Function Pointer

( * Function Pointer )

Hello Friends,

Today I am going to discuss about function pointer.

OVERVIEW :

What is Pointer to function?

Just like pointer to variable, pointer to array and pointer to structure, we can also have pointer to function. We can say that function will be stored somewhere inside memory and we can declare pointer to that function.

DECLARATION :

We can declare function pointer same as declaration of simple function. Suppose we have function add() which takes int a and int b as parameters and it returns integer value. And we want to declare pointer to this function. So we can do it this way :

suppose function is,

int add(int a,int b)

{

      return a+b;

}

and now we will declare pointer to this function . . .

void main()

{

      int (*ptr_func) ( int , int );

      ptr_func = &add;

// this will also work

      ptr_func = add;

}

steps to create function pointer :

1) first of all write return type of function. -> int

2) then write name of pointer in '( )' -> int (*ptr_func)

3) then write type of arguments -> int (*ptr_func) (int,int)

Third step is not necessary so we can also write

int (*ptr_func) ( )

but it's a good habit to write argument types.

PROGRAMs on function pointer:

Now let's see some programs on function pointer.

1). Passing function as a parameter

#include <stdio.h>

int add(int a,int b,void (*ptr2) ( ) )

{

       ptr2( );                             //calling func

       return a+b;

}

void func()

{

       printf("Addition !!\n");

}

void main()

{

       int ans;

       int (*ptr1) ( ) = add;                      //pointer to function add

       ans = ptr1( 3 , 4 , func );               //passing func as parameter

       printf( " ANS : %d \n ", ans );

}

o/p :

Addition !!

ANS : 7

Explanation :

This was a very simple example to understand passing of function as a parameter.

But here you may have one question. Why do we need to pass function as a parameter?

Because in this program use of function pointer doesn't make sense. best example for this is library function qsort(). 

For small program like this, this seems to be unnecessary.

2). we can made array of pointer to function.

Before going for this program let's see another one.

This is program for simple calculator.

#include <stdio.h >

int add (int a , int b) { return a+b; }

int sub (int a , int b) { return a-b; }

int mul (int a , int b) { return a*b; }

int div (int a , int b) { return a/b; }

void main()

{

   int input, ans;

   printf("1. Add \n 2. Substract \n 3. Multiply \n 4. Divide \n 5. Exit \n Enter Your Choice : ");

   scanf("%d",&input);

   switch(input)

  {

     case 1:

        ans = add( 3 , 4 );

        break;

     case 2:

        ans = sub ( 3 , 4 );

        break;

     

     case 3:

        ans = mul ( 3 , 4 );

        break;

     case 4:

        ans = div( 3 , 4 ); 

        break;

     default:
         break;

 }

 printf(“ANS : %d \n ”, ans );

}

Now let's use array of function pointer for the same program.

#include <stdio.h >

int add (int a , int b) { return a+b; }

int sub (int a , int b) { return a-b; }

int mul (int a , int b) { return a*b; }

int div (int a , int b) { return a/b; }

void main()

{

    int input, ans;

    int (*ptr_fun [4] ) ( int , int ) = { add, sub, mul, div };

    printf("1. Add \n 2. Substract \n 3. Multiply \n 4. Divide \n 5. Exit \n Enter Your Choice : ");

    scanf(" %d ", &input );

    ptr_fun [input-1] ( 3 , 4 ); //replacing whole switch case !!!

}

Just like array of pointer we have array of pointer to function.

So this way we can replace switch case with only a single line.

• typedef WITH FUNCTION POINTER :

As we know typedef is used for simple naming convention, we can use it for function pointer to make it more easy to read. When we have more than one function having same return type and same parameters we can make a simple way of declaring pointer to this function.

Suppose we want pointer to function for all functions add, sub, mul and div , we can do it like this,

int (*ptr1_fun) (int, int) = add;

int (*ptr2_fun) (int, int) = sub;

int (*ptr3_fun) (int, int) = mul;

int (*ptr4_fun) (int, int) = div;

now let's make it simple using typedef.

typedef int (*ptr_fun ) ( int , int);

now we can use ptr_fun to declare new pointers to function having return type integer and both parameters also integer.

ptr_fun ptr1 =add;

ptr_fun ptr2 = sub;

ptr_fun ptr3 = mul;

ptr_fun ptr4 = div;

OHH!! It looks very clean.

Thanks,

Akash Patel

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Link for Pointer in C :
http://letsmakeceasy.blogspot.in/2014/09/pointer-in-c.html

Link for Array in C :
http://letsmakeceasy.blogspot.in/2014/09/array-in-c.html

Link for Preprocessor in C :
http://letsmakeceasy.blogspot.in/2014/09/preprocessor-directives.html