Callback type things

This is a discussion on Callback type things within the C++ Programming forums, part of the General Programming Boards category; I have this code currently: Code: void (*OnPaint)(WindowArgs, PaintArgs); It works like I want it to, the user creates a ...

  1. #1
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    Callback type things

    I have this code currently:

    Code:
    void (*OnPaint)(WindowArgs, PaintArgs);
    It works like I want it to, the user creates a void function with the arguments it says there (they are alreayd defined structs) and than the user simple does
    Code:
    OnPaint = func_name;
    And they decalre the function like this:

    Code:
    void paint(WindowArgs args, PaintArgs paint)
    {
    //Do stuff here
    }
    But the thing is I want the user know what kind of arguments to put inside the callback without looking at the callback code. It is a very vague question so I hope someone can help me. Ask if you need any more info.

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    If you're wondering how to call a function pointer:
    Code:
    void (*funcp1)(int) = function, (*funcp2)(int) = &function;
    
    (*funcp1)(5);
    funcp1(5);
    As you can see, there are two ways to assign and call function pointers.
    dwk

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  3. #3
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    ahh ok I see, I think I got some ideas now. Thanks.

  4. #4
    Registered User Tonto's Avatar
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    The user can not be completely ignorant of the implementation details, they will have to make a callback function with the specified parameters before you can appropriately pass a function pointer around as a callback, bowever you can ease the complex syntax using typedef's. For example, take the API function BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam); The WNDENUMPROC is a user-defined callback, WNDENUMPROC is typedef'd as "typedef BOOL (CALLBACK* WNDENUMPROC)(HWND, LPARAM);" This means a user would define their callback like:

    Code:
    BOOL MyEnumWindowsCallback(HWND hWnd, LPARAM lParam)
    {
            return 0;
    }
    And then call the function with:

    Code:
    EnumWindows(&MyEnumWindowsProc, someSortOflParamValue);

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