NULL是否保证为0?

问题描述

我想知道在C ++中是否保证 NULL 为 0 ，所以我进行了搜索并发现了以下内容:

I was wondering if NULL is guaranteed to be 0 in C++, so I searched and came across these:

此答案指出:

似乎可以确认 NULL 始终为0.

Which seems to confirm that NULL is always 0.

但是根据 cppreference.com :

#define NULL /*implementation-defined*/

宏NULL是实现定义的空指针常量，可能是:

The macro NULL is an implementation-defined null pointer constant, which may be:

->整数类型的整数常量表达式右值，其值是到零(直到C ++ 11)

-> an integral constant expression rvalue of integer type that evaluates to zero (until C++11)

->整数文字，其值为零或prvalue类型std :: nullptr_t(自C ++ 11起)

-> an integer literal with value zero, or a prvalue of type std::nullptr_t (since C++11)

这显然表明 NULL 是依赖于实现的.

And that clearly says NULL is implementation dependent.

此答案是:

f(int);
f(foo *);

再次暗示 NULL 是整数 0 ，这可能会引起歧义

Which again implies that NULL is the integer 0 and that might cause ambiguity

我遇到了其他问题和答案，但它们大多与C语言有关，而不与C ++有关.此评论说:>

There are other questions and answers I encountered, but they are mostly about the C language, not C++. This comment says:

但同样，那是关于C的.

But again, that's about C.

总而言之，在C ++中， NULL 是否总是 0 ?C呢?(对于每个标准实现)是否都与

To sum it all up, is NULL always 0 in C++? What about C? Is it (for every standard implementation) the same as:

#define NULL 0

注意:这个问题与空指针无关，问题在于是否保证C ++中的 NULL 是整数 0 .是否依赖于实现?

Note: this question is not about the null pointer, the question is if NULL in C++ is guaranteed to be 0 the integer. Is it implementation dependent?

推荐答案

根据标准， NULL 是一个空指针常量(即文字).究竟是哪个定义了实现.

According to the standard, NULL is a null pointer constant (i.e. literal). Exactly which one, is implementation defined.

在C ++ 11之前，空指针常量是整数值，其整数值等于0，因此 0 或 0l 等.

Prior to C++11, null pointer constants were integral constants whose integral value is equal to 0, so 0 or 0l etc.

自C ++ 11起，有一个新的空指针文字 nullptr ，并且 NULL 可以定义为 nullptr .(因此，对Bjarne的报价的字面解释已经过时了.)

Since C++11, there is a new null pointer literal nullptr and NULL may be defined as being nullptr. (And thus literal interpretation of Bjarne's quote has become obsolete).

在标准化之前: NULL 在C中可以定义为(void *)0 .由于C ++基于C，所以某些C ++方言可能会约会标准可能使用了该定义，但此定义不符合标准C ++.

Prior to standardisation: NULL may be defined as (void*)0 in C. Since C++ was based on C, it is likely that some C++ dialects pre-dating the standard might have used that definition, but such definition is not conformant with standard C++.

为了完整起见:正如下面的注释中链接的SO post中所详细说明的那样，空指针常量为0并不一定意味着空指针地址的值为0(尽管地址为0是很典型的)

And for completeness: As explained in more detail in SO post linked in a comment below, null pointer constant being 0 does not necessarily mean that the value of the null pointer address is 0 (although the address being 0 is quite typical).

对此可以得出什么结论

• 不要使用 NULL 表示数字零(如果合适，请使用带有适当类型后缀的 0 )，也不要表示一个以空字符结尾的字符(使用>'\ 0').
• 不要假定 NULL 会解决指针重载.
• 要表示空指针，请不要使用 NULL ，而如果您的标准是> = C ++ 11，请使用 nullptr .在较旧的标准中，如果您需要使用它来解决过载，则可以使用(T *)NULL 或(T *)0 .用指针重载整数毫无意义.
• 考虑到从C转换为C ++时的定义可能不同，反之亦然.
• 不要将零位存储(或键入pun)零位到指针中.不能保证它是空指针.

• Don't use NULL to represent the number zero (use 0 with appropriate type suffix if appropriate), nor to represent a null-terminator character (use '\0').
• Don't assume that NULL resolves to a pointer overload.
• To represent a null pointer, don't use NULL but instead use nullptr if your standard is >= C++11. In older standard you can use (T*)NULL or (T*)0 if you need it for overload resolution... that said there are probably very few cases where overloading integers with pointers makes any sense.
• Consider that the definition may differ when converting from C to C++ and vice versa.
• Don't memset (or type pun) zero bits into a pointer. That's not guaranteed to be the null pointer.

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