Introduction to using function pointers in C (or C++)

[Page updated: 2008-11-08 (hesa)]

Why function pointers anyway?

In C (C++) you can use function pointers to implement plugins or to change, at one place, the function to call, no matter where you use the function (through the pointer), or to remove “expensive” if statements in tight loops.

But instead of spending too much energy on talking we’ll continue with an example. Throughout this introduction we’ll use a feedback functionality. Your program wants to give feedback to the user and instead of using a variable which we switch over to choose the right feedback implementation we are going to use function pointers. Depending on what environment the user is using we can use a gui feedback or console based feedback. Later on, in introduction to writing plugins, we’ll dive into more practical use of them pointers.

Let’s settle for a feedback function prototype (stripped down functionality, no variable sized argument list etc):

int default_feedback(char *str)

The function returns an int. The first argument is the string to print. Let’s add an implementation to the function:

int default_feedback(char *str)

{

return fprintf(stdout, “%s”, str);

}

Please note, we haven’t used function pointers yet.We can use the function by calling it like this:

default_feedback(”Ordinary call\n”);

So far so good. Let’s define our first function pointer. Let’s create a variable feedback_fun that stores the pointer to the function we want to call.

int (*feedback_fun)(char *str) ;

We can see that this looks a bit like the function above. We’ve just added some “(*” and an extra “)”. What the live above says is: the function pointed to by feedback_fun returns an int and takes char *str as argument. Piece of cake, isn’t it? We can now use the variable to store the address to a function we want to use.

Let’s store the address in our function pointer:

feedback_fun = default_feedback ;

and call the function through the function pointer

feedback_fun(”Called using function pointer call\n”);

Was that it? Yes, neat isn’t it? So if we turn this into a program, including a main function and some comments, we get this:

/*
* Copyright (c) 2008, Henrik Sandklef
*
*/

#include <stdio.h>

/*
* Global varible poiting to th feedback function choosen
*
*/
int (*feedback_fun)(char *str) ;

/*
* Name: default_feedback
* Args: str - string to print
* Description: Prints a string to given file
*/
int
default_feedback(char *str)
{
return fprintf(”%s”, str);
}

int main(int argc, char **argv)
{

/* Set feedback_fun to point at default feedback function */
feedback_fun = default_feedback ;

/* Call the default_feedback in the ordinary way */
default_feedback(”Ordinary call\n”);

/* Call the default_feedback through the feedback_fun variable */
feedback_fun(”Called using function pointer call\n”);

return 0;
}

31 Comments to Introduction to using function pointers in C (or C++)

e8johan
October 9, 2008

There are two interesting features that can be used with function pointer. The first one is that it is possible to break out an if-clause selecting what to do from a tight loop. Given the example above:

if( special_case )
feedback_fun = special_feedback;
else
feedback_fun = default_feedback;

for( /* tight loop over items */ )
feedback_fun( /* item pointer */ );

Given the code above, we can save quite a lot of evaluations (beware, though, today’s compilers are probably smart enough to do this).

The second feature is what C++ folks calls virtual methods, but an example of that would consume too much space to show here.

hesa
October 9, 2008

i am planning to write a follow up tonight (sleep is a poor substitute for coffee) where I’ll show how to simplify the use of function pointers by using a simple typedef

… after we (the two of us) can dive into the C++ field

hesa
October 10, 2008

i did sleep instead….

hesa
November 8, 2008

Just wrote ‘More on function pointers’:
http://www.sandklef.com/hesa/index.php/written-words/more-on-function-pointers/

jeremiah
November 19, 2008

Very cool, I would like to see a more “real world” example because this seems quite useful.

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