The relevance of pointer to funtions

This is a discussion on The relevance of pointer to funtions within the C Programming forums, part of the General Programming Boards category; I've been searching google for some info on this but I won't be making a finite state machine any time ...

  1. #1
    Registered User caroundw5h's Avatar
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    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.

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by caroundw5h
    So the question is: what other uses for pointer to functions are there and have any of you used it?
    Comparing functions.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
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    ... 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:
    int some_algorithm( struct State, (*pfn_cost)(int, int)) {
      if(pfn_cost(blah1, blah2) < pfn_cos(blah1, blah2) {
        ...
      }
      ...
    }
    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:
    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]();

    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:
      if(true == mmx_support)
        matrix_mult_mmx(...);
      else if(true == 3dnow_support)
        matrix_mult_3dnow(...);
      else
        matrix_mult_fallback(...);
    your code would have hundrets of conditionals whenever youd call matrix multiply

    instead try this:

    Code:
    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(...);

    is it clearer now?
    Last edited by Raven Arkadon; 02-27-2005 at 08:23 PM.
    signature under construction

  4. #4
    Registered User caroundw5h's Avatar
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    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:
    #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;}
    i feel woefuly inadequate.

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