Signals are emitted by objects when they change their state in a way that may be interesting to other objects. They are completely type safe.Īll classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals and slots can take any number of arguments of any type. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in. A slot is a function that is called in response to a particular signal. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A signal is emitted when a particular event occurs. In Qt, we have an alternative to the callback technique: We use signals and slots. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call. We can never be certain that the processing function will call the callback with the correct arguments. Callbacks have two fundamental flaws: Firstly, they are not type-safe. The processing function then calls the callback when appropriate. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function.

Older toolkits achieve this kind of communication using callbacks. For example, if a user clicks a Close button, we probably want the window's close() function to be called. More generally, we want objects of any kind to be able to communicate with one another. In GUI programming, when we change one widget, we often want another widget to be notified.

The signals and slots 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 used for communication between objects.

Qt multiple signals to one slot