Compiler Error

This is a discussion on Compiler Error within the C++ Programming forums, part of the General Programming Boards category; Hi, would anyone be able to help us on this problem. I have got a callback function declared within my ...

  1. #1
    Registered User subdene's Avatar
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    Compiler Error

    Hi, would anyone be able to help us on this problem. I have got a callback function declared within my class, under protected..

    Code:
    VOID CALLBACK Line(int X, int Y, LPARAM lpData){};
    I haven't put any code within this yet, as i'm just trying to get past this error message from the compiler.

    The callback function relates to
    Code:
    LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, (long)Welcome);
    which is called from within a draw function in my class. I keep getting this following error message related to the 5th parameter within
    LineDDA() function
    [C++ Error] Ass1.cpp(128): E2235 Member function must be called or its address taken
    I've also tried including the get address of operator &, but i get a different error message then.

    However, when i make
    Code:
    VOID CALLBACK Line(int X, int Y, LPARAM lpData){};
    global, i.e. outside of my class it compiles. This callback function has got to be within my class though as it needs to access certain variables. Any help on the problem would be appreciated.
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  2. #2
    °Amo fķtbol!
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    Can you show the function and how you call it?

  3. #3
    Registered User subdene's Avatar
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    Code:
    #ifndef DrawX
    #define DrawX
    #include <vcl.h>
    
    class DrawX
    {
      protected:
        long WelcomeShown; 
        VOID CALLBACK Line(int X, int Y, LPARAM lpData){};
    
    
      public:
        void Draw(void);
    
    };
    
    void DrawX::Draw()
    {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, WelcomeShown);
    }
    #endif
    I've also tried including the callback function directly within a c++ app, i.e. within the apps header file, and simply call LineDDA on an event, but i still get the same error message. Yet again, when i make callback global it compiles...???
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  4. #4
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    You have a semicolon at the end of the protected member function definition, you need to remove it.

    If that is not the problem, make the function global to the class as you did before and then give it friend status within the class...


    Code:
    #ifndef _DrawX_
    #define _DrawX_
    #include <vcl.h>
    
    
    VOID CALLBACK Line(int X, int Y, LPARAM lpData){}
    
    
    class DrawX {
    protected:
      long WelcomeShown; 
      friend VOID CALLBACK Line(int, int, LPARAM);
    public:
      void Draw(void);
    };
    
    
    void DrawX::Draw() {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, WelcomeShown);
    }
    
    
    #endif // _DrawX_
    Watch the preprocessor include guard flag as well, it should not be exactly the same name as the class.
    Last edited by DarkStar; 01-07-2003 at 10:06 PM.

  5. #5
    Registered User subdene's Avatar
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    Right, so because you've made callback a friend of the class, does that mean it will be able to access the other variables declared within that class. i.e. if i have another variable called int temp; within the protected part, will the callback function be able to use that variable?
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  6. #6
    Registered User subdene's Avatar
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    Is there no way to include the callback function within my class so it can access it's variables?
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  7. #7
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    Originally posted by subdene
    Right, so because you've made callback a friend of the class, does that mean it will be able to access the other variables declared within that class. i.e. if i have another variable called int temp; within the protected part, will the callback function be able to use that variable?
    Yes. A global function, or a member function of a different class that has friend status inside of this class can access all data members within it.

    Is there no way to include the callback function within my class so it can access it's variables?
    I really feel now that your compile time error was caused by the preprocessor include guard surrounding your class. Make sure that it does not have exactly the same name as your class. Try defining the CALLBACK function inside the DrawX class again with a different include guard like _DRAWX_ instead.
    Last edited by DarkStar; 01-08-2003 at 01:23 PM.

