callback function

# What Is a Callback Function?
a callback function is a function that is called through a function pointer. If you pass the pointer (address) of a function as an argument to another, when that pointer is used to call the function it points to it is said that a call back is made.

# Why Should You Use Callback Functions?
Because they uncouple the caller from the callee. The caller doesn't care who the callee is; all it knows is that there is a callee with a certain prototype and probably some restriction (for instance, the returned value can be int, but certain values have certain meanings).

If you are wondering how is that useful in practice, imagine that you want to write a library that provides implementation for sorting algorithms (yes, that is pretty classic), such as bubble sort, shell short, shake sort, quick sort, and others. The catch is that you don't want to embed the sorting logic (which of two elements goes first in an array) into your functions, making your library more general to use. You want the client to be responsible to that kind of logic. Or, you want it to be used for various data types (ints, floats, strings, and so on). So, how do you do it? You use function pointers and make callbacks.

A callback can be used for notifications. For instance, you need to set a timer in your application. Each time the timer expires, your application must be notified. But, the implementer of the time'rs mechanism doesn't know anything about your application. It only wants a pointer to a function with a given prototype, and in using that pointer it makes a callback, notifying your application about the event that has occurred. Indeed, the SetTimer() WinAPI uses a callback function to notify that the timer has expired (and, in case there is no callback function provided, it posts a message to the application's queue).

Another example from WinAPI functions that use callback mechanism is EnumWindow(), which enumerates all the top-level windows on the screen. EnumWindow() iterates over the top-level windows, calling an application-provided function for each window, passing the handler of the window. If the callee returns a value, the iteration continues; otherwise, it stops. EnumWindows() just doesn't care where the callee is and what it does with the handler it passes over. It is only interested in the return value, because based on that it continues its execution or not.

However, callback functions are inherited from C. Thus, in C++, they should be only used for interfacing C code and existing callback interfaces. Except for these situations, you should use virtual methods or functors, not callback functions.



[ref: http://www.codeguru.com/cpp/cpp/cpp_mfc/callbacks/article.php/c10557/#more]