![]() ![]() ![]() This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses. Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object’s client or owner. ![]() While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments. 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. Other 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. ![]()
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