Instead Of NULL, Should I Write `0x0` Or `0`? - C++ - Stack Overflow

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Learn more about Labs Instead of NULL, should I write `0x0` or `0`? Ask Question Asked 15 years ago Modified 5 years, 3 months ago Viewed 39k times 18

I know that you are supposed to use 0 instead of NULL in c++ (even though NULL is defined as 0 in C++ most of the time).

Recently I came across some code where 0x0 was used instead, though.

What is the difference?

• c++
• null

Share Improve this question Follow edited Dec 13, 2011 at 17:46 Lightness Races in Orbit 384k7777 gold badges663663 silver badges1.1k1.1k bronze badges asked Nov 19, 2009 at 17:42 mokamoka 36511 gold badge55 silver badges1010 bronze badges 5

• Who says you're supposed to use 0 instead of NULL? Personally, I think you should use "(void*)0". – Paul Tomblin Commented Nov 19, 2009 at 17:47
• 9 @Paul: (void*)0 will not work similarly in C++. C++ compilers get angry when they see something like: int* x = (void*)0; while int* x = 0; is OK. – Mehrdad Afshari Commented Nov 19, 2009 at 17:53
• Yeah, I was more opining about the design of the language than suggesting you actually do it that way. – Paul Tomblin Commented Nov 19, 2009 at 17:56
• 1 @Paul: Not allowing implicit conversion of void* to other pointer types is one place where C++ broke compatibility with C (and for a reason). – UncleBens Commented Nov 19, 2009 at 21:31
• @Paul: "I think you should use" "I was more [..] than suggesting you actually do it that way" – Lightness Races in Orbit Commented Dec 13, 2011 at 17:47

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8 Answers 8

Sorted by: Reset to default Highest score (default) Trending (recent votes count more) Date modified (newest first) Date created (oldest first) 29

0x0 is just 0 written in hexadecimal notation. There is no difference between the two:

016 = 010 :)

NULL is usually #defined to 0 somewhere and does the same thing.

Share Improve this answer Follow answered Nov 19, 2009 at 17:43 Mehrdad AfshariMehrdad Afshari 421k9292 gold badges859859 silver badges794794 bronze badges 2

• And = 0_8, which is what the question asks about. – Lightness Races in Orbit Commented Dec 13, 2011 at 17:48
• Also, as 0b0(binary) and 00(octal). Although, most compilers' preprocessor changes these to 0. – Artfaith Commented Aug 7, 2019 at 18:56

Add a comment | 19

There is no difference. In C, NULL is often defined to be (void *)0, but in C++ that's not allowed. A few ancient compilers got this wrong, but they really are ancient.

IMO, it's better to use NULL, as it portrays the intent more clearly, and gives you a nice, easy symbol to S&R when your compiler gets updated to C++ 0x, which will include nullptr.

Share Improve this answer Follow answered Nov 19, 2009 at 17:49 Jerry CoffinJerry Coffin 489k8383 gold badges647647 silver badges1.1k1.1k bronze badges 1

• +1. Up to very recently I was party to using 0 instead of NULL, but the inclusion of nullptr in C++0x made me change my mind. – Gorpik Commented Nov 26, 2009 at 16:55

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By the way, everyone,

0x0 is hex, as you have all mentioned, but

0 is Octal, not decimal! :-)

ie any number starting with 0 (and not followed by x) is octal:

0 = 0 01 = 1 02 = 2 ... 07 = 7 010 = 8 011 = 9 012 = 10 ...

:-)

Share Improve this answer Follow answered Nov 19, 2009 at 18:37 tonytony 3,85711 gold badge2020 silver badges1818 bronze badges 3

• 2 I was going to down vote you for sowing confusion. But I had to check the standard first ... and you're right. Learn something new everyday (not that this is worth remembering, but I will anyways). – deft_code Commented Nov 19, 2009 at 22:26
• 1 that's exactly how it lives in my brain - I know it is totally useless, but yet I remember it. – tony Commented Nov 20, 2009 at 2:27
• 1 Correct, but not an answer to the question. – Lightness Races in Orbit Commented Dec 13, 2011 at 17:48

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0x0 and 0 represent the same value, so you can use the one that best suits your eye. When we get C++0x, we'll have the keyword nullptr to represent null pointers of any type.

