为什么用于空指针地址为零？ [英] Why is address zero used for the null pointer?

问题描述

在C（或C ++为此事），指针是特殊的，如果他们有值为零：我劝他们释放内存后置指针为零，因为这意味着释放的指针又没有危险;当我调用malloc返回零值的指针，如果它不能让我记忆;我使用如果（P！= 0）所有的时间，以确保传递指针是有效的，等等。

In C (or C++ for that matter), pointers are special if they have the value zero: I am adviced to set pointers to zero after freeing their memory, because it means freeing the pointer again isn't dangerous; when I call malloc it returns a pointer with the value zero if it can't get me memory; I use if (p != 0) all the time to make sure passed pointers are valid, etc.

不过，由于内存寻址0开始，不为0，就像其他任何一个有效的地址？如何能0用于处理空指针，如果是这样的话？为什么不是负数空呢？

But since memory addressing starts at 0, isn't 0 just as a valid address as any other? How can 0 be used for handling null pointers if that is the case? Why isn't a negative number null instead?

编辑：

一堆好答案。我会总结一下已经pssed作为我自己的心间$ P $点的答案前$ P $有人说，并希望社会各界能够纠正我，如果我误解了。

A bunch of good answers. I'll summarize what has been said in the answers expressed as my own mind interprets it and hope that the community will correct me if I misunderstand.

• 就像一切都在规划否则它是一个抽象。只是一个常数，没有真正涉及到地址0的C ++ 0x通过添加关键字 nullptr 强调了这一。

它甚至不是一个地址的抽象，它是由C标准的编译器可以，只要把它翻译为其它数字指定的常数，因为它可以确保它永远不会等于一个真实的地址，等于其它空如果指针0是不使用该平台最大的价值。

It's not even an address abstraction, it's the constant specified by the C standard and the compiler can translate it to some other number as long as it makes sure it never equals a "real" address, and equals other null pointers if 0 is not the best value to use for the platform.

在情况下，它不是一个抽象的，这是在早期的情况下，地址0由系统使用和关闭限制程序员。

In case it's not an abstraction, which was the case in the early days, the address 0 is used by the system and off limits to the programmer.

我的负数的建议是有点野的头脑风暴，我承认。使用地址的符号的整数是有点浪费，如果它意味着，除了空指针（-1或其他）的价值空间被平分正整数，使有效的地址和那些只是浪费负数之间。

My negative number suggestion was a little wild brainstorming, I admit. Using a signed integer for addresses is a little wasteful if it means that apart from the null pointer (-1 or whatever) the value space is split evenly between positive integers that make valid addresses and negative numbers that are just wasted.

如果任何数量总是由一个数据类型重新presentable，这是0.（大概1也是。我觉得一个位整数这将是0或1，如果未签名，或只是签署如果位有符号，或者两个位整数这将是[-2,1]。不过，你可能只是去为0是零，1表示在内存中唯一可字节）。

If any number is always representable by a datatype, it's 0. (Probably 1 is too. I think of the one-bit integer which would be 0 or 1 if unsigned, or just the signed bit if signed, or the two bit integer which would be [-2, 1]. But then you could just go for 0 being null and 1 being the only accessible byte in memory.)

不过有一些东西是在我的脑海解决。堆栈＆NBSP;溢出问题的 指向一个特定的固定地址 的告诉我，即使0空指针抽象，其他指针值不一定。这使我张贴另一个堆栈＆NBSP;溢出问题的 我所能要访问的地址零 的

Still there is something that is unresolved in my mind. The Stack Overflow question Pointer to a specific fixed address tells me that even if 0 for null pointer is an abstraction, other pointer values aren't necessarily. This leads me to post another Stack Overflow question, Could I ever want to access the address zero?.

推荐答案

2点：

• 仅在源$ C ​​$ c中的常数0为空指针 - 编译器实现可以使用它想要或者在运行code需求的任何值。一些平台有一个特殊的指针值就是这样实现可能为空指针使用无效。的C常见问题有一个问题，说真的，有什么实际的机器真正使用非零空指针，或不同的再presentations为指针不同类型的？，即指出，正在使用的C源空指针，而重新在运行时不同psented $ p $ 0这个属性几个平台。 C ++标准有一张纸条，上面明确规定转换整型常量前pression与零值总是产生一个空指针，但转换其他前pressions碰巧具有零值不必产生一个空指针

• only the constant value 0 in the source code is the null pointer - the compiler implementation can use whatever value it wants or needs in the running code. Some platforms have a special pointer value that's 'invalid' that the implementation might use as the null pointer. The C FAQ has a question, "Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?", that points out several platforms that used this property of 0 being the null pointer in C source while represented differently at runtime. The C++ standard has a note that makes clear that converting "an integral constant expression with value zero always yields a null pointer, but converting other expressions that happen to have value zero need not yield a null pointer".

负值可能只是由平台为地址为可用 - C标准索性就选择了一些用来表示一个空指针和被选择为零。我真的不知道，如果其他定点值进行了审议。

a negative value might be just as usable by the platform as an address - the C standard simply had to chose something to use to indicate a null pointer, and zero was chosen. I'm honestly not sure if other sentinel values were considered.

有关空指针的唯一要求是：

The only requirements for a null pointer are:

• 它保证比较不等于一个指向一个实际的对象


• 任何两个空指针将相当于（C ++提炼这使得该只需要为指针，坚持同一类型）

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