曾经有人告诉我……我将把NULL重新定义为69。从那以后，我不再使用它:P

它使您的代码非常脆弱。

编辑:

标准中并非所有内容都是完美的。宏NULL是一个实现定义的c++空指针常量，与C NULL宏不完全兼容，除了类型隐藏隐式转换它在一个无用的和容易出错的工具。

NULL不是一个空指针，而是一个O/OL文字。

告诉我下一个例子不会让人困惑:

void foo(char *); 
void foo(int); 
foo(NULL); // calls int version instead of pointer version! 

是不是因为这一切，在新标准中出现了std::nullptr_t

如果你不想等待新标准的出台，想要使用nullptr，至少要使用一个像Meyers建议的那样像样的nullptr(参见jonh comment)。

下面是Stroustrup对此的看法:c++风格和技术常见问题解答

In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days. If you have to name the null pointer, call it nullptr; that's what it's called in C++11. Then, nullptr will be a keyword.

也就是说，不要为小事烦恼。

我认为有一些论点(其中一个是最近的)与Bjarne在这方面的立场相矛盾。

Documentation of intent Using NULL allows for searches on its use and it also highlights that the developer wanted to use a NULL pointer, irrespective of whether it is being interpreted by the compiler as NULL or not. Overload of pointer and 'int' is relatively rare The example that everybody quotes is: void foo(int*); void foo (int); void bar() { foo (NULL); // Calls 'foo(int)' } However, at least in my opinion, the problem with the above is not that we're using NULL for the null pointer constant: it's that we have overloads of foo() which take very different kinds of arguments. The parameter must be an int too, as any other type will result in an ambiguous call and so generate a helpful compiler warning. Analysis tools can help TODAY! Even in the absence of C++0x, there are tools available today that verify that NULL is being used for pointers, and that 0 is being used for integral types. C++ 11 will have a new std::nullptr_t type. This is the newest argument to the table. The problem of 0 and NULL is being actively addressed for C++0x, and you can guarantee that for every implementation that provides NULL, the very first thing that they will do is: #define NULL nullptr For those who use NULL rather than 0, the change will be an improvement in type-safety with little or no effort - if anything it may also catch a few bugs where they've used NULL for 0. For anybody using 0 today... well, hopefully they have a good knowledge of regular expressions...

我通常用0。我不喜欢宏，而且不能保证您使用的某些第三方头文件不会将NULL重新定义为一些奇怪的东西。

你可以使用一个由Scott Meyers和其他人提出的nullptr对象，直到c++获得一个nullptr关键字:

const // It is a const object...
class nullptr_t 
{
public:
    template<class T>
    operator T*() const // convertible to any type of null non-member pointer...
    { return 0; }

    template<class C, class T>
    operator T C::*() const   // or any type of null member pointer...
    { return 0; }

private:
    void operator&() const;  // Can't take address of nullptr

} nullptr = {};

谷歌"nullptr"获取更多信息。

    cerr << sizeof(0) << endl;
    cerr << sizeof(NULL) << endl;
    cerr << sizeof(void*) << endl;

    ============
    On a 64-bit gcc RHEL platform you get:
    4
    8
    8
    ================

这个故事的寓意。在处理指针时应该使用NULL。

1)它声明了你的意图(不要让我搜索你所有的代码，试图找出一个变量是指针还是一些数字类型)。

2)在某些需要变量参数的API调用中，它们将使用null指针来指示参数列表的结束。在这种情况下，使用“0”而不是NULL可能会导致问题。在64位平台上，va_arg调用需要一个64位指针，但您将只传递一个32位整数。在我看来，你是在依靠其他32位来为你归零?我见过某些编译器(例如英特尔的icpc)不是那么友好——这导致了运行时错误。

我曾经在一台机器上工作，其中0是一个有效地址，NULL被定义为一个特殊的八进制值。在该机器上(0 != NULL)，因此代码如

char *p;

...

if (p) { ... }

不会如你所愿。你必须写

if (p != NULL) { ... }

虽然我相信现在大多数编译器都将NULL定义为0，但我仍然记得那些年前的教训:NULL不一定是0。