Callback Functions Tutorial

http://www.codeguru.com/cpp/cpp/cpp_mfc/callbacks/article.php/c10557/Callback-Functions-Tutorial.htm

为什么call back函数有用？比如，在实现一个库函数的时候，库函数内部需要调用客户端程序员所写的函数。但库函数没办法知晓客户端函数的具体内容是什么。这时，库函数可以在函数内部用一个指代客户端程序的东西来代替库函数想用的函数，这个指代的东西可以称作prototype, 这个prototype可以作为库函数的参数。这种库函数定义时用prototype，而prototype的实现交给客户端程序员的方案，叫做callback.

Introduction

If you are reading this article, you probably wonder what callback functions are. This article explains what callback functions are, what are they good for, why you should use them, and so forth. However, before learning what callback functions are, you must be familiar with function pointers. If you aren't, consult a C/C++ book or consider reading the following:

What Is a Callback Function?

The simple answer to this first question is that 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.

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.

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