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    <p><strong><em>What is a Callbak function?</em></strong><br> In simple terms, a <strong>Callback function</strong> is one that is not called explicitly by the programmer. Instead, there is some mechanism that continually waits for events to occur, and it will call selected functions in response to particular events.<br> This mechanism is typically used when a operation(function) can take long time for execution and the caller of the function does not want to wait till the operation is complete, but does wish to be intimated of the outcome of the operation. Typically, Callback functions help implement such an <strong><em>asynchronous mechanism</em></strong>, wherein the caller registers to get inimated about the result of the time consuming processing and continuous other operations while at a later point of time, the caller gets informed of the result.</p> <p><strong><em>An practical example:</em></strong><br> Windows event processing:<br> virtually all windows programs set up an event loop, that makes the program respond to particular events (eg button presses, selecting a check box, window getting focus) by calling a function. The handy thing is that the programmer can specify what function gets called when (say) a particular button is pressed, even though it is not possible to specify when the button will be pressed. The function that is called is referred to as a callback.</p> <p><strong><em>An source Code Illustration:</em></strong></p> <pre><code>//warning: Mind compiled code, intended to illustrate the mechanism #include &lt;map&gt; typedef void (*Callback)(); std::map&lt;int, Callback&gt; callback_map; void RegisterCallback(int event, Callback function) { callback_map[event] = function; } bool finished = false; int GetNextEvent() { static int i = 0; ++i; if (i == 5) finished = false; } void EventProcessor() { int event; while (!finished) { event = GetNextEvent(); std::map&lt;int, Callback&gt;::const_iterator it = callback_map.find(event); if (it != callback_map.end()) // if a callback is registered for event { Callback function = *it; if (function) { (*function)(); } else { std::cout &lt;&lt; "No callback found\n"; } } } } void Cat() { std::cout &lt;&lt; "Cat\n"; } void Dog() { std::cout &lt;&lt; "Dog\n"; } void Bird() { std::cout &lt;&lt; "Bird\n"; } int main() { RegisterCallBack(1, Cat); RegisterCallback(2, Dog); RegisterCallback(3, Cat); RegisterCallback(4, Bird); RegisterCallback(5, Cat); EventProcessor(); return 0; } </code></pre> <p>The above would output the following: </p> <pre><code>Cat Dog Cat Bird Cat </code></pre> <p>Hope this helps! </p> <p><sup> <strong>Note:</strong> This is from one of my previous answers, <strong><a href="https://stackoverflow.com/questions/6183847/callback-functions-in-c-c-c/6184236#6184236">here</a></strong> </sup></p>
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    1. POwhy do we need to call these functions at run time using function pointers. we can as well call them directly
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    1. POwhy do we need to call these functions at run time using function pointers. we can as well call them directly
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    1. COThanks Als. This explains the use of function pointer but callback is still elusive. Where the callback happens here. It seems we are just invoking the function we wanted to call.
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    2. COThe point is that in the example above, all but Cat, Dog and Bird can be part of something that you did not write yourself, that you don't have the source code of, or that you wanted to write in a generic way, i.e. that you don't have to modify it when suddenly you also decide to call Fish (or any of the other thousands of annimals)
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    3. CO@user52875 one more thing I understood that function pointer is a way to achieve abstraction.. I got your point. But what about callback?
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