Function Pointers in Structs and Classes?

This is a discussion on Function Pointers in Structs and Classes? within the C++ Programming forums, part of the General Programming Boards category; Is it possible to have function pointers in structs and classes? Here is the structure of the code I am ...

  1. #1
    LPP
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    Function Pointers in Structs and Classes?

    Is it possible to have function pointers in structs and classes? Here is the structure of the code I am trying to write:

    Code:
    struct function_pointer
    {
            int id;
            int *func_1(int,...);
    };
    
    int func_2(int argument,...);
    {
        va_list arguments;
        va_start(arguments,argument);
        for (int i=0;i<argument;i++)
        {
              cout << va_arg(arguments,int);
        }
        va_end(arguments);
    }
    
    int main()
    {
        function_pointer x;
        x.id = 10;
        func_2(2,4,5);
        x.func_1 = &func_2; //error line
        x.func_1(2,2,5);
        cin.get();
    }

    There is this error: invalid use of member (did you forget the `&' ?)
    I've never seen it before now. Suggestions?
    Last edited by Ken Fitlike; 09-07-2006 at 12:47 PM. Reason: code tags added

  2. #2
    Devil's Advocate SlyMaelstrom's Avatar
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    First, you have a semicolon after your arguements in func_2 that has to go. Second, you don't have a function pointer in that struct you have a function that returns a pointer to an int. Why you have it returning an int, I don't know, you aren't returning anything in the function, anyway.

    Code:
    // This is a function pointer
    void (*func_1)(int,...);
    };
    
    void func_2(int argument,...)
    {
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  3. #3
    and the hat of wrongness Salem's Avatar
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    > int *func_1(int,...);
    Would be
    int (*func_1)(int,...);

    > x.func_1 = &func_2;
    x.func_1 = func_2;
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  4. #4
    LPP
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    Thanks very much! That was just a dumbed-down example of what I had. Ready for the next one?

    Can I have a function like so:

    Code:
    void arg_func(...)
    Where I use the va_ functions to dissect the argument list? The problem I am having is that va_start takes two arguments, and the second one must be the name of the first variable... I don't have one! Is there another choice of function?

  5. #5
    and the hat of wrongness Salem's Avatar
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    > Where I use the va_ functions to dissect the argument list?
    You can't, va_args needs at least one fixed argument.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
    LPP
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    And there is no option, other than va_arg?

    On an entirely different note, can I create argument lists as variables and then pass them to a function, ie:

    Code:
    void function(int a, int b){define function};
    _arglisttype arg_list x;
    // Add two ints to x
    function(x);
    Or, even:

    Code:
    void function(_arglisttype){define function};
    _arglisttype arg_list x;
    // Add two ints to x
    function(x);
    Last edited by LPP; 09-07-2006 at 01:32 PM.

  7. #7
    and the hat of wrongness Salem's Avatar
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    Once you have an arglist, you can write your own functions like vsprintf()
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
    Super Moderator VirtualAce's Avatar
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    Class function pointers are much different than C function pointers. I recommend googling it which will give you a host of good sites to browse on the topic.

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