Function Pointers...
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  1. #1
    Join Date
    Sep 2003
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    India
    Posts
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    Function Pointers...

    Hey gurus,

    I am completely confussed with the functionality of the Function Pointers.

    Can anyone help me out in knowing where exactly this function pointers are used.

    Here is the simple example which i have copied from one of the books.

    /****************************************************************************/
    #include <iostream>
    using namespace std;
    void func()
    {
    cout << "func() called..." << endl;
    }

    int main()
    {
    void (*fp)(); // Define a function pointer

    fp = func; // Initialize it

    printf("value in *fp after initialize is %d\n",(*fp));
    printf("after initialize fp is %d\n",fp);

    (*fp)(); // Dereferencing calls the function

    void (*fp2)() = func; // Define and initialize
    (*fp2)();

    return 0;
    }
    /****************************************************************************/

    In this example what exactly is the need of the function pointer "fp". As we can directly call the function
    "func()".

    /*****************************************************************************/
    void (*fp)(); // Define a function pointer
    /*****************************************************************************/

    Is this the correct way to define. As i know that this is something called declaration
    but not definition.

    /*****************************************************************************/
    printf("value in *fp after initialize is %d\n",(*fp));
    printf("after initialize fp is %d\n",fp);
    /*****************************************************************************/

    what will be the output of the above statement.

    Thanks in advance.

    venky

  2. #2
    Join Date
    Sep 2002
    Posts
    1,747
    Function pointers are a way to pass around function instances when all you specify is the function type (the collection of return and parameter types). So you can tell a distant function somewhere that when it gets to a particular point you want it to call a particular function instance, and that distant function never needs to make any assumptions except the type. This is common in callback mechanisms.
    */*/*/*/*/*/*/*/*/*/*/*/*/*/*/*/*/*/

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  3. #3
    Join Date
    Feb 2002
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    5,758
    In this example, fp is a type, not a variable.

    fp OneFunctionPointer = ...

    Function pointer is an essential solution in many situations. One example is the use of it as part of a function parameter. Another situation could be a replacement for many if/else and switch statements.

    Kuphryn

  4. #4
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    Bilbao
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    Basically you can use function pointers when you want to make some kind of dynamic process...

    For example... Imagine that you are developing a script interpreter... You read a text file and want to call a function with every key word in the script. (For example, READ, WRITE, OPEN, GOUP, GODOWN...)

    Of course you can make a giant switch with all the keywords, but I would prefer to do a linked list of structures that contain the keyword and the function pointer to handle it...
    Caronte
    Si tiene solución... ¿por qué te preocupas?
    Si no tiene solución... ¿por qué te preocupas?

  5. #5
    Join Date
    Oct 2002
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    Singapore
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    Originally posted by kuphryn
    In this example, fp is a type, not a variable.

    fp OneFunctionPointer = ...
    I am quite positive that the original example for fp is a variable. Only when typedef is being used, fp becomes a type.

    Code:
    typedef void (*fp)();    // Declaring a type
    fp OneFunctionPointer = ...
    
    void (*fp1)();    // Declaring a variable
    fp1 = ...

  6. #6
    Join Date
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    Location
    Altrincham, England
    Posts
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    I'd take the book back if they can't come up with a better example than that.

    Try this:
    Code:
    #include <iostream>
    using namespace std;
    void func1()
    {
       cout << "func1() called..." << endl;
    }
    
    void func2()
    {
       cout << "func2() called..." << endl;
    }
    
    int main()
    {
        void (*fp)();
    
        fp = func1;
    
        printf("value in *fp after initialize is %d\n",(*fp));
        printf("after initialize fp is %d\n",fp);
    
        (*fp)();
    
        fp = func2;
    
        printf("value in *fp after initialize is %d\n",(*fp));
        printf("after initialize fp is %d\n",fp);
    
        (*fp)();
    
        return 0;
    }
    Note here how the same function pointer is used to call two different functions.

    Incidentally, when I said "take the book back": the use of printf() when it's already included <iostream> simply confirms that the author shouldn't be trusted to give good advice. And I don't see the purpose of the statement:

    printf("value in *fp after initialize is %d\n",(*fp));
    Last edited by Graham; January 8th, 2004 at 02:48 PM.
    Correct is better than fast. Simple is better than complex. Clear is better than cute. Safe is better than insecure.
    --
    Sutter and Alexandrescu, C++ Coding Standards

    Programs must be written for people to read, and only incidentally for machines to execute.

    --
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    -- Gordon Bell


  7. #7
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    Posts
    5,758
    Okay. I did not pay attention to the typedef. My mistake.

    Kuphryn

  8. #8
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    Location
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    Posts
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    qsort is a good exemple

    A good exemple of functions pointers use can be found on qsort and bsearch functions. On bsort for exemple, that sorts an array of elements defined by the user, all the work of sorting is made internally but qsort don´t know how compare two elements of this array. This funcionallity is provide by a user´s function and a pointer to this function is passed to qsort.

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