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I hope its an extremely repetitive question. And my advance excuse to all the viewers who find it annoying.

Although I am bit experienced programmer, but I cannot justify the use of function pointer over direct call. Scenarios where I unable to find the differences are -

1) callbacks - same can be achieved by direct call.

2) Asynchronous or synchronous event handling - anyway event has to be identified, based on which element no. in function pointer array got updated. But the same can be also done via direct call.

3) In some post I had seen people commenting it is to be used when it is not known which function to call. I didn't get any proper justification for this.

I really appreciate if someone can explain me using above scenarios with practical and really simple realistic example.

Function Pointer in C, A function pointer is used when you have to switch between various versions of some implementation depending on what the user chooses for this particular  A function pointer is a variable that stores the address of a function that can later be called through that function pointer. This is useful because functions encapsulate behavior. For instance, every time you need a particular behavior such as drawing a line, instead of writing out a bunch of code, all you need to do is call the function.

1) callbacks - same can be achieved by direct call.

Not true. For a direct call, the caller must know the function name and signature when the code is compiled, and can only ever call that one function. A callback is defined at runtime and can be changed dynamically, while the caller need only know the signature, not the name. Moreover each instance of an object may have a different callback, whereas with a direct call, all instances must call the same function.

2) Asynchronous or synchronous event handling - anyway event has to be identified, based on which element no. in function pointer array got updated. But the same can be also done via direct call.

Not sure what you mean, but an event handler is simply a kind of callback. The event may be identified by the caller and different call-back handlers called through pointers. Your point only stands if there is one event handler for all event types and the user is to be responsible for identification.

3) In some post I had seen people commenting it is to be used when it is not known which function to call. I didn't get any proper justification for this.

See (1) and (2) above. Often it is a means to hook platform independent third-party library code into a specific platform without having to deliver source-code or for system events that require user/application-defined handlers.

I would not sweat it however - if all your application requirements can be resolved without using a pointer to a function, then you don't need a pointer to a function. When you need one, you will probably know. You will most likely encounter it when you have to use an API that requires it before you ever implement an interface yourself that does. For example in the standard library the qsort() function requires a pointer to a function in order to define how two objects of arbitrary type are to be ordered - allowing qsort() to support any type of object - it is a way in C of making a function "polymorphic". C++ supports polymorphism directly, so there is often less need for explicit function-pointers in C++ - although internally polymorphism is implemented using function pointers in any case.

Function Pointers in C and C++, Using Callback functions! The user's click should cause the interface to call a function that you wrote to handle the event. To get a sense for when you might do​  The create() function receives a pointer to an array of ten integers and fills that array with random values in the range of 0 through 9. The show() function receives the same array and displays all ten elements. How to return a pointer from a function. Functions are known by their types, such as int or char or even void.

Without function pointers, how would you implement a function which calculates the integral of any real-valued function?

typedef double (*Function)(double);

double Integral(Function f, double a, double b);

What is the difference between 'function pointer' and 'pointer to a , Now it is time to do something even more interesting with pointers, using them to point to and call functions. Why point to a function? The first question that may  As you can see, using a function pointer in this context provides a nice way to allow a caller to “hook” their own functionality into something you’ve previously written and tested, which helps facilitate code reuse!

There is a concept in programming called DRY -- don't repeat yourself.

Suppose you have 121 buttons in your UI. Each one of them behaves much the same, except when you press the button, a different operation happens.

You can (A) use virtual inheritance to dispatch to the right operation (requiring a class per button), or (B) use a function pointer (or a std::function) stored in the class in order to call the right "on click" handler, or (C) have every single button be a distinct type.

A virtual function is implemented in every compiler I have examined as a complex table that, in the end, is a collection of function pointers.

So your choices are function pointers or generating 121 completely distinct buttons that happen to mostly behave the same.

In any situation where you want to decouple the caller and the called, you must use something akin to a function pointer. There are a ridiculous number of cases, from work queues to thread off tasks, callbacks, etc.

In tiny programs where everything is hard coded, hard coding every call can work. But hard coded stuff like this doesn't scale. When you want to update those 121 buttons each hand-implemented, knowing their points of customization is going to be ridiculously difficult. And they will fall out of sync.

And 121 is a modest number of buttons. What about an app with 10,000? And you want to update every button's behavior to handle touch-based input?

Even more, when you type erase, you can reduce binary size significantly. 121 copies of a class implementing a button is going to take more executable space than 1 class, each of which stores a function pointer or two.

Function pointers are but one type of "type erasure". Type erasure reduces binary size, provides clearer contracts between provider and consumer, and makes it easier to refactor behavior around the type erased data.

Pointers to functions, A function pointer, also called a subroutine pointer or procedure pointer, is a pointer that points The following C program illustrates the use of two function pointers: func1 takes one double-precision (double) parameter and returns another  There are two major uses for function pointers: callbacks - used for event handlers, parser specialization, comparator function passing plugins and extensions - the pointers to functions provided by plugins or library extensions are gatherd by a standard functio GetProcAddress, dlsym or similar,

What's the use of a function pointer?, Function pointers are similar, except that instead of pointing to variables, they The other primary thing you can do with a function pointer is use it to actually call​  In C, we can use function pointers to avoid code redundancy. For example a simple qsort () function can be used to sort arrays in ascending order or descending or by any other order in case of array of structures. Not only this, with function pointers and void pointers, it is possible to use qsort for any data type.

Function Pointers - Learn C, Notice that in this call, we are not calling strcmp immediately; we are generating a pointer to strcmp, and passing that pointer as the third argument in our call to  A function pointer in c is one of the most important pointer tools which is often ignored and misunderstood by the people. Generally, people face the problem with function pointer due to an improper declaration, assignment and dereferencing the function pointer.

Function pointer, C - Function Pointers with examples: In C programming, we have a concept of Pointer to functions, using which we can call the function. Function pointers can be used to simplify code by providing a simple way to select a function to execute based on run-time values. Function pointers are supported by third-generation programming languages (such as PL/I, COBOL, Fortran, dBASE dBL, and C) and object-oriented programming languages (such as C++ and D ).

7.8, There are many use of function pointer but all they are summing around callback construct, so here i write two use cases for function pointers upon callback  The statement result = ope[choice](x, y); runs the appropriate function according to the choice made by the user The two entered integers are the arguments passed to the function. Functions Using void Pointers. Void pointers are used during function declarations. We use a void * return type permits to return any type.