casting problem

This is a discussion on casting problem within the C++ Programming forums, part of the General Programming Boards category; Hello.. What am I doing wrong with casting? Code: class someclass { public: char member; }; int main() { someclass ...

  1. #1
    l2u
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    casting problem

    Hello..

    What am I doing wrong with casting?

    Code:
    class someclass {
    public:
    	char member;
    };
    
    int main() {
    	someclass *sc = new someclass();
    	ULONG_PTR sc_pointer = static_cast<ULONG_PTR>(sc);	
    	return 0;
    }
    Thanks for help

  2. #2
    and the hat of wrongness Salem's Avatar
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    > What am I doing wrong with casting?
    At the moment, everything.

    C++ has 4 different kinds of cast, maybe read up on what each kind is really meant for.
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  3. #3
    Registered User hk_mp5kpdw's Avatar
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    Yeah, you're using the wrong cast. Try the one that starts out reint...
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  4. #4
    l2u
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    Thanks guys.. I used reinterpret_cast and works now..

    One more question..

    People often use c style cast in c++ code, is this a 'smart way' to do it? Do you use c style casts? If so, when and when not?

  5. #5
    Registered User hk_mp5kpdw's Avatar
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    You should definitely read this very recent post.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
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  6. #6
    Frequently Quite Prolix dwks's Avatar
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    Or the version that got added to the FAQ: FAQ: Difference between C and C++ style casting
    dwk

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  7. #7
    l2u
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    I have another question.
    What is the right way to cast someclass (sc) so somefunc can change the pointer?

    Code:
    class someclass {
    public:
    	char member;
    };
    
    void somefunc(ULONG_PTR *ptr) { }
    
    int main() {
    	someclass *sc;
    	somefunc(reinterpret_cast<ULONG_PTR*>(sc));
    	return 0;
    }

  8. #8
    Cat without Hat CornedBee's Avatar
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    What do you mean? If you want to change the pointer, you need to pass a pointer to it. In other words:
    Code:
    void somefunc(ULONG_PTR ptr)
    {
      someclass **p = reinterpret_cast<someclass **>(ptr);
      *p = new someclass;
    }
    
    int main() {
    	someclass *sc;
    	somefunc(reinterpret_cast<ULONG_PTR>(&sc));
    	return 0;
    }
    The other option would be to use your prototypes, but that code is 1) ugly and 2) useless. It would look like this:
    Code:
    void somefunc(ULONG_PTR *ptr)
    {
      someclass *p = new someclass;
      *ptr = reinterpret_cast<ULONG_PTR>(p);
    }
    
    int main() {
    	someclass *sc;
    	ULONG_PTR sccast = reinterpret_cast<ULONG_PTR>(sc);
    	somefunc(&sccast);
    	return 0;
    }
    Edit: Just realized this won't actually change sc. Just sccast.

    However, as I said before, this is useless. The only valid reason to cast a pointer this way in the first place is if you use a WinAPI function that requires the ULONG_PTR or similar type: callbacks, SetWindowLongPtr, SendMessage. None of these takes an ULONG_PTR, so this approach is out of the question.

    In all cases where you have the choice to use a ULONG_PTR*, you also have the choice to use a someclass* or someclass** or someclass*&. And all of those are far better alternatives. (Although not as good as a smart pointer.)
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  9. #9
    l2u
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    I use WinAPI function GetQueuedCompletionStatus thats why I need this.

  10. #10
    CSharpener vart's Avatar
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    GetQueuedCompletionStatus want to receive the pointer to ULONG_PTR variable to be filled with the completion key value. Why do you want instead pass the pointer to your class?
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  11. #11
    l2u
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    Because completion key is address of client structure? I want to change the pointer so it would point to this address.

  12. #12
    CSharpener vart's Avatar
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    Why do you want to cast outside the function? I think better to hide the cast inside the function
    Code:
    someclass* getCompl()
    {
    	ULONG_PTR temp;
    	if(GetQueuedCompletionStatus(...,&temp,...))
    	{
    		return reinterpret_cast<someclass*>(temp);
    	}
    	return 0;
    }
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  13. #13
    l2u
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    The problem is I would have to return OVERLAPPED structure too, so it would be better to pass both as argument and then set it.

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