Share Improve this answer Follow answered Nov 19, 2009 at 17:48 Michael KristofikMichael Kristofik 35.1k1616 gold badges7777 silver badges127127 bronze badges Add a comment | 2

0x0 is just an expression of the value 0 in hexadecimal. I doubt the compiler will care about the difference.

Share Improve this answer Follow answered Nov 19, 2009 at 17:44 postfuturistpostfuturist 22.6k1111 gold badges6666 silver badges8585 bronze badges Add a comment | 2

No difference

In my opinion, 0x0 is more explicit.

Some people may confuse 0 with "0"(0x30 -I´ve seen it happen).

Share Improve this answer Follow answered Nov 19, 2009 at 17:44 TomTom 45k3030 gold badges141141 silver badges171171 bronze badges 4

• 4 0x0000 is even more explicit! – Roger Pate Commented Nov 19, 2009 at 17:59
• 1 @Roger: nah, that's only more explicit on a 16bit machine. On a 32bit machine it's still not explicit enough ;-) – Steve Jessop Commented Nov 19, 2009 at 18:09
• 2 +1 for giving me a laugh. But if you are running a 64 bit compiler you should use 0x0000000000000000 for maximum explicitness. – Gunther Piez Commented Nov 19, 2009 at 18:11
• Actually, on a 64 bit machine it may make sense to define NULL as 0LL, so sizeof(NULL)==sizeof((void*)NULL). – MSalters Commented Nov 20, 2009 at 9:06

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Simply put, its a matter of taste. People like to write memory addresses in hexa decimal, so writing NULL as 0x0 would be more classic. For ordinary ones like me, 0 will suffice.

Share Improve this answer Follow answered Nov 19, 2009 at 18:25 Shailesh KumarShailesh Kumar 6,88788 gold badges3737 silver badges6060 bronze badges Add a comment | -8

So, this site has some really good information on NULL: http://c-faq.com/null/index.html

NULL is actually the "null pointer" - so it might be of type, for example, "(void*)0". This tells the compiler that NULL is not to be treated as an int, but rather as a "pointer to an int". And in C, the compiler will give warnings/errors if the types don't match.

The value "0" and "0x0" are equivalent in C++. Whenever a number is prefixed by "0x", it means that that value is the hexadecimal representation of that number. For example, 0x5 == 5.

Also, 0xA [in hexadecimal, base 16) == 10 [in base 10].

edit

Here are some sources. In glibc, NULL seems to be defined differently in different places. (why should this be?)

#define NULL ((void *)0) (stdlib/gmp-impl.h)

#define NULL 0 (posix/getopt1.c)

edit2

well, it looks like i've missed the mark on this one! sorry! (CW'd. feel free to keep downvoting!)

Share Improve this answer Follow edited Nov 26, 2009 at 16:48 community wiki 3 revspoundifdef 3

• 6 In C++, NULL will not be (for example) "(void*)0" -- that's clearly not allowed. Though it's probably a typo, that would be treated as point to void, not pointer to int. – Jerry Coffin Commented Nov 19, 2009 at 18:11
• This answer is wrong. NULL is not the "null pointer". It merely converts to a "null pointer" of a particular type. Also you are linking to a C site on a C++ question. (even in C, NULL is not necessarily a "null pointer"). – Johannes Schaub - litb Commented Nov 26, 2009 at 11:15
• Notice that your linked site says "NULL is defined as a null pointer constant" - not as a null pointer! – Johannes Schaub - litb Commented Nov 26, 2009 at 11:16

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