The relevance of pointer to funtions

I've been searching google for some info on this but I won't be making a finite state machine any time soon and I haven't come up where I would need to assign a function as a parameter for another function. So the question is: what other uses for pointer to functions are there and have any of you used it? Are there any optimization benefits in using them?
thanks.

Quote:

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Originally Posted by caroundw5h

So the question is: what other uses for pointer to functions are there and have any of you used it?

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Comparing functions.

... if youve searcher with google you should have come across

www.function-pointer.org

anyway: YES!!! function pointers are VERY useful.
for example when some function requires some cost-function.

Code:

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int some_algorithm( struct State, (*pfn_cost)(int, int)) {
  if(pfn_cost(blah1, blah2) < pfn_cos(blah1, blah2) {
    ...
  }
  ...
}

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if you have different cost functions you just have to pass a cost function to this - instead of writing multiple versions of your algorithm.

also: you can map values to function

a byte code interpreter might use this mapping - depending on the byte value the appropriate action is performed.

(note that a switch statement would be faster but uglier here - as long as the compiler creates a jump table)

Code:

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void f1() {
  printf("hi, im function 1\n");
}


void f2() {
  printf("hi, im function 2\n");
}

void (*(a_pfn_my_map[2]))();


a_pfn_my_map[0] = &f1;
a_pfn_my_map[1] = &f2;

int i = blah;
if(i >= 0 && i <= 2)
  a_pfn_my_map[i]();

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another useful thing about function pointers is that you might have function which all do the same - but the implementation differs.

for example matrix multiplication can be done using the ordinary instruction set, or 3dnow or mmx extension or whatever.

consider that:

Code:

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  if(true == mmx_support)
    matrix_mult_mmx(...);
  else if(true == 3dnow_support)
    matrix_mult_3dnow(...);
  else
    matrix_mult_fallback(...);

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your code would have hundrets of conditionals whenever youd call matrix multiply

instead try this:

Code:

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Matrix (*pfn_matrix_multiply)(Matrix *p_m1, Matrix *p_m2);

  if(true == mmx_support)
    pfn_matrix_multiply = &matrix_mult_mmx;
  else if(true == 3dnow_support)
    pfn_matrix_multiply = &matrix_mult_3dnow;
  else
    pfn_matrix_multiply = matrix_mult_fallback;


  // now we can always use the fastest implementation without having to worry which implementation actually "hides" behind the pointer
  pfn_matrix_multiply(...);

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is it clearer now?

Nice!! the array of pointers to functions is nice!! Bit manipulation is where my book is taking me next perhaps once i've done that, i might see some other uses for pointers to functions. As it stands i just play around with previous code and muck around like this:

Code:

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#include <stdio.h>
#include <math.h>

double twice(double x);
double half(double x);
double thrice(double x);
void showmenu(void);
#define NUM 4
int main(void)
{
    double (*pf[NUM])(double)  = {twice, half, thrice, sqrt};
    double val;
    double ans;
    int sel;
   
    printf("Enter a number (negative to quit): ");
    while (scanf("%lf", &val) && val >= 0)
    {
        showmenu();
        while (scanf("%d", &sel) && sel >= 0 && sel <= 3)
        {
            //ans = (*pf[sel])(val);
            ans = pf[sel](val);//BSD way
            printf("answer = %f\n", ans);
            showmenu();
        }
        printf("Enter next number (negative to quit): ");
       
    }   
    puts("bye");
    return 0;
}


void showmenu(void)
{
    puts("Enter one of the following choices:");
    puts("0) double the value        1) halve the value");
    puts("2) triple the value        3) squareroot the value");
    puts("4) next number");
}

double twice(double x) {return 2.0 * x;}
double half(double x) {return x / 2.0;}
double thrice(double x) {return 3.0 * x;}

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i feel woefuly inadequate. :o