function pointer

[mallik]

Hi,

what is the actual use of function pointer and at which stage is useful.

what is the difference , by calling the function directly instead of thru function pointer.

how we will distinguish these ...........

Pls help me.....

[Cabhan]

Function pointers are useful in that they allow us to pass around functions, which enables a certain level of abstraction. For instance, imagine the following:

#include <stdio.h>

static void printfirst(char *s)
{
    putchar(s[0]);
    putchar('\n');
}

static void printsecond(char *s)
{
    putchar(s[1]);
    putchar('\n');
}

void doToArray(char **strings, int length, void (*func)(char *))
{
    int i;
    for(i = 0; i < length; i++)
        func(strings[i]);
}

int main()
{
    char *strings[] = { "hello", "goodbye", "hi there!", "testing", "yohoho" };

    puts("I will now print the FIRST letter of each string");
    doToArray(strings, 5, printfirst);
    putchar('\n');

    puts("I will now print the SECOND letter of each string");
    doToArray(strings, 5, printsecond);

    return 0;
}

Which gives the following output:

alex@danu ~/test/c $ ./a.out 
I will now print the FIRST letter of each string
h
g
h
t
y

I will now print the SECOND letter of each string
e
o
i
e
o

So you see how I can write a single function that does something to every element of an array, and what it does is determined by the function I give it.

I could use a similar thing to sort a list of structs using different comparisons (say, if we have a struct that contains file information, we could sort by filename, by inode, by type, etc.). Or maybe I use it to copy a list of void pointers, where the person who made the list passes me the function to use for each individual copy.

See how this works?