difference between 0 and null

This is a discussion on difference between 0 and null within the C++ Programming forums, part of the General Programming Boards category; Hi What's the difference between these two statements - I'm not sure, and I'm using one of them in srand(): ...

  1. #1
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    difference between 0 and null

    Hi

    What's the difference between these two statements - I'm not sure, and I'm using one of them in srand():

    time(0)

    time(NULL)

    Thanks

  2. #2
    Magically delicious LuckY's Avatar
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    There is no difference. NULL is defined as 0.

  3. #3
    Registered User major_small's Avatar
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    NOOOOOOOOOO!

    NULL is a macro... when you use time(0), it's expecting a pointer, so when it gets a 0, it kinda sends the data nowhere... but when you pass NULL, the compiler defines NULL as what it wants... usually it is 0, but it can also be something else...

    NULL != 0
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  4. #4
    Code Goddess Prelude's Avatar
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    >NOOOOOOOOOO!
    You're overreacting just a smidge.

    >NULL is a macro...
    Good so far

    >when you use time(0), it's expecting a pointer
    Still good

    >so when it gets a 0, it kinda sends the data nowhere...
    Eh? When 0 is used in a pointer context, it is converted to a null pointer. Note that a null pointer need not be all bits zero, even when the literal 0 is used.

    >but when you pass NULL, the compiler defines NULL as what it wants...
    Technically, NULL is defined well before you use it. However, it is true that NULL may not be all bits zero, the representation of a null pointer is up to the implementation, as is the definition of NULL.

    >NULL != 0
    NULL != 0 when NULL is not defined as 0. Most modern implementations define NULL like so:
    Code:
    #define NULL 0
    However, the use of NULL is considered unsafe practice in C++ because it could also be defined as
    Code:
    #define NULL ((void*)0)
    This causes a world of trouble, while the literal 0 is guaranteed to work correctly.
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  5. #5
    Registered User major_small's Avatar
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    ^ I've always been told that getting into the habit of saying that "NULL is the same as 0" is as bad as writing void main()...

    I reacted like that because the first response was short and (from what I've been taught) was exactly the wrong answer to give...
    Last edited by major_small; 12-10-2003 at 10:03 AM.
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  6. #6
    Code Goddess Prelude's Avatar
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    >I've always been told that getting into the habit of saying that "NULL is the same as 0" is as bad as writing void main()...
    That attitude is a little bit extreme. void main is explicitly stated by the standard to be ill-formed. A null pointer constant is any expression that evaluates to zero, and NULL is an implementation-defined macro that is highly likely to be an expression that evaluates to zero. So "NULL is the same as 0" is likely to be correct, while writing void main is almost never correct. As with everything, there are no hard and fast rules that apply in all cases. For example, even void main can be legal C++ in certain situations.
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    #define NULL (void*)0

  8. #8
    Registered User cyberCLoWn's Avatar
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    Maybe a dumb question, but wouldn't (void*)0 give you 0 in any case?

  9. #9
    and the hat of wrongness Salem's Avatar
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    > but wouldn't (void*)0 give you 0 in any case?
    Whilst (void*)0 is freely usable as a NULL pointer in C (because C treats void* pointers as special), it presents no problem.

    However, having (void*)0 as NULL in C++ raises a lot of needless warnings about casting between pointer types. Since C++ (in a pointer context) converts 0 into (whatever_type*)0 for you, there is no point in decorating
    #define NULL 0
    with
    #define NULL (void*)0
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    I use NULL all the time, especially for clearing flags (my flags mostly). Its just clearer to read that you're setting that value to a non-accepted value. I use zero only for numbers, null for pointers and flags.

  11. #11
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    NULL the macro is even worse for flags. (bits & NULL) is fine where NULL is 0 but where NULL is ((void *)0) the results are at best an error in C++, quite possibly an implementation defined value even equivilent to (bits & 0xFFFFFFFF) in C Thus it could be exactly the oppisite of what you wanted. 0 in C++ is automagical, as is evaluating a pointer in a boolean context. Years ago while(p && p->next) p = p->next; could fail, today in C++ use of NULL has all of the problems that the original implied cast to int had in C. The problem is much more rare as to be using C headers, yet compiling as C++, and those headers need to be somewhat unaware of C++, and you need (int)(NULL) != 0, and that's excedingly rare, but if you are going to pick nit's NULL is more nitworthy than 0

  12. #12
    and the hat of wrongness Salem's Avatar
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    > Years ago while(p && p->next) p = p->next; could fail,
    News to me - I've never seen that one before
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  13. #13
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    In C, prior to ansi if NULL was non-zero when cast as an int the loop could continue forever. if() while() etc took an int and evaluated true if non zero, C++ takes a bool where ints are cast to true if non-zero and pointers are cast to true if non-null.

  14. #14
    and the hat of wrongness Salem's Avatar
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    The internal representation of a NULL pointer has always been irrelevant at the level of C source code.

    if ( p )
    if ( p != 0 )
    if ( p != NULL )
    are all the same whether the internal representation of NULL is all bits zero or not.

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    http://www.eskimo.com/~scs/C-faq/s5.html

    Sounds to me more like a case of crappy coding on a DOS machine where it really didn't matter whether you were any good at allocating memory or not. For a while at least, it usually worked anyway.
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  15. #15
    Guest Sebastiani's Avatar
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    NULL and 0 : the former is used as a visual cue that something is null (invalidated, negated, denied) whereas the latter might indicate that something is simply integrally zero (though it's perfectly clear what (p!=0) means when p is a pointer, of course).



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