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

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.

In fact, a quick reading of the standards reveals its location to be standard defined. It's in <cstddef>

Thanks for the help.

~~Chris

Why not just use 0?

http://www.research.att.com/~bs/bs_faq2.html#null

Quote:

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Why not just use 0?

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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:

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Foo *fo = new Foo;

delete fo;
fo = NULL;

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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 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:

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#include <cstdlib>
int main() { int p = NULL; }

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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 :)

>> 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.

Quote:

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we will all be using nullptr in the future anyway.

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Good. Im glad about that, it is to the point and you can see imediatley what the statement does