When MSDN says NULL, is it okay to use nullptr?

In various places, MSDN will talk about the behavior corresponding to the case where a handle type has the value NULL
. A customer wanted to know whether it was safe to use nullptr
in such cases, or whether they have to use NULL

Although the programming languages used by MSDN for documenting Windows are putatively C and C++, MSDN understands that a lot of people write code for Windows in other languages, and therefore it tries to avoid relying on language subtleties

Esoteric definitions for the term NULL
is one of those language subtleties.

Formally, the C and C++ languages permit the following definitions for the NULL

NULL 0 (void*)0 nullptr
Callowedallowednot allowed¹
C++allowednot allowed²allowed

is defined as (void*)0
in C or as nullptr
in C++, then it can be assigned only to a pointer type. And since MSDN cannot control how the C and C++ header files define NULL
, it needs to work with any definition that is permitted by the corresopnding standards. Which means that saying NULL
implies that the underlying type is a pointer type.

Therefore, you are welcome to write nullptr
instead of NULL
if you’re writing C++ code. You’re also welcome to write anything else that produces a null pointer, such as

HMUMBLE h5 = 3 - 3;

But most people would probably prefer you to write NULL
or nullptr

As noted, MSDN understands that a significant portion of its readership is not fluent in the subtleties of C and C++. When it writes NULL
, it means the obvious thing: A null pointer. You can translate that into the appropriate construction for the language you are using. For example, for C#, you can use null
, or if you are operating in raw IntPtr
s, you can use IntPtr.Zero

Bonus chatter
: When MSDN says NULL
, is it okay to use 0
? Yes, but you probably don’t want to. Using 0
as a null pointer constant is permitted by the C and C++ languages for backward compatbility reasons, but it’s not considered modern style.

Bonus bonus chatter
: I’m told that the Visual C++ folks occasionally entertain the possibility of changing the definition of NULL
to nullptr
, which is permitted by the standard. However, this ends up breaking a lot of code which assumed that NULL
is an integral constant evaluating to zero. For example:

void foo(char* p)
  char c = NULL; // would not work if NULL were defined as nullptr
  *p = NULL;     // would not work if NULL were defined as nullptr

Although that code is technically already broken, it manages to work if NULL
is defined as 0
, and updating the definition in the language header files would break existing (albeit poorly-written) code.

¹ C does not have the nullptr

² C++ does not allow NULL
to be defined as (void*)0
because C++ does not permit implicit conversion from void*
to arbitrary T*

int* p = (void*)0; // allowed in C but not C++
稿源:The Old New Thing (源链) | 关于 | 阅读提示

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