What are callback functions?

Answer 1

In computer science, a callback is executable code that is passed as an argument to other code. It allows a low level software layer to call a function occurring in a higher level layer. Usually the higher level code first calls a function within the lower level code passing to it a pointer or handle to another function. Then the lower level function in the course of executing may call the passed-in function any number of times to perform some subtask. Another option is that the lower level function registers the passed-in function as a handler that is to be called asynchronously by the lower level at a later time in reaction to something. A callback can be used as a simpler alternative to polymorphism and generic programming, in that the exact behavior of a function can be dynamically determined by passing different (yet compatible) function pointers or handles to the lower level function. This can be a very powerful technique for code reuse.

Answer 2

In computer programming, a callback is a mechanism that allows one function (the callback function) to be passed to another function (the callee) via an argument. The callback function may be a (pointer to) function, a function object reference or a llambda expression. The caller can then invoke the callee with different callbacks to obtain different results from the same function.

As an example, a sorting algorithm will sort a sequence container in ascending order by default. It achieves this by invoking the less-than operator upon the container's elements to compare them. But if the element's do not have a less-than operator, or the user wants to sort the elements by some other order (such as descending order), they must provide a user-defined callback because the less-than operator is the default callback.

Answer 3

A callback is a return telephone call, an official product recall, or the return of a situation to the previous situation.