Will C++11 implementations define NULLas nullptr?
Would this be prescribed by the new C++ standard?
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2The standard says it's allowed; though few implementations are likely to actually do it.– Billy ONealCommented Aug 26, 2011 at 6:52
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4 Answers
From the horse's mouth
C.3.2.4 Macro NULL [diff.null]
1/ The macro NULL, defined in any of
<clocale>,<cstddef>,<cstdio>,<cstdlib>,<cstring>,<ctime>, or<cwchar>, is an implementation-defined C++ null pointer constant in this International Standard (18.2).
It is up to each implementation to provide its own definition, gcc if I recall correctly defines it to __nullptr for which it has special checks (verifies that it is not used in arithmetic contexts for example).
So it is possible to define it as nullptr, you will have to check your compiler/Standard Library documentation to see what has been done.
answered Aug 26, 2011 at 6:25
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On the other hand, the standard doesn't say that is has to be
nullptr, even though that change was proposed. Commented Aug 26, 2011 at 6:54 -
@Bo: I am not much of a standardista nor is English my mother-tongue so I assumed that implementation-defined meant exactly that, ie the Standard does not impose anything. Commented Aug 26, 2011 at 7:02
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4My point is that it was proposed that the standard should require NULL to be nullptr, but that didn't happen. Commented Aug 26, 2011 at 7:16
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@Bo: Ah okay, I wish it had, it could probably have uncovered a number of bugs. I may be a type/compiler freak, but I must admit I never quite understand people who refuse such changes. Commented Aug 26, 2011 at 8:37
No, NULL is still the same as before. Too many people used the NULL macro in surprising ways, redefining it to nullptr would have broken a lot of code.
To elaborate: people have used NULL for example for many kinds of handle typedefs. If the real type behind such a typedef is not a pointer, defining NULL as nullptr would be a problem. Also, it seems some people have indeed used NULL to initialize numeric types.
At least that is what Microsoft found when they added the nullptr to MSVC10, and why they decided to keep NULL as it always was. Other compilers might choose a different path, but I don't think they would.
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1I've seen code that uses NULL to initialize an element of an array of char (it should have used
'\0'). Personally I wouldn't mind breaking such code, but backward compatibility is a harsh mistress. Commented Aug 26, 2011 at 6:57 -
"... Microsoft found ... they decided to keep NULL ..." - nice info. Do you have any reference link? Commented Aug 26, 2011 at 8:24
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1@Martin: I have that information from one of the Channel 9 videos with Stephan T. Lavavej (the Microsoft guy responsible for their STL implementation) about the C++0x features. I don't remember the exact video though.– TeaWolfCommented Aug 26, 2011 at 8:43
FDIS of the upcoming standard C++11, integral expression is still a null pointer constant. NULL macro is still implementation defined but must be a null pointer constant. So in practice it means it is good as 0 or can be nullptr.
Your code that used either 0 or NULL will work just as before.
Read the details here.
answered Aug 26, 2011 at 6:23
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Its not "upcoming" standard anymore. It is already a current standard. Commented Aug 26, 2011 at 7:18
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1It has become the ISO Standard on 12th August 2011. See this which says
The next revision of C++ that we’ve been calling “C++0x” is now an International Standard!”Commented Aug 26, 2011 at 7:35 -
@Nawaz: I hate to break it to you, but All we have is the C++0x FDIS, which is identical in content to what will be the C++11 standard in a few weeks. The FDIS was approved. It is not a standard until it is published. Your link is One of those rare ocassions where Herb erred, or even lied. Commented Aug 26, 2011 at 7:54
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4@Als: Herb is right. Per the ISO/IEC Directives Part 1, 2.7.8, voting is what makes an IS out of an FDIS. Publication is covered in 2.8– MSaltersCommented Aug 26, 2011 at 8:14
NULL comes from C, so its definition must be compatible with both C and C++. So it can't be nullptr, because that is C++ keyword, not C, unless it is defined differently for C and C++ (we can use #ifdef __cplusplus to distinguish between them). So NULL is usually defined as smth compiler specific, like __nullptr in gcc, or just ((void*)0).
