Naming pointer to function in a struct...

This is a discussion on Naming pointer to function in a struct... within the C Programming forums, part of the General Programming Boards category; Okay, I'm writing a program where I need to use structs which contain two pointers to functions. I want to ...

  1. #1
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    Exclamation Naming pointer to function in a struct...

    Okay, I'm writing a program where I need to use structs which contain two pointers to functions. I want to refer to these pointers as choose and decide... I'd like to know now if this code is the right way...
    Code:
    struct
    {
     int ident;
     int (*choose) (int);
     int (*decide) (int);
    } personality[NUM];
    And is there any other way? Like maybe...
    Code:
    struct
    {
     int ident;
     int (*) (int) choose;
     int (*) (int) decide;
    } personality[NUM]
    Also, I'd like to make sure that the following is a valid use of these function pointers.
    Code:
    j = (*personality[i].choose)(i);
    I've never really used something like this before so... help me save a little time here anyone who already knows what the proper syntax is?
    Callou collei we'll code the way
    Of prime numbers and pings!

  2. #2
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    And oh yea, is there any way to name the parameters of the function.. you know, just for documentation? Like...
    Code:
    struct
    {
     int ident;
     int (*choose) (int self);
     int (*decide) (int judgeMe);
    } personality[NUM];
    Callou collei we'll code the way
    Of prime numbers and pings!

  3. #3
    the hat of redundancy hat nvoigt's Avatar
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    Hannover, Germany
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    3,138
    Yes, the first one is the right way. However, you can make it cleaner and easier to understand with a typedef:

    ( hope it's correct, I don't have a compiler here... )

    PHP Code:

    #include <stdio.h>

    typedef int (*FUNCTION_POINTER_INT2INT)(int);

    const 
    unsigned int NUM 2;

    struct
    {
        
    int ident;
        
    FUNCTION_POINTER_INT2INT choose;
        
    FUNCTION_POINTER_INT2INT decide;
    personality[NUM];

    int testint i )
    {
        return 
    i*2;
    }

    int main()
    {
        
    personality[0].choose test;

        
    printf"%d"personality[0].choose) );

        return 
    0;

    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  4. #4
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    Ehhe, okay, now I have another problem...
    Code:
    enum {kind, usurper, egoist, NUM};
    
    struct
    {
     int ident;
     int (*choose) (void);
     int (*decide) (int);
    } personality[NUM];
    
    int kind_choose (void);
    int kind_decide (int judgeMe);
    
    personality[kind] = {kind, kind_choose, kind_decide};
    Let me be the first to say that this did not work.

    I'm sure there's other problems with this, but the first problem is that
    personality[kind] = {kind, kind_choose, kind_decide};
    looks like a declaration of an array of kind elements, not the assignment of values struct personality[kind] (admittedly, this probably isn't even a legal way to assign values to a struct).

    This leaves me in kind of a problem, because I would like a nice and easy way to add and remove personalities. I considered using something like this...
    Code:
    #define ASSIGNPERSONALITY(A) \\
    { personality[A].ident = A; \\
      personality[A].choose = A ## _choose; \\
      personality[A].decide = A ## _decide; }
    But at first I'm not even sure this will work, since each A is going to be an enum constant, and I think that this is a pretty confusing construct anyhow.

    So anyways, does anyone have a suggestion for a good way for me to -easily- add/remove personalities when I compile?
    Callou collei we'll code the way
    Of prime numbers and pings!

  5. #5
    ATH0 quzah's Avatar
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    The only thing wrong with your last example is failure to include 'struct' before it, that and you haven't named your structure. I'd do something like:
    Code:
    struct personalityTypes {
        int ident;
        int (*chose)(void);
        int (*decide)(int);
    };
    Then build a table, I think you could use your macro, but let me clarify, you want a compile-time list change, not a runtime? A compile time change could use something like:

    Non-macro version:
    Code:
    struct personalityTypes personalityTable[] =
    {
        { kind, kind_choose, kind_decide },
        { usurper, usurper_choose, usurper_decide },
        ... more ... 
    };
    Or, with slight modification, your macro might work, I personally haven't tried, but I believe it should do the trick:

    Macro version:

    Code:
    #define PERSONALITY(A) { A, A ## _chose, A ## _decide }
    
    struct personalityTypes personalityTable[] =
    {
        PERSONALITY(kind),
        PERSONALITY(usurper),
        PERSONALITY(egoist)
    }
    That should do the trick.

    Quzah.
    Hope is the first step on the road to disappointment.

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