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Its used for implementing asynchronous behaviour among two independent/related modules/tasks e.g: the master-slave, server-client, wherein a service (in terms of a function call) is issued by the master to the slave and the response/result is not expected immediately or in other words, the caller and the called are in two separate threads of execution

for event notification

Here the function pointer comes into the picture..

for the purpose of event indication or asynchronous mechanism:

void showGreen()
{

cout<<""Green";
}

typedef void (*fp)();

void callCallBack( fp* FunPointer)
{
(*FunPointer)();
}
int main()
{

callCallBack(showGreen);

}

A callback function is the function which is delivered to an API which is to be called later on and is used as the Event Handler in cases.
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