What is a callback function?

Question

What is a callback function?

Answer 1

A callback function is a function you pass (as a reference or a pointer) to a certain function or object. This function or object will call this function back any time later, possibly multiple times, for any kind of purpose :

• notifying the end of a task
• requesting comparison between two item (like in c qsort())
• reporting progress of a process
• notifying events
• delegating the instanciation of an object
• delegating the painting of an area

...

So describing a callback as a function being called at the end of another function or task is overly simplifying (even if it's a common use case).