NULL是指针吗? [英] Is NULL a pointer?

问题描述

因此，我与教授较早地争论过，认为NULL不是指针，但他一直坚持认为这是因为存在诸如NULL指针之类的东西.所以，现在我有点困惑NULL是否真的是一个指针

So, I had an argument with my professor earlier defending that NULL is not a pointer, but he kept on insisting that it is because there is such a thing as NULL pointer. So, here I am now a little bit confused if NULL is really a pointer or not

我已经尝试通过互联网搜索，但是找不到任何答案，所以我的最后选择是在这里

I already tried search over the internet but couldn't find any answer, so my last resort is here

推荐答案

在C中，NULL是一个扩展为空指针常量的宏.

In C, NULL is a macro that expands to a null pointer constant.

7.19p3

宏是

NULL，它扩展为实现定义的空指针常量； ...

NULL which expands to an implementation-defined null pointer constant; ...

空指针常量是具有值0( 例如0，1-1，42*0LL等)或强制转换为(void*)的表达式.

A null pointer constant is an integer constant expression with the value 0 ( e.g., 0, 1-1, 42*0LL, etc.) or such an expression cast to (void*).

6.3.2.3p3

值为0的整数常量表达式或此类表达式 强制转换为void *类型，称为空指针常量.66)如果为空 将指针常量转换为指针类型，结果 指针，称为空指针，保证将不等于与 指向任何对象或函数的指针.

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

大多数常见的C实现将NULL定义为0，0L或((void*)0).

Most common C implementations define NULL to be 0, 0L, or ((void*)0).

所以您是是正确的. NULL不必是指针.

So you are correct. NULL need not be a pointer.

(IIRC，C ++甚至不允许将(void*)强制转换为NULL，这意味着C ++中的NULL始终具有整数类型.因此，并且由于void*指针 不要与C ++中的常规指针进行比较，因此C ++> = 11现在有一个特殊的nullptr关键字.)

(IIRC, C++ doesn't even allow the (void*) cast in NULL, meaning NULL in C++ always has integer type. Because of that and because void* pointers do not compare with regular pointers so readily in C++, C++>=11 now has a special nullptr keyword.)

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