它是安全的#define NULL nullptr？

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问题描述

我在许多最顶层的头文件中看到下面的宏：

I have seen below macro in many topmost header files:

#define NULL 0  // C++03

在所有代码中， NULL 和 0 可互换使用。如果我将它更改为。

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

#define NULL nullptr  // C++11

会产生不良的副作用吗？我可以想到唯一的（良好的）副作用，如下面的用法会变得不合理：

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;

推荐答案

您不应该看到，标准库定义在 ; cstddef> （和< stddef.h> ）。并且，根据标准，IIRC重新定义由标准头文件定义的名称将导致未定义的行为。因此，从纯粹的标准角度来看，你不应该这样做。

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.

无论什么原因他们的想法思想：

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

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

（与[错误地]：一样，将虚拟表指针设置为 NULL ）

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

只有在定义 NULL as 0 ，因为 = 0 是纯虚函数的有效标记（§9.2 [class.mem] ）。

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

也就是说， if NULL 正确使用作为空指针常量。

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

但是，请注意，，这将改变：

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*)

然而，如果是这种情况，无论如何。

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

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