callback function v. regular function

[7stud]

Hi,

Every time I see the term callback function, my eyes glaze over, and I know I no longer really understand what the author is talking about.

In three sentences or less, can someone explain what distinguishes a call back function from a regular function?

Thanks.

[misplaced]

real quick. it's when you pass the address of a function to another function and it gets called.

[misplaced]

ok. i'm bored. i'll explain some more. (i could be mistaken, but....)

an example is, if you're using MFC, there's a class which describes a Button. it's kind of a dumb class. it only knows when the button is pressed, mouse-overed, etc. at some point, when a button is clicked, you want it to actually do something (such as submit a form). since the button class is already defined and needs to be used over and over again for many different situations, you can't just build the functionality into the button. but, you can have a variable within the class which holds the address of another function. this is a function you create (typically). it's the function that you want to be executed when the button is pressed.

[7stud]

I liked your first answer the best, but I've read about the important implications contained in your second answer.

Thanks.

[BMJ]

It's a kind of relationship between the program and the OS, the program tells the OS when to call a certain function (a callback), and the OS will then (hopefully ) do so when the time comes.

[Micko]

It's a kind of relationship between the program and the OS, the program tells the OS when to call a certain function (a callback), and the OS will then (hopefully ) do so when the time comes.

That's right
Consider this :

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
....
}

CALLBACK is defined as __stdcall which refers to a special calling sequence for function calls that occur between Windows itself and your application.

[misplaced]

*stands corrected* (for the most part)

[Raven Arkadon]

>> calling sequence

isnt it calling convention?
like by conventions arguments are passed in right to left order and the callee (called function) cleans up the stack?

[Hunter2]

It's a user-specified function, usually set via a function pointer, that gets called by the inner workings of an independent module when a given event occurs.

I don't think 'callback' is defined in relation to the calling convention; CALLBACK is just what Microsoft likes to use for their callback functions. Also, a callback doesn't necessarily involve the OS. For an example, see the thread on reference-counting smart pointers here - my implementation uses a callback function to know what to do when the reference count hits zero, whether to use delete, delete[], or to take some other user-defined action.

**EDIT**
NVM, all of the implementations seem to use a callback of some sort; mine and codeplug's original use function pointers, while some others used a functor class as a template parameter instead.