  8. #8
    Registered User subdene's Avatar
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    I really appreciate your help on this. When i try to access one of the members in the class i get the compiler error: DrawX E2451 Undefined Symbol FormatOk. I've even tried DrawX::FormatOk = true;

    Code:
    #ifndef _DrawX_
    #define _DrawX_
    #include <vcl.h>
    
    
    VOID CALLBACK Line(int X, int Y, LPARAM lpData)
    {
      FormatOk = true;
    }
    
    
    class DrawX {
    protected:
      long WelcomeShown; 
      bool FormatOk;
      friend VOID CALLBACK Line(int, int, LPARAM);
    public:
      void Draw(void);
    };
    
    
    void DrawX::Draw() {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, WelcomeShown);
    }
    
    
    #endif // _DrawX_
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  9. #9
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    Try defining the global after the class declaration...

    Code:
    #ifndef _DrawX_
    #define _DrawX_
    #include <vcl.h>
    
    
    
    class DrawX 
    {
    protected:
      long WelcomeShown; 
      bool FormatOk;
      friend VOID CALLBACK Line(int, int, LPARAM);
    public:
      void Draw(void);
    };
    
    
    
    void DrawX::Draw()
    {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, WelcomeShown);
    }
    
    
    
    VOID CALLBACK Line(int X, int Y, LPARAM lpData)
    {
      FormatOk = true;
    }
    
    
    
    #endif // _DrawX_
    Compile errors are quite a challenge to figure out.

  10. #10
    Registered User subdene's Avatar
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    Just copied and pasted that, still doesn't work. I give up.
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  11. #11
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    Im getting the same compile error too, I will try to figure this one out, it looks difficult. The use of CALLBACK is just not being allowed.

  12. #12
    Registered User subdene's Avatar
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    ok, thanks for your help.
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  13. #13
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    Try this subdene...

    Code:
    #ifndef _DrawX_
    #define _DrawX_
    #include <vcl.h>
    
    
    
    class DrawX 
    {
    protected:
      long WelcomeShown; 
      bool FormatOk;
      friend VOID CALLBACK Line(int, int, LPARAM);
    public:
      void Draw(void);
    };
    
    // modified global friend...
    VOID CALLBACK Line(int X, int Y, LPARAM lpData)
    {
      DrawX dx; // Here is what was needed
      dx.FormatOk = true;
    }
    
    void DrawX::Draw()
    {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, WelcomeShown);
    }
    
    
    
    #endif // _DrawX_
    Yes you can give a function external to a class the ability to modify protected and private data members, but you have to create an instance of that class before you can access those data members. STUPID ME, sorry.

    Your program should compile now...
    Last edited by DarkStar; 01-08-2003 at 03:18 PM.

  14. #14
    Registered User subdene's Avatar
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    Darkstar - sorry for the late reply Iíve just been trying to get my class fully working. I took your idea, and just altered it slightly. When the callback functionvcreating an instance of the DrawX class, i had a few problems with the deconstructor. As other class initialize objects, and since these methods weren't being called, it was throwing up an error as these objects were trying to be deleted, but no memory had been allocated to them. Therefore, instead of creating an instance of the class, I passed the actual address of the class by casting the this pointer to a long.

    Code:
    #ifndef _DrawX_
    #define _DrawX_
    #include <vcl.h>
    
    
    
    class DrawX 
    {
    protected:
      long WelcomeShown; 
      bool FormatOk;
      friend VOID CALLBACK Line(int, int, DrawX *);
    public:
      void Draw(void);
    };
    
    // modified global friend...
    VOID CALLBACK Line(int X, int Y, DrawX *Draw)
    {
      Draw.FormatOk = true;
    }
    
    void DrawX::Draw()
    {
      WelcomeShown = 0;
      LineDDA(0, 0, 50, 50, (LINEDDAPROC)Line, (long) this);
    }
    
    
    
    #endif // _DrawX_
    Thanks for your help, it works now.....
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  15. #15
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    Thumbs up

    Cool !

    Good to hear that you got it working.
    Last edited by DarkStar; 01-09-2003 at 08:39 PM.

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