Is it safe to #define NULL nullptr?

问题

I have seen below macro in many topmost header files:

#define NULL 0  // C++03

In all over the code, NULL and 0 are used interchangeably. If I change it to.

#define NULL nullptr  // C++11

Will it cause any bad side effect ? I can think of the only (good) side effect as following usage will become ill-formed;

int i = NULL;

回答1:

I have seen below macro in topmost header file:

You shouldn't have seen that, the standard library defines it in <cstddef> (and <stddef.h>). And, IIRC, according to the standard, redefining names defined by standard header files results in undefined behaviour. So from a purely standardese viewpoint, you shouldn't do that.

---

I've seen people do the following, for whatever reason their broken mind thought of:

struct X{
  virtual void f() = NULL;
}

(As in [incorrectly]: "set the virtual table pointer to NULL")

This is only valid if NULL is defined as 0, because = 0 is the valid token for pure-virtual functions (§9.2 [class.mem]).

That said, if NULL was correctly used as a null pointer constant, then nothing should break.

However, beware that, even if seemingly used correctly, this will change:

void f(int){}
void f(char*){}

f(0); // calls f(int)
f(nullptr); // calls f(char*)

However, if that was ever the case, it was almost certainly broken anyways.

回答2:

Far better is to search and replace NULL with nullptr throughout the code.

It may be syntactically safe, but where would you put the #define? It creates code organisation problems.

回答3:

No. You're not allowed to (re)define standard macros. And if you see

#define NULL 0

at the top of any file other than a standard header (and even there, it should be in include guards, and typically in additional guards as well), then that file is broken. Remove it.

Note that good compilers will typically define NULL with something like:

#define NULL __builtin_null

, to access a compiler builtin which will trigger a warning if it is used in a non-pointer context.

回答4:

You shouldn't be defining it at all, unless you're writing your own version of <cstddef>; it certainly shouldn't be in "many topmost header files".

If you are implementing your own standard library, then the only requirement is

18.2/3 The macro NULL is an implementation-defined C++ null pointer constant

so either 0 or nullptr is acceptable, and nullptr is better (if your compiler supports it) for the reason you give.

回答5:

Maybe Not

If you have a particular format of overloading behaviour:

void foo(int);
void foo(char*);

Then the behaviour of the code:

foo(NULL);

will change depending on whether NULL is changed to nullptr or not.

Of course, there's another question as to whether it's safe to write such code as is present in this answer...

回答6:

While it might break backwards-compatibility with older stuff that was badly written (either that, or overly clever...), for your newer code, this is a non-issue. You should use nullptr, and not NULL, where you mean nullptr. Also, you should use 0 where you mean zero.

来源：https://stackoverflow.com/questions/9003425/is-it-safe-to-define-null-nullptr