicked()));

When the user clicks the button, the buttonClicked() slot will be called. We can also do this in Qt Designer.

Select the button and go to the “Signals & Slots” tab in theProperty Editor. Click on the “clicked()” signal and drag it to your form:.

This will automatically generate the code for you. Now let’s take a look at how signals and slots work under the hood.

When you connect a signal to a slot, Qt creates a connection between them. When you emit a signal, the slots connected to it are executed.

This is very convenient because it allows us to write code without having to worry about when or how the slots are going to be called. .

But how does Qt know which slots to call when a signal is emitted? This is where the meta-object system comes in. When you compile your code, Qt generates meta-objects for all your classes.

The meta-object contains information about signals and slots, as well as other information used by Qt for introspection (e.g. property system).

When you connect a signal to a slot, Qt uses the meta-object system to dynamically figure out which slot should be called when the signal is emitted. This means that you don’t have to write any boilerplate code or do any manual connects in order for signals and slots to work; it all happens automatically behind the scenes.

The meta-object system also provides other features such as dynamic property system and introspection, but we’ll leave that for another time.

So that’s basically how signals and slots work in Qt. They are very convenient and they make it easy to write code without having to worry about when or how slots are going to be called.