What is the use of callback functions?

In short, it is a function that is called. If you pass the pointer (address) of the function as an argument to another function, when the pointer is used to invoke the function it points to, we say this is
。
Why should I use it?
Because the caller can be separated from the callee. The caller does not care who the callee is, all it needs to know, but there is a called function that has a certain prototype, some restrictions (such as a return value of int).
If you want to know what the callback function does in practice, let's assume that there is a case where we're going to write a library that provides some
The implementation, such as
, quick sort, shell sort, shake sort, etc., but to make the library more generic, do not want to embed the sorting logic in the function, but let the user implement the corresponding logic, or, want to make the library available for a variety of
(int, float, string), what should I do now? can use
, and make callbacks.
Callbacks can be used to notify the mechanism, for example, sometimes to set a timer in the program, each time, the program will be notified, but the notification mechanism of the implementation of our program is ignorant. At this point, there is a need for a specific prototype
, use this pointer to make a callback to notify us that the program event has occurred. In fact, the SetTimer () API uses a callback function to notify the timer, and in the event that a callback function is not provided, it also sends a message to the program's
。
Another use of the callback mechanism
is Enumwindow (), which enumerates all the top-level windows on the screen, invokes a program-provided function for each window, and passes the window's handlers. If the callee returns a value, the iteration continues, otherwise, exits. Enumwindow () does not care where the callee is, nor does it care what the callee does with the handlers it passes, it only cares about the return value, because it will continue to execute or exit based on the return value.
In any case, the callback function continues from the C language, so in C + +, you should only use the callback function if you are establishing an interface with C code or dealing with an existing callback interface. In addition to the above, you should use virtual methods or function characters (functor) instead of callback functions in C + +.