pointer state, how to test?

This is a discussion on pointer state, how to test? within the C++ Programming forums, part of the General Programming Boards category; Hello, An object passes a pointer as a parameter to a function call. Is there a was to find if ...

  1. #1
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    pointer state, how to test?

    Hello,

    An object passes a pointer as a parameter to a function call.

    Is there a was to find if the memory was freed by the calling function and the pointer is not valid?

    BR,
    Cristian

  2. #2
    Iamregistered
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    you can test if the pointer is valid by seeing if it is NULL or not. If it is, this does not however mean that the memory was released - the pointer may just have been assigned to a different address.

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    Re: pointer state, how to test?

    Originally posted by Cristian Negres
    Hello,

    An object passes a pointer as a parameter to a function call.

    Is there a was to find if the memory was freed by the calling function and the pointer is not valid?

    BR,
    Cristian
    No, that's something you have to keep track of yourself.

  4. #4
    Confused Magos's Avatar
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    Re: pointer state, how to test?

    Originally posted by Cristian Negres
    Hello,

    An object passes a pointer as a parameter to a function call.

    Is there a was to find if the memory was freed by the calling function and the pointer is not valid?

    BR,
    Cristian
    This is the reason NULL pointers exists. NULL pointer => Whatever it pointed at has been freed/doesn't exist any more.
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    Hi,

    Thank you for your help.

    BR,
    Cristian

  6. #6
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    Originally posted by Magos
    This is the reason NULL pointers exists. NULL pointer => Whatever it pointed at has been freed/doesn't exist any more.
    Really? I thought that delete only deallocated memory...not change the pointer.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  7. #7
    Confused Magos's Avatar
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    Originally posted by pianorain
    Really? I thought that delete only deallocated memory...not change the pointer.
    I never said that delete sets the pointer to NULL. That is up to you. You should always set the pointer to NULL after deletion to make sure the adress cannot be accessed or a call to delete again doesn't screw something up.
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  8. #8
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    The NULL pointer always (to my knowledge... there may be exceptions) points to address 0 in memory. Most operating systems never assign any object there. It's strictly off-limits. A pointer set to this place will never contact a real object.

    Also, any attempt to access this space in memory automatically results in a Seg Fault. If anything attempts to dereference the pointer, the program will tell them immediately, and the program will crash.

    If the pointer wasn't reallocated, the memory the pointer's accessing could be garbage, and the program would still run. This makes finding the error much harder.

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    Unfortunately, the instance of the pointer that is deallocated with delete is not necessarily the one that will cause a seg fault. Most likely, it will be another copy of the pointer that is unaware that the memory is freed and has no way of knowing. The common solution to this is reference counted "smart" pointers.
    The crows maintain that a single crow could destroy the heavens. Doubtless this is so. But it proves nothing against the heavens, for the heavens signify simply: the impossibility of crows.

  10. #10
    Confused Magos's Avatar
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    Originally posted by IfYouSaySo
    The common solution to this is reference counted "smart" pointers.
    A good structure in your program would never put you in this position.
    Last edited by Magos; 02-22-2003 at 03:59 PM.
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    >A good structure in your program would bever put
    >you in this position.

    If you say so.

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