Where's NULL?

This is a discussion on Where's NULL? within the C++ Programming forums, part of the General Programming Boards category; In a program I'm writing I tried to initialize some pointers to NULL. When I tried to compile, I received ...

  1. #1
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    Where's NULL?

    In a program I'm writing I tried to initialize some pointers to NULL. When I tried to compile, I received the following message:

    error: 'NULL' was not declared in this scope


    The file this is occurring has no #includes. What needs to be imported for NULL to work? Where is the macro defined?

    Thanks

  2. #2
    (?<!re)tired Mario F.'s Avatar
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    It is implementation defined. A logical place to have it defined is probably <cstdlib>.

    Or you can define it yourself with #define NULL 0, or #define NULL 0L, or any other arithmetic integral type.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  3. #3
    (?<!re)tired Mario F.'s Avatar
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    In fact, a quick reading of the standards reveals its location to be standard defined. It's in <cstddef>
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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    Smile Thanks...

    Thanks for the help.

    ~~Chris

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    Its hard... But im here swgh's Avatar
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    Why not just use 0?
    Yes, I know it is the equivilent of NULL, but many books advise using assigning NULL to
    a pointer to show it has been safely deleted from allocation with delete.

    Code:
    Foo *fo = new Foo;
    
    delete fo;
    fo = NULL;
    My assumption behind this is that zero '0' appears to be purley integer based,
    where as NULL is better used on pointer operations.

    I am not saying you are wrong daved, but I wonder why many books teach pointer
    allocation to NULL and not zero as you suggested? I am intersted on your view on many
    books teachings.
    I'm just trying to be a better person - My Name Is Earl

  7. #7
    (?<!re)tired Mario F.'s Avatar
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    I personally prefer 0.

    MinGW does not even define NULL correctly, adopting instead ((void*)0). For some reason the __cplusplus macro is not defined when reaching stddef.h and the correct C++ form is not used.

    This code, compiled with mingw32-g++.exe, gives me "warning: converting to non-pointer type `int' from NULL"

    Code:
    #include <cstdlib>
    int main() { int p = NULL; }
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

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    using NULL or 0 is just preference. It's both correct. The books probably suggest doing it because when you see NULL you immediately see you're assigning to a pointer.

    I dont care if somebody uses 0 or NULL, I usually just adopt the style used by the one before me. when I write things just for me I just use 0, less typing

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    >> why many books teach pointer allocation to NULL and not zero as you suggested?
    There are a lot of practices suggested by C++ books that are not exactly how I (or Bjarne Stroustrup) would do it. Part of this is because many books use C style or older C++ style idioms and practices in their C++ code. Since NULL is actually different than 0 in C, there is more reason to use it there. As has been said, it is perfectly fine to use in C++, I just prefer 0. And as the link says, we will all be using nullptr in the future anyway.

  10. #10
    Its hard... But im here swgh's Avatar
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    we will all be using nullptr in the future anyway.
    Good. Im glad about that, it is to the point and you can see imediatley what the statement does
    I'm just trying to be a better person - My Name Is Earl

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