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NULL vs 0

Jamin Grey · 2012-12-16T02:40:23

[[size=2]forked from Problems with my custom linked list class] In another thread, I gave this advice: Don't compare to 0, compare to NULL (or if using C++11, compare to nullptr). It describes your intent better.[/quote] Another forummer responded, ...I always thought the same thing you just said there. Though I posted a basic Linked List code couple weeks ago and was using NULL instead of 0 and someone who said he was a TA at a University kept saying that I should not be using NULL in C++, use 0. Good to know that what I learned to use NULL still seems to be right. Also I do use nullptr when I can but the University Compilers don't support C++11 yet.[/quote] So I'd like to hear other people's thoughts on it as well. [hr] My thoughts are purely based on describing your code intent. 0 implies working with numbers, whereas NULL implies working with pointers; even though pointers are a number, their usage and purpose is different enough that instantly discerning what is what makes code (very slightly) easier to read. Example: this->object = 0; Is 'object' an integer index into an array where the object is found, or is it a pointer to the object itself? Granted, better naming also helps alleviate this confusion, but there is no denying that strict usage of NULL and 0 would help where improper naming causes confusion. Looking for an alternative perspective, Bjarne Stroustrup says this: In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days. If you have to name the null pointer, call it nullptr; that's what it's called in C++11. Then, "nullptr" will be a keyword.[/quote] So his complaints are: Macros are evil People are ignorant Older compilers are broken All valid-ish points. Here are suggested fixes: Macros are evil The problems usually associated with macro mis-use doesn't apply to a single '0'. 0 cannot be accidentally be used in any way that I can think of, which might mess up operator precedence or anything like that. But if you are absolutely intent on avoiding macros, use a constant: const size_t MyNull = 0; People are ignorant Perhaps, but the key then is to educate them. Either that, or don't let them near your code, since there are more dangerous issues in C++ that ignorance of NULL. Older compilers are broken So use a modern compiler? Older compilers mess up in other ways as well. Shall we not use the parts of the standard that older compilers got wrong? However, he does make a good point about "If you have to name the null pointer, call it nullptr; that's what it's called in C++11." If, for some reason, you are set against using NULL as a macro, and decided to use a constant, you might as well use this as your constant: #if __cplusplus < 201103L const size_t nullptr = 0; #endif It'll only be defined (I think) if C++11's nullptr is not available. [hr] When you add C++11's nullptr into the mix, nullptr is obviously the superior choice because not only does it declare your intent, but it also provides additional type-safe protections. But between the outdated NULL and 0, what are your thoughts on the matter?

General and Gameplay Programming Programming

Started by Servant of the Lord December 14, 2012 07:29 PM

43 comments, last by wqking 11 years, 4 months ago

wqking

778

December 15, 2012 04:50 PM

[font=courier new,courier,monospace]NULL[/font] is [font=courier new,courier,monospace]#define[/font]d as [font=courier new,courier,monospace]0[/font], meaning there's absolutely no difference except style/personal preference, and Bjarne "C++" Stroustrup was quoted earlier as preferring 0 (or nullptr in C++11 where available) over the NULL macro.

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https://github.com/wqking/eventpp eventpp -- C++ library for event dispatcher and callback list

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Bruno Sofiato

407

December 15, 2012 04:55 PM

IMO using NULL makes the intent clearer.

Cornstalks

7,033

December 15, 2012 06:02 PM

[quote name='Hodgman' timestamp='1355570648' post='5010908']
[font=courier new,courier,monospace]NULL[/font] is [font=courier new,courier,monospace]#define[/font]d as [font=courier new,courier,monospace]0[/font], meaning there's absolutely no difference except style/personal preference, and Bjarne "C++" Stroustrup was quoted earlier as preferring 0 (or nullptr in C++11 where available) over the NULL macro.

As far as I understand, C++ does not require NULL to be zero.
It can be any integer beside 0.
I can not remember where I read, but on some embed system, NULL can be a value different than zero.
So he is correct.
[/quote]
Not in a standard complying C++ implementation. The C++ standard requires [font=courier new,courier,monospace]NULL[/font] to evaluate to zero (that is, [font=courier new,courier,monospace]NULL[/font] == 0 is always true). Embedded systems implementations sometimes break the rules of C++, so they're not exactly the best reference. If you don't believe me, read the C++ Standard:

Section 18.2, paragraph 3: The macro NULL is an implementation-de?ned C++ null pointer constant in this International Standard
Section 4.10, paragraph 1: A null pointer constant is an integral constant expression prvalue of integer type that evaluates to zero or a prvalue of type [font=courier new,courier,monospace]std::nullptr_t[/font]

Since [font=courier new,courier,monospace]NULL[/font] is required to be a "null pointer constant" and a "null pointer constant is an integral constant expression [...] that evaluates to zero" we can deduct that [font=courier new,courier,monospace]NULL[/font] evaluates to zero.

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SiCrane

11,840

December 15, 2012 07:13 PM

I can not remember where I read, but on some embed system, NULL can be a value different than zero.

What you might be thinking of is the actual representation on the machine being different from zero, which is allowed by the standard. It's also legal for some pointers to have a zero binary representation, but others have a non-zero binary representation. For instance, with MSVC most pointers use 0 for the actual representation of a null pointer, but pointers to members have the value 0xFFFFFFFF on 32-bit builds. However, no matter what value the machine actually sees, you can't use a non-zero integral constant to represent the null pointer.

iMalc

2,466

December 15, 2012 10:17 PM

Macros are a fact of C & C++ programming. e.g. You don't get compiler independent include guards, or a whole heap of other useful stuff like half of boost, without them. Claiming all macros are evil is like any other claim that all XYZ are bad. There are always degrees of bad, and there are always exceptions.

NULL is like the least evil macro there is.
Use the tools you are provided with, in the way that they were intended to be used, that shows the most clear intentions.
The real problems are caused when you fight against the language / tools, and write unclear code.

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wqking

778

December 16, 2012 02:40 AM

Not in a standard complying C++ implementation. The C++ standard requires [font=courier new,courier,monospace]NULL[/font] to evaluate to zero (that is, [font=courier new,courier,monospace]NULL[/font] == 0 is always true). Embedded systems implementations sometimes break the rules of C++, so they're not exactly the best reference. If you don't believe me, read the C++ Standard:

Section 18.2, paragraph 3: The macro NULL is an implementation-de?ned C++ null pointer constant in this International Standard
Section 4.10, paragraph 1: A null pointer constant is an integral constant expression prvalue of integer type that evaluates to zero or a prvalue of type [font=courier new,courier,monospace]std::nullptr_t[/font]

Since [font=courier new,courier,monospace]NULL[/font] is required to be a "null pointer constant" and a "null pointer constant is an integral constant expression [...] that evaluates to zero" we can deduct that [font=courier new,courier,monospace]NULL[/font] evaluates to zero.

Thanks for pointing it out.
Knowing standard better is always good.

https://www.kbasm.com -- My personal website

https://github.com/wqking/eventpp eventpp -- C++ library for event dispatcher and callback list

https://github.com/cpgf/cpgf cpgf library -- free C++ open source library for reflection, serialization, script binding, callbacks, and meta data for OpenGL Box2D, SFML and Irrlicht.

NULL vs 0

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NULL vs 0

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