function pointer tutorial may be helpful..

[devesh]

function pointer is a variable which points to the address
of a function.
it is basically used for removing/replacing switch statement and if
else statement.
it is very usefull to implement callback function.callback functions
are mostely
used in driver writing.it can also be used insted of virtual function
to save some time...


now consider this example to understand how function pointer works...


#include<stdio.h>
static int my_function(int a)
{

printf("my_function :%d",a);
return(2*a+3);
}
int main(void)
{
int (*new_function)(int)= my_function; /* here a function
new_function is decleared, the address of the function named
my_function is assigned to it, and function is invoked by
dereferencing the pointer.*/
int x;
x=(*new_function)(10);
return 0;
}


now consider another example..

void young(int);
void old(int);

int main(void) {
void (*fp)(int);
int age;
printf("How old are you? ");
scanf("%d", &age);
fp = (age > 30) ? old : young;
fp(age);
return 0;
}
void young(int n) {
printf("Being only %d, you sure are young.\n", n);
}
void old(int m) {
printf("Being already %d, you sure are old.\n", m);
}
here the function pointer *fp is declared here as void (*fp)(int),
which specifies both the return type (void) and the types of arguments
(int) of the function. We then assign the pointer to a particular
function, and having done so, can then call the function just as we
normally would.


Another example of the function pointer is..

void young(int);
void old(int);
void greeting(void (*)(int), int); // function pointer is used to call
the function old/young
int main(void) {
int age;
printf("How old are you? ");
scanf("%d", &age);
if (age > 30) {
greeting(old, age);
}
else {
greeting(young, age);
}
return 0;
}
void greeting(void (*fp)(int), int k) {
fp(k);
}
void young(int n) {
printf("Being only %d, you sure are young.\n", n);
}
void old(int m) {
printf("Being already %d, you sure are old.\n", m);


now come to the million dollar question
I have function like arithmetic( int1, int2, int3)
Int3: Arg for select the operation 1. Add 2. Mul 3.
Div 4. Sub
Write the program for above condition without using
switch and if-else

Void arithematic(float a,float b,float (*operation_arithematic)
(float,float));
{
float result=operation_arithematic(a,b);
printf("the result is = %f",result);
}
float plus(float a,float b)
{
return(a+b);
}
float minus(float a,float b)
{
return(a-b);
}
float multiply(float a,float b)
{
return(a*b);
}
float divide(float a,float b)
{
return(a/b);
}
void main( )
{
arithematic(3,5,&plus);
printf("%f",result);

arithematic(7,5,&minus);
printf("%f",result);

arithematic(7,5,&multiply);
printf("%f",result);

arithematic(7,5,&divide);
printf("%f",result);

}

I think this will work fine And this will be very useful to
understand .

 

[Richard Heathfield]

devesh said:

function pointer is a variable which points to the address
of a function.

A function pointer is a value that is the address of a function. A function
pointer is said to "point to a function" rather than "point to the address
of a function".

it is basically used for removing/replacing switch statement and if
else statement.

That is one possible use.

it is very usefull to implement callback function.callback functions
are mostely
used in driver writing.

Actually, they are used much more widely than that.

it can also be used insted of virtual function
to save some time...

C doesn't have virtual functions.

now consider this example to understand how function pointer works...


#include<stdio.h>
static int my_function(int a)
{

printf("my_function :%d",a);
return(2*a+3);
}
int main(void)
{
int (*new_function)(int)= my_function; /* here a function
new_function is decleared,

No. There is no function by that name; new_function is not a function, but
a function pointer.

the address of the function named
my_function is assigned to it, and function is invoked by
dereferencing the pointer.*/

Right.

now consider another example..

void young(int);
void old(int);

int main(void) {
void (*fp)(int);
int age;
printf("How old are you? ");

When you use any variadic function (and that certainly includes printf),
you need to provide a valid function prototype for it. You can do this by
adding:

#include <stdio.h>

at the top of your code. If you don't provide such a prototype, the
behaviour of your program is undefined. (The same applies to scanf and
other variadic functions.)

scanf("%d", &age);

Check that this scanf call succeeded. The scanf function returns a useful
value that you should test to ensure that it's what you expect.

Void arithematic(float a,float b,float (*operation_arithematic)
(float,float));

C has no Void type. If you don't define it yourself, it doesn't exist.
(Hint: C is case-sensitive.)

void main( )

In C, main returns int.

<snip>